Metabolism of carbohydrates –1. Glycolysis, aerobic oxidation of glucose;
alternative ways of monosaccharides metabolism
Most tissues have at least some requirement for glucose. In brain, the requirement is substantial. Glycolysis, the major pathway for glucose metabolism, occurs in the cytosol of all cells. It is unique in that it can function either aerobically or anaerobically. Erythrocytes, which lack mitochondria, are completely reliant on glucose as their metabolic fuel and metabolize it by anaerobic glycolysis.
However, to oxidize glucose beyond pyruvate (the end product of glycolysis) requires both oxygen and mitochondrial enzyme systems such as the pyruvate dehydrogenase complex, the citric acid cycle, and the respiratory chain.
Glycolysis is both the principal route for glucose metabolism and the main pathway for the metabolism of fructose, galactose, and other carbohydrates derived from the diet. The ability of glycolysis to provide ATP in the absence of oxygen is especially important because it allows skeletal muscle to perform at very high levels when oxygen supply is insufficient and because it allows tissues to survive anoxic episodes. However, heart muscle, which is adapted for aerobic performance, has relatively low glycolytic activity and poor survival under conditions of ischemia. Diseases in which enzymes of glycolysis (eg, pyruvate kinase) are deficient are mainly seen as hemolytic anemias or, if the defect affects skeletal muscle (eg, phosphofructokinase), as fatigue. In fast-growing cancer cells, glycolysis proceeds at a higher rate than is required by the citric acid cycle, forming large amounts of pyruvate, which is reduced to lactate and exported. This produces a relatively acidic local environment in the tumor which may have implications for cancer therapy. The lactate is used for gluconeogenesis
in the liver, an energy-expensive process responsible for much of the hypermetabolism seen in cancer cachexia. Lactic acidosis results from several causes, including impaired activity of pyruvate dehydrogenase.
Common features and differences of aerobic and unaerobic oxidation of carbohydrates .
The anaerobic degradation of glucose to yield lactic acid is called glycolysis. The first 10 reactions (to the stage of pyruvate formation) of carbohydrates aerobic oxidation are identical to that of glycolysis. But then in aerobic conditions the pyruvate is decarboxylated to acetyl-CoA (in anaerobic conditions the pyruvate is converted to lactate).
Another difference between aerobic and unaerobic oxidation is the amount of energy produced in these processes. In unaerobic oxidation of 1 molecule of glucose 4 molecules of ATP are formed, in aerobic conditions - 38 molecules.
Stages of aerobic oxidation of glucose (specific way and including into the general catabolic way).
There are 4 stages of aerobic
oxidation of carbohydrates. During the first stage glucose is converted to pyruvate. It is the specific pathway of glucose oxidation.
Then pyruvate undergoes oxidative decarboxylation
and acetyl-CoA is formed (2 stage). Acetyl-CoA enters tricarboxylic acid cycle in stage
GLYCOLYSIS CAN FUNCTION UNDER
When a muscle contracts in an anaerobic medium, ie, one from which oxygen is excluded, glycogen disappears and lactate appears as the principal end product.
When oxygen is admitted, aerobic recovery takes place and lactate disappears. However, if contraction occurs under aerobic conditions, lactate does not accumulate and pyruvate is the major end product of glycolysis.
Pyruvate is oxidized further to CO2 and water. When oxygen is in short supply, mitochondrial reoxidation of NADH formed from NAD+ during glycolysis is impaired, and NADH is reoxidized by reducing pyruvate to lactate, so permitting glycolysis to proceed. While glycolysis can occur under anaerobic conditions, this has a price, for it limits the amount of ATP formed per mole of glucose oxidized, so that much more glucose must be metabolized under anaerobic than under aerobic conditions.
THE REACTIONS OF GLYCOLYSIS CONSTITUTE THE MAIN PATHWAY OF GLUCOSE UTILIZATION
The overall equation for glycolysis from glucose to lactate is as follows:
All of the enzymes of glycolysis are found in the cytosol. Glucose enters glycolysis by phosphorylation to glucose 6-phosphate, catalyzed by hexokinase, using ATP as the phosphate donor. Under physiologic conditions, the phosphorylation of glucose to glucose 6-phosphate can be regarded as irreversible.
Hexokinase is inhibited allosterically by its product, glucose 6-phosphate. In tissues other than the liver and pancreatic B islet cells, the availability of glucose for glycolysis (or glycogen synthesis in muscle and lipogenesis in adipose tissue) is controlled by transport into the cell, which in turn is regulated by insulin. Hexokinase has a high affinity (low Km) for its substrate, glucose, and in the liver and pancreatic B islet cells is saturated under all normal conditions and so acts at a constant rate to provide glucose 6-phosphate to meet the cell’s need. Liver and pancreatic B islet cells also contain an isoenzyme of hexokinase, glucokinase, which has a Km very much higher than the normal intracellular concentration of glucose. The function of glucokinase in the liver is to remove glucose from the blood following a meal, providing glucose 6-phosphate in excess of requirements for glycolysis, which will be used for glycogen synthesis and lipogenesis. In the pancreas, the glucose 6-phosphate formed by glucokinase signals increased glucose availability and leads to the secretion of insulin. Glucose 6-phosphate is an important compound at the junction of several metabolic pathways (glycolysis, gluconeogenesis, the pentose phosphate pathway, glycogenesis, and glycogenolysis). In glycolysis, it is converted to fructose 6-phosphate by phosphohexoseisomerase, which involves an aldose-ketose isomerization. This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructokinase (phosphofructokinase-1), forming fructose 1,6- bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions; it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are interconverted by the enzyme phosphotriose isomerase. Glycolysis continues with the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate. The enzyme catalyzing this oxidation, glyceraldehyde 3-phosphate dehydrogenase, is NAD-dependent. Structurally, it consists of four identical polypeptides (monomers) forming a tetramer. SH groups are present on each polypeptide, derived from cysteine residues within the polypeptide chain. One of the SH groups at the active site of the enzyme combines with the substrate forming a thiohemiacetal that is oxidized