Metabolism of lipids: digestion, absorption, resynthesis in the intestinal wall.Metabolism of lipids: oxidation and biosynthesis of fatty acids, triacylglycerols and phospholipids.
Lipids are water-insoluble organic biomolecules that can be extracted from cells and tissues by nonpolar solvents, e.g., chloroform, ether, or benzene.
Lipids are an amphiphilic class of hydrocarbon-containing organic compounds. Lipids are categorized by the fact that they have complicated solvation properties, giving rise to lipid polymorphism. Lipid molecules have these properties because they consist largely of long hydrocarbon tails which are lipophilic in nature as well as polar headgroups (e.g. phosphate-based functionality, and/or inositol based functionality). In living organisms, lipids are used for energy storage, serve as the structural components of cell membranes, and constitute important signalling molecules. Although the term lipid is often used as a synonym for fat, the latter is in fact a subgroup of lipids called triglycerides.
There are several different families or classes of lipids but all derive their distinctive properties from the hydrocarbon nature of a major portion of their structure.
Biological functions of lipids
Biological molecules that are insoluble in aqueous solutions and soluble in organic solvents are classified as lipids. The lipids of physiological importance for humans have four major functions:
Lipids have several important biological functions, serving
(1) as structural components of membranes,
(2) as storage and transport forms of metabolic fuel,
(3) as a protective coating on the surface of many organisms, and
(5) as cell-surface components concerned in cell recognition, species specificity, and tissue immunity. Some substances classified among the lipids have intense biological activity; they include some of the vitamins and hormones.
Although lipids are a distinct class of biomolecules, we shall see that they often occur combined, either covalently or through weak bonds, with members of other classes of biomolecules to yield hybrid molecules such as glycolipids, which contain both carbohydrate and lipid groups, "and lipoproteins, which contain both lipids and proteins. In such biomolecules the distinctive chemical and physical properties of their components are blended to fill specialized biological functions.
Classification of lipids
Lipids have been classified in several different ways. The most satisfactory classification is based on their backbone structures:
1. Simple lipids:
2. Complex lipids:
Lipids usually contain fatty acids as components. Such lipids are called saponifiable lipids since they yield soaps (salts of fatty acids) on alkaline hydrolysis. The other great group of lipids which do not contain fatty acids and hence are nonsapomfiable.
Let us first consider the structure and properties of fatty acids, characteristic components of all the complex lipids.
Fatty acids and glycerides
Fatty acids fill two major roles in the body:
· 1. as the components of more complex membrane lipids.
· 2. as the major components of stored fat in the form of triacylglycerols.
Fatty acids are long-chain hydrocarbon molecules containing a carboxylic acid moiety at one end. The numbering of carbons in fatty acids begins with the carbon of the carboxylate group. At physiological pH, the carboxyl group is readily ionized, rendering a negative charge onto fatty acids in bodily fluids.
Fatty acids that contain no carbon-carbon double bonds are termed saturated fatty acids; those that contain double bonds are unsaturated fatty acids. The numeric designations used for fatty acids come from the number of carbon atoms, followed by the number of sites of unsaturation (eg, palmitic acid is a 16-carbon fatty acid with no unsaturation and is designated by 16:0). The site of unsaturation in a fatty acid is indicated by the symbol and the number of the first carbon of the double bond (e.g. palmitoleic acid is a 16-carbon fatty acid with one site of unsaturation between carbons 9 and 10, and is designated by 16:19).
Saturated fatty acids of less than eight carbon atoms are liquid at physiological temperature, whereas those containing more than ten are solid. The presence of double bonds in fatty acids significantly lowers the melting point relative to a saturated fatty acid.
The majority of body fatty acids
are acquired in the diet. However, the lipid biosynthetic capacity of the body
(fatty acid synthase and other fatty acid modifying enzymes) can supply the
body with all the various fatty acid structures needed. Two key exceptions to
this are the highly unsaturated fatty acids know as linoleic acid and linolenic
acid, containing unsaturation sites beyond carbons 9 and 10. These two fatty
acids cannot be synthesized from precursors in the body, and are thus
considered the essential fatty acids;
essential in the sense that they must be provided in the diet. Since plants are
capable of synthesizing linoleic and linolenic acid humans can aquire these
fats by consuming a variety of plants or else by eating the meat of animals
that have consumed these plant fats.
Chemically, fatty acids can be described as long-chain monocarboxylic acids and have a general structure of CH3(CH2)nCOOH. The length of the chain usually ranges from 12 to 24, always with an even number of carbons. When the carbon chain contains no double bonds, it is a saturated chain. If it contains one or more such bonds, it is unsaturated. The presence of double bonds generally reduces the melting point of fatty acids. Furthermore, unsaturated fatty acids can occur either in cis or trans geometric isomers. In naturally occurring fatty acids, the double bonds are in the cis-configuration.
Glycerides are lipids possessing a glycerol (propan-1, 2, 3-triol) core structure with one or more fatty acyl groups, which are fatty acid-derived chains attached to the glycerol backbone by ester linkages. Glycerides with three acyl groups (triglycerides or neutral fats) are the main storage form of fat in animals and plants.
An important type of glyceride-based molecule found in biological membranes, such as the cell's plasma membrane and the intracellular membranes of organelles, are the phosphoglycerides or glycerophospholipids. These are phospholipids that contain a glycerol core linked to two fatty acid-derived "tails" by ester or, more rarely, ether linkages and to one "head" group by a phosphate ester linkage. The head groups of the phospholipids found in biological membranes are phosphatidylcholine (also known as PC, and lecithin), phosphatidylethanolamine (PE), phosphatidylserine and phosphatidylinositol (PI). These phospholipids are subject to a variety of functions in the cell: for instance, the lipophilic and polar ends can be released from specific phospholipids through enzyme-catalysed hydrolysis to generate secondary messengers involved in signal transduction. In the case of phosphatidylinositol, the head group can be enzymatically modified by the addition of one, two or three phosphate groups, this constituting another mechanism of cell signalling. While phospholipids are the major component of biological membranes, other non-glyceride lipid components like sphingolipids and sterols (such as cholesterol in animal cell membranes) are also found in biological membranes.
A biological membrane is a form of lipid bilayer, as is a liposome. Formation of lipid bilayers is an energetically-favoured process when the glycerophospholipids described above are in an aqueous environment. In an aqueous system, the polar heads of lipids orientate towards the polar, aqueous environment, while the hydrophobic tails minimise their contact with water. The lipophilic tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). Other aggregations are also observed and form part of the polymorphism of amphiphile (lipid) behaviour. The polar heads (P) face the aqueous environment, curving away from the water. Phase behaviour is a complicated area within biophysics and is the subject of current academic research.
Micelles and bilayers form in the polar medium by a process known as the lipophilic effect. When dissolving a lipophilic or amphiphilic substance in a polar environment, the polar molecules (i.e. water in an aqueous solution) become more ordered around the dissolved lipophilic substance, since the polar molecules cannot form hydrogen bonds to the lipophilic areas of the amiphphile. So, in an aqueous environment the water molecules form an ordered "clathrate" cage around the dissolved lipophilic molecule.
The self-organisation depends on the concentration of the lipid present in solution. Below the critical micelle concentration, the lipids form a single layer on the liquid surface and are (sparingly) dispersed in the solution. At the first critical micelle concentration (CMC-I), the lipids organise in spherical micelles, at given points above this concentration, other phases are observed (see lipid polymorphism).
Although fatty acids occur in very large amounts as building-block components of the saponifiable lipids, only traces occur in free (unesterified) form in cells and tissues. Well over 100 different kinds of fatty acids have been isolated from various lipids of animals, plants, and microorganisms. All possess a long hydrocarbon chain and a terminal carboxyl group. The hydrocarbon chain may be saturated, as in palmitic acid, or it may have one or more double bonds, as in oleic acid; a few fatty acids contain triple bonds. Fatty acids differ from each other primarily in chain length and in the number and position of their unsaturated bonds. They are often symbolized by a shorthand notation that designates the length of the carbon chain and the number, position, and configuration of the double bonds. Thus palmitic acid (16 carbons, saturated) is symbolized 16:0 and oleic acid [18 carbons and one double bond (cis) at carbons 9 and 10] is symbolized 18:1. It is understood that the double bonds are cis (see below) unless indicated otherwise.
Some generalizations can be made on the different fatty acids of higher plants and animals. The most abundant have an even number of carbon atoms with chains between 14 and 22 carbon atoms long, but those with 16 or 18 carbons predominate. The most common among the saturated fatty acids are palmitic acid (Cis) and stearic acid (Cis) and among the unsaturated fatty acids oleic acid (Cis). Unsaturated fatty acids predominate over the saturated ones, particularly in higher
plants and in animals living at low temperatures. Unsaturated fatty acids have lower melting points than saturated fatty acids of the same chain length. In most monounsaturated (monoenoic) fatty acids of higher organisms there is a double bond between carbon atoms 9 and 10. In most polyunsaturated (polyenoic) fatty acids one double bond is between carbon atoms 9 and 10; the additional double bonds usually occur between the 9,10 double bond and the methyl-terminal end of the chain. In most types of polyunsaturated fatty acids the double bonds are separated by one methylene group, for example, —CH=CH—CH2—CH=CH—; only in a few types of plant fatty acids are the double bonds in conjugation, that is, —CH=CH—CH=CH—. The double bonds of nearly all kinds of naturally occurring unsaturated fatty acids are in the cis geometrical configuration; only a very few are trans.
There are two kinds of fats, saturated and unsaturated. Unsaturated fats have at least one double
bond in one of the fatty acids. A double bond happens when two electrons are
shared or exchanged in a bond. They are much stronger than single bonds. Saturated fats have no double bonds.
Fats have a lot of energy stored up in their molecular bonds. That's why the human body stores fat as an energy source. When it needs extra fuel, your body breaks down the fat and uses the energy. Where one molecule of sugar only gives a small amount of energy, a fat molecule gives off many times more.
Some naturally occuring fatty acids
Saturated fatty acid
Unsaturated monoenic fatty acid
Unsaturated polienic fatty acid
All Lipids are hydrophobic: that’s the one property they have in common. This group of molecules includes fats and oils, waxes, phospholipids, steroids (like cholesterol), and some other related compounds.
Structure of Fatty Acids
Fats and oils are made from two kinds of molecules: glycerol (a type of alcohol with a hydroxyl group on each of its three carbons) and three fatty acids joined by dehydration synthesis. Since there are three fatty acids attached, these are known as triglycerides. “Bread” and pastries from a “bread factory” often contain mono- and diglycerides as “dough conditioners.” Can you figure out what these molecules would look like? The main distinction between fats and oils is whether they’re solid or liquid at room temperature, and this, as we’ll soon see, is based on differences in the structures of the fatty acids they contain.
Essential fatty acids
When weanling or immature rats are placed on a fat-free diet, they grow poorly, develop a scaly skin, lose hair, and ultimately die with many pathological signs. When linoleic acid is present in the diet, these conditions do not develop. Linolenic acid and arachidonic acid also prevent these symptoms. Saturated and monounsaturated fatty acids are inactive. It has been concluded that mammals can synthesize saturated and monounsaturated fatty acids from other precursors but are unable to make linoleic and linolenic acids. Fatty acids required in the diet of mammals are called essential fatty acids. The most abundant essential fatty acid in mammals is linoleic acid, which makes up from 10 to 20 percent of the total fatty acids of their triacylglycerols and phosphoglycerides. Linoleic and linolenic acids cannot be synthesized by mammals but must be obtained from plant sources, in which they are very abundant. Linoleic acid is a necessary precursor in mammals for the biosynthesis of arachidonic acid, which is not found in plants.
The terms saturated, mono-unsaturated, and poly-unsaturated refer to the number of hydrogens attached to the hydrocarbon tails of the fatty acids as compared to the number of double bonds between carbon atoms in the tail. Fats, which are mostly from animal sources, have all single bonds between the carbons in their fatty acid tails, thus all the carbons are also bonded to the maximum number of hydrogens possible. Since the fatty acids in these triglycerides contain the maximum possible amouunt of hydrogens, these would be called saturated fats. The hydrocarbon chains in these fatty acids are, thus, fairly straight and can pack closely together, making these fats solid at room temperature. Oils, mostly from plant sources, have some double bonds between some of the carbons in the hydrocarbon tail, causing bends or “kinks” in the shape of the molecules. Because some of the carbons share double bonds, they’re not bonded to as many hydrogens as they could if they weren’t double bonded to each other. Therefore these oils are called unsaturated fats. Because of the kinks in the hydrocarbon tails, unsaturated fats can’t pack as closely together, making them liquid at room temperature. Many people have heard that the unsaturated fats are “healthier” than the saturated ones. Hydrogenated vegetable oil (as in shortening and commercial peanut butters where a solid consistency is sought) started out as “good” unsaturated oil. However, this commercial product has had all the double bonds artificially broken and hydrogens artificially added (in a chemistry lab-type setting) to turn it into saturated fat that bears no resemblance to the original oil from which it came (so it will be solid at room temperature).
Although the specific functions of essential fatty acids in mammals were a mystery for many years, one function has been discovered. Essential fatty acids are necessary precursors in the biosynthesis of a group of fatty acid derivatives called prostaglandins, hormonelike compounds which in trace amounts have profound effects on a number of important physiological activities.
Physical and chemical properties of fatty acids
Saturated and unsaturated fatty acids have quite different conformations. In saturated fatty acids, the hydrocarbon tails are flexible and can exist in a very large number of conformations because each single bond in the backbone has complete freedom of rotation. Unsaturated fatty acids, on the other hand, show one or more rigid kinks contributed by the nonrotating double bond(s).
Unsaturated fatty acids undergo addition reactions at their double bonds. Quantitative titration with halogens, e.g., iodine or bromine, can yield information on the relative number of double bonds in a given sample of fatty acids or lipid.
Fat is also known as a triglyceride. It is made up of a molecule known as glycerol that is connected to one, two, or three fatty acids. Glycerol is the basis of all fats and is made up of a three-carbon chain. It connects the fatty acids together. A fatty acid is a long chain of carbon atoms connected to each other.
Fatty acid esters of the alcohol glycerol are called acylglycerols or glycerides; they are sometimes referred to as "neutral fats," a term that has become archaic. When all three hydroxyl groups of glycerol are esterified with fatty acids, the structure is called a triacylglycerol:
Although the name "triglyceride" has been traditionally used to designate these compounds, an international nomenclature commission has recommended that this chemically inaccurate term no longer be used. Triacylglycerols are the most abundant family of lipids and the major components of depot or storage lipids in plant and animal cells. Triacylglycerols that are solid at room temperature are often referred to as "fats" and those which are liquid as "oils." Diacylgiycerols (also called diglycerides) and monoacylgiycerols (or monoglycerides) are also found in nature, but in much smaller amounts.
Triacylglycerols occur in many different types, according to the identity and position of the three fatty acid components esterified to glycerol. Those with a single kind of fatty acid in all three positions, called simple triacylglycerols, are named after the fatty acids they contain. Examples are tristearoylglycerol, tripalmitoylglycerol, and trioleoylglycerol; the trivial and more commonly used names are tristearin, tripalmitin, and trioiein, respectively. Mixed triacylglycerols contain two or more different fatty acids. The naming of mixed triacylglycerols can be illustrated by the example of 1-palmitoyldi-stearoylglycerol (trivial name, 1-palmitodistearin). Most natural fats are extremely complex mixtures of simple and mixed triacylglycerols.
Properties of triacylglycerols
The melting point of triacylglycerols is determined by their fatty acid components. In general, the melting point increases with the number and length of the saturated fatty acid components. For example, tripalmitin and tristearin are solids at body temperature, whereas triolein and trilinolein are liquids. All triacylglycerols are insoluble in water and do not tend by themselves to form highly dispersed micelles. However, diacylglycerols and monoacylglycerols have appreciable polarity because of their free hydroxyl groups and thus can form micelles. Diacyl- and monoacylglycerols find wide use in the food industry in the production of more homogeneous and more easily processed foods; they are completely digestible and utilized biologically. Acylglycerols are soluble in ether, chloroform, benzene, and hot ethanol. Their specific gravity is lower than that of water. Acylglycerols undergo hydrolysis when boiled with acids or bases or by the action of lipases, e.g., those present in pancreatic juice. Hydrolysis with alkali, called saponification, yields a mixture of soaps and glycerol.
Steroids occur in animals in
something called hormones. The basis of a steroid molecule is a four-ring
structure, one with five carbons and three with six carbons in the rings. You
may have heard of steroids in the news. Many body builders and athletes use
anabolic steroids to build muscle mass. The steroids make their body want to
add more muscle than they normally would be able to. The body builders wind
up stronger and bulkier (but not faster).
Never take drugs to enhance your body. Those body builders are actually hurting their bodies. They can't see it because it is slowly destroying their internal organs and not the muscles. When they get older, they can have kidney and liver problems. Some even die.
The important class of lipids called steroids are actually metabolic derivatives of terpenes, but they are customarily treated as a separate group. Steroids may be recognized by their tetracyclic skeleton, consisting of three fused six-membered and one five-membered ring, as shown in the diagram to the right. The four rings are designated A, B, C & D as noted, and the peculiar numbering of the ring carbon atoms (shown in red) is the result of an earlier misassignment of the structure. The substituents designated by R are often alkyl groups, but may also have functionality. The R group at the A:B ring fusion is most commonly methyl or hydrogen, that at the C:D fusion is usually methyl. The substituent at C-17 varies considerably, and is usually larger than methyl if it is not a functional group. The most common locations of functional groups are C-3, C-4, C-7, C-11, C-12 & C-17. Ring A is sometimes aromatic. Since a number of tetracyclic triterpenes also have this tetracyclic structure, it cannot be considered a unique identifier.
Steroids are widely distributed in animals,
where they are associated with a number of physiological processes. Examples of
some important steroids are shown in the following diagram. Different kinds of
steroids will be displayed by clicking the "Toggle Structures"
button under the diagram. Norethindrone is a synthetic steroid, all the other
examples occur naturally. A common strategy in pharmaceutical chemistry is to
take a natural compound, having certain desired biological properties together
with undesired side effects, and to modify its structure to enhance the desired
characteristics and diminish the undesired. This is sometimes accomplished by
trial and error.
The generic steroid structure drawn above has seven chiral stereocenters (carbons 5, 8, 9, 10, 13, 14 & 17), which means that it may have as many as 128 stereoisomers. With the exception of C-5, natural steroids generally have a single common configuration. This is shown in the last of the toggled displays, along with the preferred conformations of the rings.
Chemical studies of the steroids were very important to our present understanding of the configurations and conformations of six-membered rings. Substituent groups at different sites on the tetracyclic skeleton will have axial or equatorial orientations that are fixed because of the rigid structure of the trans-fused rings. This fixed orientation influences chemical reactivity, largely due to the greater steric hindrance of axial groups versus their equatorial isomers. Thus an equatorial hydroxyl group is esterified more rapidly than its axial isomer.
Steroids are complex ethers of cyclic spirits sterols and fatty acids. Sterols are
derivatives of the saturated tetracylic hydrocarbon cyclopentanoperhydrophenanthrene:
The general structure of cholesterol consists of two six-membered rings side-by-side and sharing one side in common, a third six-membered ring off the top corner of the right ring, and a five-membered ring attached to the right side of that.
The central core of this molecule, consisting of four fused rings, is shared by all steroids, including estrogen (estradiol), progesterone, corticosteroids such as cortisol (cortisone), aldosterone, testosterone, and Vitamin D. In the various types of steroids, various other groups/molecules are attached around the edges. Know how to draw the four rings that make up the central structure.
Cholesterol is not a “bad guy!” Our bodies make about 2 g of cholesterol per day, and that makes up about 85% of blood cholesterol, while only about 15% comes from dietary sources. Cholesterol is the precursor to our sex hormones and Vitamin D. Vitamin D is formed by the action of UV light in sunlight on cholesterol molecules that have “risen” to near the surface of the skin. At least one source I read suggested that people not shower immediately after being in the sun, but wait at least ½ hour for the new Vitamin D to be absorbed deeper into the skin. Our cell membranes contain a lot of cholesterol (in between the phospholipids) to help keep them “fluid” even when our cells are exposed to cooler temperatures.
Many people have hear the claims that egg yolk
contains too much cholesterol, thus should not be eaten. An interesting study
was done at
A great many different steroids, each with a distinctive function or activity, have been isolated from natural sources. Steroids differ in the number and position of double bonds, in the type, location, and number of substituent functional groups, in the configuration of the bonds between the substituent groups and the nucleus, and in the configuration of the rings in relation to each other. Cholesterol is the most abundant steroid in animal tissues. Cholesterol and lanosterol are members of a large subgroup of steroids called the sterols. They are steroid alcohols containing a hydroxyl group at carbon 3 of ring A and a branched aliphatic chain of eight or more carbon atoms at carbon 17. They occur either as free alcohols or as long-chain fatty acid esters of the hydroxyl group at carbon 3; all are solids at room temperature. Cholesterol melts at 150 °C and is insoluble in water but readily extracted from tissues with chloroform, ether, benzene, or hot alcohol. Cholesterol occurs in the plasma membranes of many animal cells and in the lipoproteins of blood plasma. Lanosterol was first found in the waxy coating of wool in esterified form before it was established as an important intermediate in the biosynthesis of cholesterol in animal tissues.
Cholesterol is the precursor of many other steroids in animal tissues, including the bile acids, detergentlike compounds that aid in emulsification and absorption of lipids in the intestine; the androgens, or male sex hormones; the estrogens, or female sex hormones; the progestational hormone progesterone; and the adrenocortical hormones. Among the most important steroids are a group of compounds having vitamin D activity.
Waxes are water-insoluble, solid esters of higher fatty acids with long-chain monohydroxylic fatty alcohols or with sterols. They are soft and pliable when warm but hard when cold. Waxes are found as protective coatings on skin, fur, and feathers, on leaves and fruits of higher plants, and on the exoskeleton of many insects. The major components of beeswax are palmitic acid esters of long-chain fatty alcohols with 26 to 34 carbon atoms. Lanolin, or wool fat, is a mixture of fatty acid esters of the sterols lanosterol and agnosterol.
Waxes are used to coat and protect
things in nature. Bees make wax. Your ears make wax. Plant leaves even have wax
on the outside of their leaves. It can be used for structures such as the bees'
honeycombs. Waxes can also be used for protection. Plants use wax to stop
evaporation of water from their leaves.
Prostaglandins Thromboxanes & Leukotrienes
The members of this group of structurally related natural hormones have an extraordinary range of biological effects. They can lower gastric secretions, stimulate uterine contractions, lower blood pressure, influence blood clotting and induce asthma-like allergic responses. Because their genesis in body tissues is tied to the metabolism of the essential fatty acid arachadonic acid (5,8,11,14-eicosatetraenoic acid) they are classified as eicosanoids. Many properties of the common drug asprin result from its effect on the cascade of reactions associated with these hormones.
The metabolic pathways by which arachidonic acid is converted to the various eicosanoids are complex and will not be discussed here. A rough outline of some of the transformations that take place is provided below. It is helpful to view arachadonic acid in the coiled conformation shown in the shaded box.
The basic structure of phospolipids is very similar to that of the triacylglycerides except that C-3 (sn3)of the glycerol backbone is esterified to phosphoric acid. The building block of the phospholipids is phosphatidic acid which results when the X substitution in the basic structure shown in the Figure below is a hydrogen atom. Substitutions include ethanolamine (phosphatidylethanolamine), choline (phosphatidylcholine, also called lecithins), serine (phosphatidylserine), glycerol (phosphatidylglycerol), myo-inositol (phosphatidylinositol, these compounds can have a variety in the numbers of inositol alcohols that are phosphorylated generating polyphosphatidylinositols), and phosphatidylglycerol.
Phosphoglycerides are characteristic major components of cell membranes; only very small amounts of phosphoglycerides occur elsewhere in cells.
Phospholipids are made from glycerol, two fatty acids, and (in place of the third fatty acid) a phosphate group with some other molecule attached to its other end. The hydrocarbon tails of the fatty acids are still hydrophobic, but the phosphate group end of the molecule is hydrophilic because of the oxygens with all of their pairs of unshared electrons. This means that phospholipids are soluble in both water and oil.
An emulsifying agent is a substance which is soluble in both oil and water, thus enabling the two to mix. A “famous” phospholipid is lecithin which is found in egg yolk and soybeans. Egg yolk is mostly water but has a lot of lipids, especially cholesterol, which are needed by the developing chick. Lecithin is used to emulsify the lipids and hold them in the water as an emulsion. Lecithin is the basis of the classic emulsion known as mayonnaise.
cell membranes are made mostly of
phospholipids arranged in a double
layer with the tails from both layers “inside” (facing toward each other) and the heads facing “out” (toward the watery environment) on both surfaces.
In phosphoglycerides one of the primary hydroxyl groups of glycerol is esterified to phosphoric acid; the other hydroxyl groups are esterified to fatty acids. The parent compound of the series is thus the phosphoric ester of glycerol.
Because phosphoglycerides possess a polar head in addition to their nonpolar hydrocarbon tails, they are called amphipathic or polar lipids. The different types of phosphoglycerides differ in the size, shape, and electric charge of their polar head groups.
The parent compound of the phosphoglycerides is phosphatidic acid, which contains no polar alcohol head group. It occurs in only very small amounts in cells, but it is an important intermediate in the biosynthesis of the phosphoglycerides.
The most abundant phosphoglycerides in higher plants and animals are phosphatidylethanoamme and phosphatidylchohne, which contain as head groups the amino alcohols ethanoiamine and choline, respectively. (The new names recommended for these phosphoglycerides are ethanolamine phosphoglyceride and choline phosphoglyceride, but they have not yet gained wide use. The old trivial names are cephalin and lecithin, respectively.) These two phosphoglycerides are major components of most animal cell membranes.
In phosphqtidylserine, the hydroxyl group of the amino acid L-serine is esterified to the phosphoric acid.
Closely related to phosphatidylglycerol is the more complex lipid cardiolipin, also called diphosphatidylglycerol, which consists of a molecule of phosphatidylglycerol in which the 3'-hydroxyl group of the second glycerol moiety is esterified to the phosphate group of a molecule of phosphatidic acid. The backbone of cardiolipin thus consists of three molecules of glycerol joined by two phosphodiester bridges; the two hydroxyl groups of both external glycerol molecules are esterified with fatty acids. Cardiolipin is present in large amounts in the inner membrane of mitochondria; it was first isolated from heart muscle, in which mitochondria are abundant.
Lipid Soluble Vitamins
The essential dietary substances called vitamins are commonly classified as "water soluble" or "fat soluble". Water soluble vitamins, such as vitamin C, are rapidly eliminated from the body and their dietary levels need to be relatively high. The recommended daily allotment (RDA) of vitamin C is 100 mg, and amounts as large as 2 to 3 g are taken by many people without adverse effects. The lipid soluble vitamins, shown in the diagram below, are not as easily eliminated and may accumulate to toxic levels if consumed in large quantity. The RDA for these vitamins are:
800 μg ( upper limit ca. 3000 μg)
Vitamin D 5 to 10 μg ( upper limit ca. 2000 μg)
Vitamin E 15 mg ( upper limit ca. 1 g)
Vitamin K 110 μg ( upper limit not specified)
From this data it is clear that vitamins A and D, while essential to good health in proper amounts, can be very toxic. Vitamin D, for example, is used as a rat poison, and in equal weight is more than 100 times as poisonous as sodium cyanide. From the structures shown here, it should be clear that these compounds have more than a solubility connection with lipids. Vitamins A is a terpene, and vitamins E and K have long terpene chains attached to an aromatic moiety. The structure of vitamin D can be described as a steroid in which ring B is cut open and the remaining three rings remain unchanged. The precursors of vitamins A and D have been identified as the tetraterpene beta-carotene and the steroid ergosterol, respectively.
Properties of phosphoglycerides
Phosphoglycerides are soluble in most nonpolar solvents containing some water and are best extracted from cells and tissues with chloroform-methanol mixtures. When phosphoglycerides are placed in water, they appear to dissolve, but only very minute amounts go into true solution; most of the "dissolved" lipid is in the form of micelles.
Phosphoglycerides have variations in the size, shape, polarity, and electric charge and it plays a significant role in the structure of various types of cell membranes.
Phosphoglycerides can be hydrolyzed by specific phospholipases, which have become important tools in the determination of phosphoglyceride structure. Phospholipase A1 specifically removes the fatty acid from the 1 position and phospholipase A2 from the 2 position. Removal of one fatty acid molecule from a phosphoglyceride yields a lysophosphoglyceride, e.g., lysophosphatidyl-ethanolamine. Lysophosphoglycerides are intermediates in phosphoglyceride metabolism but are found in cells or tissues in only very small amounts; in high concentrations they are toxic and injurious to membranes. Phospholipase B can bring about successive removal of the two fatty acids of phosphoglycerides. Phospholipase C hydrolyzes the bond between phosphoric acid and glycerol, while phospholipase D removes the polar head group to leave a phosphatidic acid.
Sphingolipids are composed of a backbone of sphingosine which is derived itself from glycerol. Sphingosine is N-acetylated by a variety of fatty acids generating a family of molecules referred to as ceramides. Sphingolipids predominate in the myelin sheath of nerve fibers. Sphingomyelin is an abundant sphingolipid generated by transfer of the phosphocholine moiety of phosphatidylcholine to a ceramide, thus sphingomyelin is a unique form of a phospholipid.
The other major class of sphingolipids (besides the sphingomyelins) are the glycosphingolipids generated by substitution of carbohydrates to the sn1 carbon of the glycerol backbone of a ceramide. There are 4 major classes of glycosphingolipids:
n Cerebrosides: contain a single moiety, principally galactose.
n Sulfatides: sulfuric acid esters of galactocerebrosides.
n Globosides: contain 2 or more sugars.
n Gangliosides: similar to globosides except also contain sialic acid.
Sphingophospholipids, complex lipids containing as their backbone sphingosine, are important membrane components in both plant and animal cells. They are present in especially large amounts in brain and nerve tissue. Only trace amounts of sphingophospholipids are found in depot fats. All sphingophospholipids contain four characteristic building-block components: one molecule of a fatty acid, one molecule of sphingosine, phosphoric acid and a polar head group, which in some sphingolipids is very large and complex.
The most abundant sphingophospholipids in the tissues of higher animals are sphingomyelins, which contain phosphorylethanolamine or phosphorylcholine as their polar head groups, esterified to the 1-hydroxyl group of ceramide. Sphingomyelins have physical properties very similar to those of phosphatidylethanolamine and phosphatidylcholine.
Glycosyldiqcylglycerols contain a sugar in glycosidic linkage with the unesterified 3-hydroxyl group of diacylglycerols. A common example is galactosyldiacylglycerol, found in higher plants and also in neural tissue of vertebrates.
This class of glycolipids contains one or more neutral sugar residues as their polar head groups and thus has no electric charge; they are called neutral glycosphingolipids. The simplest of these are the cerebrosides, which contain as their polar head group a monosaccharide bound in beta-glycosidic linkage to the hydroxyl group of ceramide. The cerebrosides of the brain and nervous system contain D-galactose and are therefore called galactocerebrosides. Cerebrosides are also present in much smaller amounts in nonneural tissues of animals, where, because they usually contain D-glucose instead of D-galactose, they are called glucocerebrosides.
Sulfate esters of galactocerebrosides (at the 3 position of the D-galactose) are also present in brain tissue; they are called sulfotides.
The neutral glycosphingolipids are important cell-surface components in animal tissues. Their nonpolar tails presumably penetrate into the lipid bilayer structure of cell membranes, whereas the polar heads protrude outward from the surface. Some of the neutral glycosphingolipids are found on the surface of red blood cells and give them blood-group specificity.
Acidic glycosphingolipids (gangliosides)
Gangliosides contain in their oligosaccharide head groups one or more residues of a sialic acid, which gives the polar head of the gangliosides a net negative charge at pH 7.0. The sialic acid usually found in human gangliosides is N-acetylneuraminic acid. Gangliosides are most abundant in the gray matter of the brain, where they constitute 6 percent of the total lipids, but small amounts are also found in nonneural tissues.
Function of glycosphingolipids
Although glycosphingolipids are only minor constituents of membranes, they appear to be extremely important in a number of specialized functions. Because gangliosides are especially abundant in nerve endings, it has been suggested that they function in the transmission of nerve impulses across synapses. They are also believed to be present at receptor sites for acetylcholine and other neurotransmitter substances. Some of the cell-surface glycosphingolipids are concerned not only in blood-group specificity but also in organ and tissue specificity. These complex lipids are also involved in tissue immunity and in cell-cell recognition sites fundamental to the development and structure of tissues. Cancer cells, for example, have characteristic glycosphingolipids different from those in normal cells.
The most important derivatives of lipids
The lipids discussed up to this point contain fatty acids as building blocks, which can be released on alkaline hydrolysis. The simple lipids contain no fatty acids. They occur in smaller amounts in cells and tissues than the complex lipids, but they include many substances having profound biological activity—vitamins, hormones, and other highly specialized fat-soluble biomolecules.
Prostaglandins are a family of fatty acid derivatives which have a variety of potent biological activities of a hormonal or regulatory nature. Prostaglandins function as regulators of metabolism in a number of tissues and in a number of ways.
All the natural prostaglandins are biologically derived by cyclization of 20-carbon unsaturated fatty acids, such as arachidonic acid, which is formed from the essential fatty acid linoleic acid. The prostaglandins differ from each other with respect to their biological activity, although all show at least some activity in lowering blood pressure and inducing smooth muscle to contract. Some, like PGE2, antagonize the action of certain hormones. PGE2 and PGE2a may find clinical use in inducing labor and bringing about therapeutic abortion.
Digestion of fats
By far the most common of the diet are the neutral fats, also known as triglycerides, each molecule of which is composed of a glycerol nucleus and three fatty acids, as illustrated. Neutral fat is found in food of both animal and and plant origin. In the usual diet are also small quantities of phospholipids, cholesterol, and cholesterol esters.
Digestion of fats in the intestine. A small amount of short chain triglycerides is digested in the stomach by gastric lipase.
Emulsification of fat by bile acids. The first in fat digestion is to break the fat globules into s sizes so that the water-soluble digestive enzymes act on the globule surfaces. This process is called emulsification of the
fat, and it is achieved under
the presence of bile acids. Bile contain a large quantity of bile salts, mainly in the form of ionized sodium salts.
The carboxyl and other parts of the bile salt molecule are highly soluble in water, whereas most of the sterol portion of the bile is highly soluble in fat. Therefore, the fat-soluble portion of the bile salt dissolves in the surface layer of the fat globule and polar portion of the bile salt is soluble in the surrounding fluids. This effect decreases the interfacial tension of the fat. When the interfacial tension of a globule is low, globule is broken up into many minute particles. The total surface area of the particles in the intestinal contents is inversely proportional to the diameters of the particles. The lipases are water-soluble compounds and can act on the fat globules only on their surfaces. Consequently, it can be readily understood how important detergent function of bile salts is for the digestion of fats.
Digestion of fats by pancreatic lipase. The most important enzyme for the digestion of fats is pancreatic lipase in the pancreatic juice. However, the cells of the small intestine also contain a minute quantity of lipase known as enteric lipase. Both liiese act alike to cause hydrolysis of fat.
Products of fat digestion. Most of the triglycerides of the diet are split into free fatty acids and monoglycerides.
Role of bile salts in accelerating fat digestion — formation of micelles. The hydrolysis of triglycerides highly reversible process; therefore, accumulation of monoglycerides and free fatty acids very quickly blocks further digestion. The bile salts play an important role in removing the monoglycerides and free fatty acids from the vicinity of the digesting fat globules almost as rapidly as these end-products of digestion are formed. This occurs in the following way: bile salts have the propensity to form micelles, which are small spherical globules composed of 20 to 40 molecules of bile salt. These develop because each bile salt molecule is composed of a sterol nucleus, most of which is highly fat-soluble, and a polar group that is highly water-soluble. The sterol nuclei of the 20 to 40 bile salt molecules of the micelle aggregate together to form a small fat globule in the middle of the micelle. This aggregation causes the polar groups to project outward to cover the surface of the micelle During triglyceride digestion, as rapidly as the monoglycerides and free fatty acids are formed they become dissolved in the fatty portion of the micelles, which immediately reduces these end-products of digestion in the vicinity of the digesting fat globules. The bile salt micelles also act as a transport medium to carry the monoglycerides and the free fatty acids, both of which would otherwise be relatively insoluble, to the brush borders of the epithelial cells. There the monoglycerides and free fatty acids are absorbed. On delivery of these substances to the brush border, the bile salts are again released
back into the chyme to be used again and again for this "ferrying" process.
Digestion of Cholesterol Esters and Phospholipids. Most of the cholesterol in the diet is in the form of cholesterol esters, which are combinations of free cholesterol and one molecule of fatty acid. And phospholipids also contain fatty acid chains within their molecules. Both the cholesterol esters and the phospholipids are hydrolyzed by lipases in the pancreatic secretion that free the fatty acids — the enzyme cholesterol ester hydrolase to hydrolyze the cholesterol ester and phospholipase A to hydrolyze the phospholipid.
The bile salt micelles play identically the same role in "ferrying" free cholesterol as they play in "ferrying" monoglycerides and free fatty acids. Indeed, this role of the bile salt micelles is absolutely essential to the absorption of cholesterol because essentially no cholesterol is absorbed without the presence of bile salts. On the other hand, as much as 60 per cent of the triglycerides can be digested and absorbed even in the absence of bile salts.
Absorption of fats
Monoglycerides and fatty acids - both of digestive end-products - become dissolved in the lipid portion of the micelles. Because of the molecular dimension of these micelles, only 2.5 nanometers, and also because of their highly charged, they are soluble in the chyme. Micelles contact with the surfaces of the brush border even penetrating into the recesses , agitating microvilli.
The micelles then diffuse back through the chyme and absorb still more monoglycerides and fatty acids, and similarly transport these also to the epithelial cells. Thus, the bile acids perform a "ferrying" function, which is highly important for fat absorption. In the presence of an abundance of bile acids, approximately 97 per cent of the fat is absorbed; in the absence of bile acids, only 50 to 60 per cent is normally absorbed.
The mechanism for absorption of the monoglycerides and fatty acids through the brush border is based entirely on the fact that both these substances are highly lipid-soluble. Therefore, they become dissolved in the membrane and simply diffuse to the interior of the cell. The undigested triglycerides and the diglycerides are both also highly soluble in the lipid membrane of the epithelial cell. However, only small quantities of these are normally absorbed because the bile acid micelles will not dissolve either triglycerides or diglycerides and therefore will not ferry them to the epithelial membrane.
After entering the epithelial cell, the fatty acids and monoglycerides are taken up by the smooth endoplasmic reticulum, and here they are mainly recombined to form new triglycerides. However, a few of the monoglycerides are further digested into glycerol and fatty acids by an epithelial cell lipase. Then, the free fatty acids are reconstituted by the smooth endoplasmic reticulum into triglycerides. Most of the glycerol that is utilized for this purpose is synthesized de novo from alpha-glycerophosphate, this synthesis requiring both energy from ATP and a complex of enzymes to catalyze the reactions. Once formed, the triglycerides aggregate within the endoplasmic reticulum into globules along with absorbed cholesterol, absorbed phospholipids, and small amounts of newly synthesized cholesterol and phospholipids. The phospholipids arrange themselves in these globules with the fatty portion of the phospholipid toward the center and the polar portions located on the surface. This provides an electrically charged surface that makes these globules miscible with the fluids of the cell. In addition, small amounts of lipoprotein, also synthesized by the endoplasmic reticulum, coat part of the surface of each globule. In this form the globule diffuses to the side of the epithelial cell and is excreted by the process of cellular exocytosis into the space between the cells; from there it passes into the lymph in the central lacteal of the villus. These globules are then called chylomicrons.
Transport of the Chylomicrons in the Lymph. From the sides of the epithelial cells the chylomicrons wend their way into the central lac-teals of the villi and from here are propelled, along with the lymph, by the lymphatic pump upward through the thoracic duct to be emptied into the great veins of the neck. Between 80 and 90 per cent of all fat absorbed from the gut is absorbed in this manner and is transported to the blood by way of the thoracic lymph in the form of chylomicrons.
Direct Absorption of fatty acids into the portal blood. Small quantities of short chain fatty acids, such as those from butterfat, are absorbed directly into the portal blood rather than being converted into triglycerides and absorbed into the lymphatics. The cause of this difference between short and long chain fatty acid absorption is that the shorter chain fatty acids are more water-soluble and are not reconverted into triglycerides by the endoplasmic reticulum. This allows direct diffusion of these fatty acids from the epithelial cells into the capillary blood of the dlood.
Catabolism of triacylglycerols
Dietary acylglycerols undergo hydrolysis in the small intestine by the action of lipases, e.g., those present in pancreatic juice. Lipase digests the triacylglycerols to 2-monoglycerols, glycerol and free fatty acids. These components are absorbed and metabolized in the enterocytes, blood and liver. In the enterocytes and liver the specific for organism acylglycerols are synthesized. Then these are accumulated in adipose tissue and in much smaller quantity in other organs.
Fermentative hydrolysis of in adipocytes and other cells is implemented in several stages. Diacylglycerols, monoacylglycerols, glycerol and free fatty acids are formed in this process:
OXIDATION OF FATTY ACIDS
Fatty acids play an extremely important part as an energy-rich fuel in higher animals and plants since large amounts can be stored in cells in the form of triacylglycerols. Triacylglycerols are especially well adapted for this role because they have a high energy content (about 9 kcal/g) and can be accumulated in nearly anhydrous form as intracellular fat droplets. In contrast, glycogen and starch can yield only about 4 kcal/g; moreover, since they are highly hydrated, they cannot be stored in such concentrated form. Fatty acids provide up to 40 percent of the total fuel requirement in man on a normal diet.
Sources of Fatty Acids.
Mammalian tissues normally contain only vanishingly small amounts of free fatty acids, which are in fact somewhat toxic. By the action of hormonally controlled lipases free fatty acids are formed from triacylglycerols in fat or adipose tissue. The free fatty acids are then released from the tissue, become tightly bound to serum albumin, and in this form are carried via the blood to other tissues for oxidation. Fatty acids delivered in this manner are first enzymatically "activated" in the cytoplasm and then enter the mitochondria for oxidation.
Long-chain fatty acids are oxidized to CO2 and H2O in nearly all tissues of vertebrates except the brain. Some tissues, such as heart muscle, obtain most of their energy from the oxidation of fatty acids. The mobilization, distribution, and oxidation of fatty acids are integrated with the utilization of carbohydrate fuels; both are under complex endocrine regulation.
The pathway of fatty acid oxidation.
Knoop postulated that fatty acids are oxidized by b-oxidation, i.e., oxidation at the b carbon to yield a b-keto acid, which was assumed to undergo cleavage to form acetic acid and a fatty acid shorter by two carbon atoms.
Outline of the fatty acid oxidation cycle.
Before oxidation, long-chain fatty acids from the cytosol must undergo a rather complex enzymatic activation, followed by transport across the mitochondrial membranes into the major compartment. There the fatty acyl group is transferred to intramitochondrial coenzyme A, yielding a fatty acyl-CoA thioester. The subsequent oxidation of the fatty acyl-CoA takes place entirely in the mitochondrial matrix. The fatty acyl-CoA is dehydrogenated by removal of a pair of hydrogen atoms from the a and b carbon atoms (atoms 2 and 3) to yield the a,b-unsaturated acyl-CoA. This is then enzymatically hydrated to form a b-hydroxyacyl-CoA, which in turn is dehydrogenated in the next step to yield the b-ketoacyl-CoA. It then undergoes enzymatic cleavage by reaction with a second molecule of CoA. One product is acetyl-CoA, derived from carbon atoms 1 and 2 of the original fatty acid chain. The other product, a long-chain saturated fatty acyl-CoA having two fewer carbon atoms than the original fatty acid, now becomes the substrate for another round of reactions, beginning at the first dehydrogenation step and ending with the removal of a second two-carbon fragment as acetyl-CoA. At each passage through this spiral the fatty acid chain loses a two-carbon fragment as acetyl-CoA. The 16-carbon palmitic acid thus undergoes a total of seven such cycles, to yield altogether 8 molecules of acetyl-CoA and 14 pairs of hydrogen atoms. The palmitate must be primed or activated only once, since at the end of each round the shortened fatty acid appears as its CoA thioester.
The hydrogen atoms removed during the dehydrogenation of the fatty acid enter the respiratory chain; as electrons pass to molecular oxygen via the cytochrome system, oxidative phosphorylation of ADP to ATP occurs. The acetyl-CoA formed as product of the fatty acid oxidation system enters the tricarboxylic acid cycle.
Activation and entry of fatty acids into mitochondria.
There are three stages in the entry of fatty acids into mitochondria from the extramitochondrial cytoplasm: (1) the enzymatic ATP-driven esterification of the free fatty acid with extramitochondrial CoA to yield fatty acyl-CoA, a step often referred to as the activation of the fatty acid, (2) the transfer of the acyl group from the fatty acyl-CoA to the carrier molecule carnitine, followed by the transport of the acyl carnitine across the inner membrane, and (3) the transfer of the acyl group from fatty acyl carnitine to intramitochondrial CoA.
Activation of fatty acids.
At least three different enzymes catalyze formation of acyl-CoA thioesters, each being specific for a given range of fatty acid chain length. These enzymes are called acyl-CoA synthetases. Acetyl-CoA synthetase activates acetic, propionic, and acrylic acids, medium-chain acyl-CoA synthetase activates fatty acids with 4 to 12 carbon atoms, and long-chain acyI-CoA synthetase activates fatty acids with 12 to 22 or more carbon atoms. The last two enzymes activate both saturated and unsaturated fatty acids. Otherwise the properties and mechanisms of all three synthetases, which have been isolated in highly purified form, are nearly identical. The overall reaction catalyzed by the ATP-linked acyl-CoA synthetases is:
RCOOH + ATP + CoA–SH Û RCO—S—CoA + AMP + PP
Fatty acids acyl-CoA
In this reaction a thioester linkage is formed between the fatty acid carboxyl group and the thiol group of CoA; the ATP undergoes pyrophosphate cleavage to yield AMP and inorganic pyrophosphate.
The acyl-CoA synthetases are found in the outer mitochondrial membrane and in the endoplasmic reticulum.
Transfer to carnitine.
Long-chain saturated fatty acids have only a limited ability to cross the inner membrane as CoA thioesters, but their entry is greatly stimulated by carnitine.
The stimulation of fatty acid oxidation by carnitine is due to the action of an enzyme carnitine acyltransferase, which catalyzes transfer of the fatty acyl group from its thioester linkage with CoA to an oxygen-ester linkage with the hydroxyl group of carnitine. The acyl carnitine ester so formed then passes through the inner membrane into the matrix, presumably via a specific transport system.
Transfer to intramitochondrial CoA.
In the last stage of the entry process the acyl group is transferred from carnitine to intramitochondrial CoA by the action of a second type of carnitine acyltransferase located on the inner surface of the inner membrane:
Acyl carnitine + CoA Û acyl-CoA + carnitine
This complex entry mechanism, often called the fatty acid shuttle, has the effect of keeping the extramitochondrial and intramitochondrial pools of CoA and of fatty acids separated. The intramitochondrial fatty acyl-CoA now becomes the substrate of the fatty acid oxidation system, which is situated in the inner matrix compartment.
The first dehydrogenation step in fatty acid oxidation.
Following the formation of intramitochondrial acyl-CoA, all subsequent reactions of the fatty acid oxidation cycle take place in the inner compartment. In the first step the fatty acyl-CoA thioester undergoes enzymatic dehydrogenation by acyl-CoA dehydrogenase at the a and b carbon atoms (carbons 2 and 3) to form enoyl-CoA as product. The double bond formed in this reaction has the trans geometrical configuration. Recall, however, that the double bonds of the unsaturated fatty acids of natural fats nearly always have the cis configuration.
There are four different acyl-CoA dehydrogenases, each specific for a given range of fatty acid chain lengths. All contain tightly bound flavin adenine dinucleotide (FAD) as prosthetic groups. The FAD becomes reduced at the expense of the substrate, a process that probably occurs through distinct one-electron steps.
The FADH2 of the reduced acyl-CoA dehydrogenase cannot react directly with oxygen but donates its electrons to the respiratory chain via a second flavoprotein, electron-transferring flavoprotein, which in turn passes the electrons to some carrier of the respiratory chain.
The hydration step.
The double bond of the enoyl-CoA ester is then hydrated to form 3-hydroxyacyl-CoA by the enzyme enoyl-CoA hydratase.
The addition of water across the trans double bond is stereo-specific and results in the formation of the L-stereoisomer of the 3-hydroxyacyl-CoA.
The second dehydrogenation step.
In the next step of the fatty acid oxidation cycle, the 3-hydroxyacyl-CoA is dehydrogenated to form 3-ketoacyl-CoA) by 3-hydroxyacyl-CoA dehydrogenase. NAD+ is the specific electron acceptor. The reaction is:
This enzyme is relatively nonspecific with respect to the length of the fatty acid chain but is absolutely specific for the l stereoisomer. The NADH formed in the reaction donates its electron equivalents to the NADH dehydrogenase of the mitochondrial respiratory chain.
The cleavage step.
In the last step of the fatty acid oxidation cycle, which is catalyzed by acetyl-CoA acetyltransferase, more commonly known as thiolase, the 3-ketoacyl-CoA undergoes cleavage by interaction with a molecule of free CoA to yield the carboxyl-terminal two-carbon fragment of the fatty acid as acetyl-CoA. The remaining fatty acid, now shorter by two carbon atoms, appears as its coenzyme A thioester.
This cleavage reaction, also called a thiolysis or a thiolytic cleavage, is analogous to hydrolysis. Since the reaction is highly exergonic, cleavage is favored. There appear to be two (perhaps three) forms of the enzyme, each specific for different fatty acid chain lengths.
The balance sheet.
We have described one turn of the fatty acid oxidation cycle, in which one molecule of acetyl-CoA and two pairs of hydrogen atoms have been removed from the starting long-chain fatty acyl-CoA. The overall equation for one turn of the cycle, starting from palmitoyl-CoA, is
Palmitoyl-CoA + CoA + FAD+ + NAD+ + H2O ®
myristoyl-CoA + acetyl-CoA + FADH2 + NADH2
We can now write the equation for the seven turns of the cycle required to convert one molecule of palmitoyl-CoA into eight molecules of acetyl-CoA:
Palmitoyl-CoA + 7CoA + 7FAD+ + 7NAD+ + 7H2O ®
8 acetyl-CoA + 7FADH2 + 7NADH2 + 7H+
Each molecule of FADH2 donates a pair of electron equivalents to the respiratory chain at the level of coenzyme Q; thus two molecules of ATP are generated during the ensuing electron transport to oxygen. Similarly, oxidation of each molecule of NADH2 by the respiratory chain results in formation of three molecules of ATP. Hence, a total of five molecules of ATP is formed by oxidative phosphorylation per molecule of acetyl-CoA cleaved.
The seven turns of the cycle required to convert one molecule of palmitoyl-CoA rsults in the formation of 5 x 7 = 35 ATP.
The eight molecules of acetyl-CoA formed in the fatty acid cycle may now enter the tricarboxylic acid cycle. The degradation of 1 molecule of acetyl-CoA in tricarboxylic acid cycle results in the formation of 12 molecules of ATP. 8 molecules of acetyl-CoA give 96 molecules of ATP.
Thus, the total output of energy in full cleavage of 1 molecule of palmitoyl-CoA is: 35 + 96 = 131 molecules of ATP.
Since one molecule of ATP is in effect utilized to form palmitoyl-CoA from palmitate, the net yield of ATP per molecule of palmitate is 130.
Oxidation of unsaturated fatty acids.
Unsaturated fatty acids, such as oleic acid, are oxidized by the same general pathway as saturated fatty acids, but two special problems arise. The double bonds of naturally occurring unsaturated fatty acids are in the cis configuration, whereas the unsaturated acyl-CoA intermediates in the oxidation of saturated fatty acids are trans, as we have seen. Moreover, the double bonds of most unsaturated fatty acids occur at such positions in the carbon chain that successive removal of two-carbon fragments from the carboxyl end yields a D3-unsaturated fatty acyl-CoA rather than the D2 fatty acyl-CoA serving as the normal intermediate in the fatty acid cycle.
These problems have been resolved with the discovery of an auxiliary enzyme, enoyl-CoA isomerase, which catalyzes a reversible shift of the double bond from the D3-cis to the D2-trans configuration. The resulting D2-trans-unsaturated fatty acyl-CoA is the normal substrate for the next enzyme of the fatty acid oxidation sequence, enoyl-CoA hydratase, which hydrates it to form L-3-hydroxyacyl-CoA. The complete oxidation of oleyl-CoA to nine acetyl-CoA units by the fatty acid oxidation cycle thus requires an extra enzymatic step catalyzed by the enoyl-CoA isomerase, in addition to those steps required in the oxidation of saturated fatty acids.
Polyunsaturated fatty acids, such as linoleic acid, require a second auxiliary enzyme to complete their oxidation, since they contain two or more cis double bonds. When three successive acetyl-CoA units are removed from linoleyl-CoA, a D3-cis double bond remains, as in the case of oleyl-CoA. This is then transformed by the enoyl-CoA isomerase described above to the D2-trans isomer. This undergoes the usual reactions, with loss of two acetyl-CoA's, leaving an eight-carbon D2-unsaturated acid. Note, however that the double bond of the latter is in the cis configuration. Although the D2-cis double bond can be hydrated by enoyl-CoA hydratase, the product is the D stereoisomer of a 3-hydroxyacyl-CoA, not the L stereoisomer normally formed during oxidation of saturated fatty acids. Utilization of the d stereoisomer requires a second auxiliary enzyme, 3-hydroxyacyl-CoA epimerase, which catalyzes epi-merization at carbon atom 3 to yield the l isomer. The product of this reversible reaction is then oxidized by the L-specific 3-hydroxyacyl-CoA dehydrogenase and cleaved by thiolase to complete the oxidation cycle. The remaining six-carbon saturated fatty acyl-CoA derived from linoleic acid can now be oxidized to three molecules of acetyl-CoA. These two auxiliary enzymes of the fatty acid oxidation cycle make possible the complete oxidation of all the common unsaturated fatty acids found in naturally occurring lipids. The number of ATP molecules yielded during the complete oxidation of an unsaturated fatty acid is somewhat lower than for the corresponding saturated fatty acid since unsaturated fatty acids have fewer hydrogen atoms and thus fewer electrons to be transferred via the respiratory chain to oxygen.
Oxidation of odd-carbon fatty acids and the fate of propionyl-CoA
Odd-carbon fatty acids, which are rare but do occur in some marine organisms, can also be oxidized in the fatty acid oxidation cycle. Successive acetyl-CoA residues are removed until the terminal three-carbon residue pro-pionyl-CoA is reached. This compound is also formed in the oxidative degradation of the amino acids valine and isoleucine. Propionyl-CoA undergoes enzymatic carboxylation in an ATP-dependent process to form Ds-methylmaionyl-CoA, a reaction catalyzed by propionyl-CoA corboxylase. This enzyme contains biotin as its prosthetic group. In the next step Ds-methylmalonyl-CoA undergoes enzymatic epimerization to LR-methylmalonyl-CoA, by action of methyimaionyl-CoA racemase. In the next reaction step, catalyzed by methylmalonyl-CoA mutase, LR-methylmalonyl-CoA is isomerized to succinyl-CoA, which may then undergo deacylation by reversal of the succinyl-CoA synthetase reaction to yield free succinate, an intermediate of the tricarboxylic acid cycle.
Methylmalonyl-CoA mutase requires as cofactor coenzyme B12. Study of this intramolecular reaction with isotope tracers has revealed that it takes place by the migration of the entire —CO—S—CoA group from carbon atom 2 of methylmalonyl-CoA to the methyl carbon atom in exchange for a hydrogen atom.
Patients suffering from pernicious anemia, who are deficient in vitamin B12 because of their lack of intrinsic factor, excrete large amounts of methylmalonic acid and its precursor propionic acid in the urine, showing that in such patients the coenzyme B12-dependent methylmalonyl-CoA mutase reaction is defective.
OXIDATION OF GLYCEROL
Glycerol formed in cleavage of tryacylglycerols enter catabolism or use for biosynthesis of glycerides again. Before including of glycerol in metabolism it is activated by ATP to glycerol-3-phosphate by action of glycerol phosphokinase:
Glycerol-3-phosphate is oxidized by glycerophosphate dehydrogenase and glyceroaldehyde-3-phosphate is produced:
Glyceroaldehyde-3-phosphate is the central metabolite of glycolysis.
The biosynthesis of lipids is a prominent metabolic process in most organisms. Because of the limited capacity of higher animals to store polysaccharides, glucose ingested in excess of immediate energy needs and storage capacity is converted by glycolysis into pyruvate and then acetyl-CoA, from which fatty acids are synthesized. These in turn are converted into triacylglycerols, which have a much higher energy content than polysaccharides and may be stored in very large amounts in adipose or fat tissues. Triacylglycerols are also stored in the seeds and fruits of many plants.
The formation of the various phospholipids and sphingolipids of cell membranes is also an important biosynthetic process. These complex lipids undergo continuous metabolic turnover in most cells.
Biosynthesis of saturated fatty acids
The biosynthesis of saturated fatty acids from their ultimate precursor acetyl-CoA occurs in all organisms but is particularly prominent in the liver, adipose tissues, and mammary glands of higher animals. It is brought about by a process that differs significantly from the opposed process of fatty acid oxidation. In the first place total biosynthesis of fatty acids occurs in the cytosol, whereas fatty acid oxidation occurs in the mitochondria. Second, the presence of citrate is necessary for maximal rates of synthesis of fatty acids, whereas it is not required in fatty acid oxidation. Perhaps the most unexpected difference is that CO2 is essential for fatty acid synthesis in cell extracts, although isotopic CO2 is not itself incorporated into the newly synthesized fatty acids. These and many other observations have revealed that fatty acid synthesis from acetyl-CoA takes place with an entirely different set of enzymes from those employed in fatty acid oxidation.
In the overall reaction of fatty acid synthesis, which is catalyzed by a complex multienzyme system in the cytosol, the fatty-acid synthetase complex, acetyl-CoA derived from carbohydrate or amino acid sources is the ultimate precursor of all the carbon atoms of the fatty acid chain. However, of the eight acetyl units required for biosynthesis of palmitic acid, only one is provided by acetyl-CoA; the other seven arrive in the form of malonyl-CoA, formed from acetyl-CoA and HCO3- in a carboxylation reaction. One acetyl residue and seven malonyl residues undergo successive condensation steps, with release of seven molecules of CO2, to form palmitic acid; the reducing power is furnished by NADPH:
Acetyl-CoA + 7 malonyl-CoA + 14NADPH + 14H+ ®
CH3(CH2)14COOH + 7CO2 + 8CoA + 14NADP+ + 6H2O
The single molecule of acetyl-CoA required in the process serves as a primer, or starter; the two carbon atoms of its acetyl group become the two terminal carbon atoms (15 and 16) of the palmitic acid formed. Chain growth during fatty acid synthesis thus starts at the carboxyl group of acetyl-CoA and proceeds by successive addition of acetyl residues at the carboxyl end of the growing chain. Each successive acetyl residue is derived from two of the three carbon atoms of a malonic acid residue entering the system in the form of malonyl-CoA; the third carbon atom of malonic acid, i.e., that of the unesterified carboxyl group, is lost as CO2. The final product is a molecule of palmitic acid.
A distinctive feature of the mechanism of fatty acid biosynthesis is that the acyl intermediates in the process of chain lengthening are thio esters, not of CoA, as in fatty acid oxidation, but of a low-molecular-weight conjugated protein called acyl carrier protein (ACP). This protein can form a complex or complexes with the six other enzyme proteins required for the complete synthesis of palmitic acid. In most eukariotic cells all seven proteins of the fatty acid synthetase complex are associated in a multienzymes cluster.
In most organisms the end product of the fatty-acid synthetase system is palmitic acid, the precursor of all other higher saturated fatty acids and of all unsaturated fatty acids.
The carbon source for fatty acid synthesis
The ultimate source of all the carbon atoms of fatty acids is acetyl-CoA, formed in the mitochondria by the oxidative decarboxylation of pyruvate, the oxidative degradation of some of the amino acids, or by the b-oxidation of long-chain fatty acids.
Acetyl-CoA itself cannot pass out of the mitochondria into the cytosol; however, its acetyl group is transferred through the membrane in other chemical forms. Citrate, formed in mitochondria from acetyl-CoA and oxaloacetate, may pass through the mitochondrial membrane to the cytoplasm via the tricarboxylate transport system. In the cytosol acetyl-CoA is regenerated from citrate by ATP-citrate lyase, also called citrate cleavage enzyme, which catalyzes the reaction:
Citrate + ATP + CoA ® acetyl-CoA + ADP + P + oxaloacetate
In a second pathway the acetyl group of acetyl-CoA is enzymatically transferred to carnitine, which acts as a carrier of fatty acids into mitochondria preparatory to their oxidation. Acetylcarnitine passes from the mitochondrial matrix through the mitochondrial membrane into the cytosol; acetyl-CoA is then regenerated by transfer of the acetyl group from acetylcarnitine to cytosol CoA.
Formation of malonyl-CoA: acetyl-CoA carboxylase
Before the acetyl groups of acetyl-CoA can be utilized by the fatty-acid synthetase complex, an important preparatory reaction must take place to convert acetyl-CoA into malonyl-CoA, the immediate precursor of 14 of the 16 carbon atoms of palmitic acid. Malonyl-CoA is formed from acetyl-CoA and bicarbonate in the cytosol by the action of acetyl-CoA carboxylase, a complex enzyme that catalyzes the reaction:
The carbon atom of the CO2 becomes the distal or free carboxyl carbon of malonyl-CoA. However, the above equation give only the overall reaction, the sum of at least three intermediate reactions.
Acetyl-CoA carboxylase contains biotin as its prosthetic group. The carboxyl group of biotin is bound in amide linkage to the e-amino group of a specific lysine residue of a subunit of the enzyme. The covalently bound biotin serves as an intermediate carrier of a molecule of CO2.
The reaction catalyzed by acetyl-CoA carboxylase, an allosteric enzyme, is the primary regulatory, or rate-limiting, step in the biosynthesis of fatty acids. Acetyl-CoA carboxylase is virtually inactive in the absence of its positive modulators citrate or isocitrate. The striking allosteric stimulation of this enzyme by citrate accounts for the fact that citrate is required for fatty acid synthesis in cell extracts without being used as a precursor.
Acetyl-CoA carboxylase occurs in both an inactive monomeric form and an active polymeric form. As it occurs in the avian liver, the inactive enzyme monomer has a molecular weight of 410,000 and contains one binding site for CO2 (that is, one biotin prosthetic group), one binding site for acetyl-CoA, and one for citrate. Citrate shifts the equilibrium between the inactive monomer and the active polymer, to favor the latter.
Polymeric acetyl-CoA carboxylase of animal tissues consists of long filaments of enzyme monomers; each monomer unit contains a molecule of bound citrate. The length of the polymeric form varies, but on the average each filament contains about 20 monomer units, has a particle weight of some 8 megadaltons, and is about 400 nm long. Such filaments have been studied in the electron microscope and have actually been observed in the cytoplasm of adipose cells.
The acetyl-CoA carboxylase reaction is complex. In fact, the monomeric unit of the enzyme contains four different subunits. The sequence of reactions in the formation of malonyl-CoA has been deduced from study of the four subunits of the monomer. One of these subunits, biotin carboxylase (BC), catalyzes the first step of the overall reaction, namely, the carboxylation of the biotin residue covalently bound to the second subunit, which is called biotin carboxyl-carrier protein (BCCP). The second step in the overall reaction is catalyzed by the third type of subunit, called carboxyl transferase (CT). In these reactions the biotin residue of the carboxyl carrier protein serves as a swinging arm to transfer the bicarbonate ion from the biotin carboxylase subunit to the acetyl-CoA bound to the active site of the carboxyltransferase subunit. The change from the inactive monomeric form of acetyl-CoA carboxylase to the polymeric, active form of the enzyme occurs when citrate is bound to the fourth subunit of each monomeric unit.
The reactions of the fatty-acid synthetase system
Once malonyl-CoA has been generated from acetyl-CoA by the action of the rather complex acetyl-CoA carboxylase reaction, the ensuing reactions of fatty acid synthesis occur in a sequence of six successive steps catalyzed by the six enzymes of the fatty-acid synthetase system. The seventh protein of this system, which has no enzymatic activity itself, is acyl carrier protein, to which the growing fatty acid chain is covalently attached.
Acyl carrier protein (ACP)
Acyl carrier protein, universally symbolized as ACP, was first isolated in pure form from E. coli and has since been studied from many other sources. The E. coli ACP is a relatively small (mol wt 10,000), heat-stable protein containing 77 amino acid residues, whose sequence has been established, and a covalently attached prosthetic group.
The single sulfhydryl group of ACP, to which the acyl intermediates are esterified, is contributed by its prosthetic group, a molecule of 4'-phosphopantetheine, which is covalently linked to the hydroxyl group of serine residue 36 of the protein. The 4'-phosphopantetheme moiety is identical with that of coenzyme A, from which it is derived. The function of ACP in fatty acid synthesis is analogous to that of CoA in fatty acid oxidation: it serves as an anchor to which the acyl intermediates are esterified.
The priming reaction
To prime the fatty-acid synthetase system, acetyl-CoA first reacts with the sulfhydryl group of ACP by the action of one of the six enzymes of the synthetase system, ACP-acyltransferase, which catalyzes the reaction:
The malonyl transfer step
In the next reaction, catalyzed by ACP malonyltransferase, malonyl-S-CoA formed in the acetyl-CoA carboxylase reaction reacts with the —SH group of the 4'-phosphopantetheine arm of ACP, with loss of free CoA, to form malonyl-S-ACP:
Malonyl—S—CoA + ACP—SH Û malonyl—S—ACP + CoA—SH
As a result of this step and of the preceding priming reaction, a malonyl group is now esterified to ACP and an acetyl group is esterified to an —SH group on the ACP molecule.
The condensation reaction
In the next reaction of the sequence, catalyzed by b-ketoacyl-ACP synthase, the acetyl group esterified to the cysteine residue is transferred to carbon atom 2 of the malonyl group on ACP, with release of the free carboxyl group of the malonyl residue as CO2:
Study of the reaction equilibrium has revealed the probable basis for the biological selection of malonyl-CoA as the precursor of two-carbon residues for fatty acid synthesis. If acetoacetyl-CoA were to be formed from two molecules of acetyl-CoA by the action of acetyl-CoA acetyltransferase,
Acetyl—S—CoA + acetyl—S—CoA Û acetoacetyl—S—CoA + CoA—SH
the reaction would be endergonic, with its equilibrium lying to the left.
The first reduction reaction
The acetoacetyl-S-ACP now undergoes reduction by NADPH to form b-hydroxybutyryl-S-ACP. This reaction is catalyzed by b-ketoacyl-ACP reductase:
The dehydration step
b-Hydroxybutyryl-S-ACP is next dehydrated to the corresponding unsaturated acyl-S-ACP, namely, crotonyl-S-ACP, by b-hydroxyacyl—ACP-dehydratase:
The second reduction step
Crotonyl-S-ACP is now reduced to butyryl-S-ACP by enoil-ACP reductase (NADPH); the electron donor is NADPH in animal tissues:
This reaction also differs from the corresponding reaction of fatty acid oxidation in mitochondria in that a pyridine nucleotide rather than a flavoprotein is involved. Since the NADPH-NADP+ couple has a more negative standard potential than the fatty acid oxidizing flavoprotein, NADPH favors reductive formation of the saturated fatty acid.
The formation of butyryl-ACP completes the first of seven cycles en route to palmitoyl-S-ACP. To start the next cycle the butyryl group is transferred from —SH group of phosphopantetheine to the —SH group of cysteine, thus allowing —SH group of ACP phosphopantetheine to accept a malonyl group from another molecule of malonyl-CoA.
Then the cycle repeats, the next step being the condensation of malonyl-S-ACP with butyryl-S-ACP to yield b-ketohexanoyl-S-ACP and CO2.
After seven complete cycles, palmitoyl-ACP is the end product. The palmitoyl group may be removed to yield free palmitic acid by the action of a thioesterase, or it may be transferred from ACP to CoA, or it may be incorporated directly into phosphatidic acid in the pathway to phospholipids and triacylglycerols.
It is remarkable that in most organisms the fatty-acid synthetase system stops with the production of palmitic acid and does not yield stearic acid, which has only two more carbon atoms than palmitic acid and thus does not differ greatly in physical properties.
Saturated fatty acids having an odd number of carbon atoms, which are found in many marine organisms, are also made by the fatty-acid synthetase complex. In this case the synthesis is primed by a starter molecule of propionyl-S-ACP (instead of acetyl-S-ACP), to which are added successive two-carbon units via condensations with malonyl-S-ACP.
We can now write the overall equation for palmitic acid biosynthesis starting from acetyl-S-CoA:
8 Acetyl—S—CoA + 14NADPH + 14H+ + 7ATP + H2O ®
palmitic acid + 8CoA + 14NADP+ + 7ADP + 7P.
The 14 molecules of NADPH required for the reductive steps in the synthesis of palmitic acid arise largely from the NADP-dependent oxidation of glucose 6-phosphate via the phosphogluconate pathway. Liver, mammary gland, and adipose tissue of vertebrates, which have a rather high rate of fatty acid biosynthesis, also have a very active 6-phosphogluconate pathway.
The enzymatic steps leading to the biosynthesis of palmitic acid differ from those involved in oxidation of palmitic acid in the following respects:
1. Their intracellular location.
2. The type of acyl-group carrier.
3. The form in which the two-carbon units are added or removed.
4. The pyridine nucleotide specificity of the b-ketoacyl-b-hydroxyacyl reaction.
5. The stereoisomeric configuration of the b-hydroxyacyl intermediate.
6. The electron donor-acceptor system for the crotonyl-butyryl step.
These differences illustrate how two opposing metabolic processes may proceed independently of each other in the cell.
Elongation of saturated fatty acids in mitochondria and microsomes
Palmitic acid, the normal end product of the fatty-acid synthetase system, is the precursor of the other long-chain saturated and unsaturated fatty acids in most organisms. Elongation of palmitic acid to longer-chain saturated fatty acids, of which stearic acid is most abundant, occurs by the action of two different types of enzyme systems, one in the mitochondria and the other in the endoplasmic reticulum.
In mitochondria palmitic and other saturated fatty acids are lengthened by successive additions to the carboxyl-terminal end of acetyl units in the form of acetyl-CoA; malonyl-ACP cannot replace acetyl-CoA. The mitochondrial elongation pathway occurs by reactions similar to those in fatty acid oxidation. Condensation of palmityl-CoA with acetyl-CoA yields b-ketostearyl-CoA, which is reduced by NADPH to b-hydroxystearyl-CoA. The latter is dehydrated to the unsaturated stearyl-CoA, which is then reduced to yield stearyl-CoA at the expense of NADPH. This system will also elongate unsaturated fatty acids.
Microsome preparations can elongate both saturated and unsaturated fatty acyl-CoA esters, but in this case malonyl-CoA rather than acetyl-CoA serves as source of the acetyl groups. The reaction sequence is identical to that in the fatty-acid synthetase system except that the microsomal system employs CoA and not ACP as acyl carrier.
Formation of monoenoic acids
Palmitic and stearic acids serve as precursors of the two common monoenoic (monounsaturated) fatty acids of animal tissues, namely, poimitoleic and oleic acids, both of which possess a cis double bond in the D9 position. Although most organisms can form palmitoleic and oleic acids, the pathway and enzymes employed differ between aerobic and anaerobic organisms.
In vertebrates (and most other aerobic organisms) the D9 double bond is introduced by a specific monooxygenase system; it is located in the endoplasmic reticulum of liver and adipose tissue. One molecule of molecular oxygen (O2) is used as the acceptor for two pairs of electrons, one pair derived from the palmitoyl-CoA or stearyl-CoA substrate and the other from NADPH, which is a required coreductant in the reaction. The transfer of electrons in this complex reaction involves a microsomal electron-transport chain which carries electrons from NADPH (or NADH) to microsomal cytochrome b5 via cytochrome b5 reductase, a flavoprotein. A terminal cyanide-sensitive factor (CSF), a protein, is required to activate the acyl-CoA and the oxygen.
The overall reaction for palmitoyl-CoA is:
Palmitoyl—CoA + NADPH + H+ + O2 ® palmitoleyl—CoA + NADP+ + 2H2O.
Formation of polyenoic acids
Bacteria do not contain polyenoic acids; however, these acids are abundant both in higher plants and in animals. Mammals contain four distinct families of polyenoic acids, which differ in the number of carbon atoms between the terminal methyl group and the nearest double bond. These families are named from their precursor fatty acids, namely, palmitoleic, oleic, linoleic, and linolenic acids. All polyenoic acids found in mammals are formed from these four precursors by further elongation and/or desaturation reactions. Two of these precursor fatty acids, linoleic and linolenic acids, cannot be synthesized by mammals and must be obtained from plant sources; they are therefore called essential fatty acids.
The elongation of chains of polyenoic acids occurs at the carboxyl end by the mitochondrial or microsomal systems described above. The desaturation steps occur by the action of the cytochrome b5-oxygenase system with NADPH as coreductant of oxygen, like the steps in the formation of palmitoleic and oleic acids, also described above.
Arachidonic acid is the most abundant polyenoic acid. When young rats are placed on diets deficient in essential fatty acids, they grow slowly and develop a scaly dermatitis and thickening of the skin. This condition can be relieved by administration not only of linoleic or linolenic acid but also of arachidonic acid. The essential fatty acids and some of their derivatives serve as precursors of the prostaglandins.
In plants linoleic and linolenic acids are synthesized from oleic acid via aerobic desaturation reactions catalyzed by specific oxygenase systems requiring NADPH as coreductant.
The double bonds of naturally occurring fatty acids do not in general undergo hydrogenation to yield more completely saturated fatty acids; only a few organisms appear to carry out this process. Unsaturated fatty acids, however, are completely oxidized by the fatty acid oxidation system.
In most organisms the conversion of saturated to unsaturated fatty acids is promoted by low environmental temperatures. This is an adaptation to maintain the melting point of the total cell lipids below the ambient temperature; unsatu-rated fatty acids have lower melting points than saturated. In some organisms the enzymes involved in fatty acid desaturation increase in concentration in response to low temperatures; in others the unsaturated fatty acids are inserted into lipids at increased rates.
Biosynthesis of triacylglycerols
The triacylglycerols, which function as depot, or storage, lipids, are actively synthesized in the cells of vertebrates, particularly liver and fat cells, as well as those of higher plants. Bacteria in general contain relatively small amounts of triacyglycerols.
In higher animals and plants two major precursors are required for the synthesis of triacylglycerols: L-glycerol 3-phosphate and fatty acyl-CoA.. L-Glycerol 3-phosphate is derived from two different sources. Its normal precursor is dihydroxyacetone phosphate, the product of the aldolase reaction of glycolysis. Dihydroxyacetone phosphate is reduced to L-glycerol 3-phosphate by the NAD-linked glycerol- 3-phosphate dehydrogenase of the cytosol:
Dihydroxyacetone phosphate + NADH + H+ ® L-glycerol 3-phosphate + NAD+
It may also be formed from free glycerol arising from degradation of triacylglycerols, through the action of glycerol kinase:
ATP + glycerol ® L-glycerol 3-phosphate + ADP
The first stage in triacyglycerol formation is the acylation of the free hydroxyl groups of glycerol phosphate by two molecules of fatty acyl-CoA to yield first a lysophosphotidic acid and then a phosphatidic acid:
Glycerol phosphate Lysophosphotidic acid
Lysophosphotidic acid Phosphatidic acid
Free glycerol is not acylated. These reactions occur preferentially with 16- and 18-carbon saturated and unsaturated acyl-CoA.
Phosphatidic acids occur only in trace amounts in cells, but they are important intermediates in the biosynthesis of triacylglycerols and phosphoglycerides.
In the pathway to triacylglycerols, phosphatidic acid undergoes hydrolysis by phosphatidate phosphatase to form a diacylglycerol:
Phosphatidic acid Diacylglycerol
The diacylglycerol then reacts with a third molecule of a fatty acyl-CoA to yield a triacylglycerol by the action of diacylglycerol acyltransferase:
In the intestinal mucosa of higher animals, which actively synthesizes triacylglycerols during absorption of fatty acids from the intestine, another type of acylation reaction comes into play. Monoacylglycerols formed during intestinal digestion may be acylated directly by acylglycerol palmitoyltrans-ferase and thus phosphatidic acid is not an intermediate:
Monoacylglycerol + palmitoyl-CoA ® diacylglycerol + CoA
In storage fats of animal and plant tissues the triacylglycerols are usually mixed, i.e., contain two or more different fatty acids.
Ketone bodies: structure.
In many vertebrates the liver has the enzymatic capacity to divert some of the acetyl-CoA derived from fatty acid or pyruvate oxidation, presumably during periods of excess formation, into free acetoacetate and b-hydroxybutyrate, which are transported via the blood to the peripheral tissues, where they may be oxidized via the tricarboxylic acid cyrcle.
These compounds, together with acetone, are collectively called the ketone bodies.
acetoacetate b-hydroxybutyrate acetone
Ketogenesis: mechanism, localization, biological role.
Free acetoacetate, which is the primary source of the other ketone bodies, is formed from acetoacetyl-CoA. Some of the acetoacetyl-CoA arises from the last four carbon atoms of a long-chain fatty acid after oxidative removal of successive acetyl-CoA residues in the mitochondrial matrix. However, most of the acetoacetyl-CoA formed in the liver arises from the head-to-tail condensation of two molecules of acetyl-CoA derived from fatty acid oxidation by the action of acetyl-CoA acetyltransferase:
acetyl-CoA acetyl-CoA acetoacetyl-CoA
The acetoacetyl-CoA formed in these reactions then undergoes loss of CoA, a process called deacylation, to yield free acetoacetate in a special pathway taking place in the mitochondrial matrix. It involves the enzymatic formation and cleavage of b-hydroxy-b-methylglutaryl-CoA, an intermediate which also serves as a precursor of sterols.
acetoacetyl-CoA acetyl-CoA b-hydroxy-b-methylglutaryl-CoA
b-hydroxy-b-methylglutaryl-CoA acetoacetate acetyl-CoA
The free acetoacetate so produced is enzymatically reduced to D-b-hydroxybutyrate by the NAD-linked b-hydroxybutyrate dehydrogenase, which is located in the inner mitochondrial membrane.
The mixture of free acetoacetate and b-hydroxybutyrate resulting from these reactions may diffuse out of the liver cells into the bloodstream, to be transported to the peripheral tissues.
The mechanism of acetoacetate utilizing in tissues (ketolysis).
In the peripheral tissues the b-hydroxybutyrate is oxidized to acetoacetate, which is then activated by transfer of CoA from succinyl-CoA. The succinyl-CoA required arises from the oxidation of a-ketoglutarate.
Another way of acetoacetate activation in peripheral tissues is the direct interaction of acetoacetate with ATP and CoA-SH:
The acetoacetyl-CoA formed in the peripheral tissues by these reactions then undergoes thiolytic cleavage to two molecules of acetyl-CoA, which then may enter the tricarboxylic acid cycle.
The mechanism of the increase of ketone bodies content in blood at diabetus mellitus and starvation.
Normally the concentration of ketone bodies in the blood is rather low (10-20 mg/l), but in fasting or in the disease diabetes mellitus, it may reach very high levels. This condition, known as ketosis, arises when the rate of formation of the ketone bodies by the liver exceeds the capacity of the peripheral tissues to utilize them, with a resulting accumulation in the blood and excretion via the kidneys (in normal the content of ketone bodies in urine is up to 50 mg/day).
The utilization of acetyl-CoA in tricarboxylic acid cycle depends on the availability of oxaloacetate in cell. The formation of oxaloacetate depends on quantity of pyruvate, which is formed from glucose. In fasting or diabetus mellitus the entering of glucose into cells is inhibited, oxaloacetate enters the gluconeogenesis process and is not available for the interaction with acetyl CoA in citrate synthase reaction. In this metabolic state acetyl-CoA is used for the ketone bodies formation. The accumulation of ketone bodies is also promotted by b-oxidation of fatty acids due to the stimulation of lipolysis in adipose tissue in glucose starvation conditions.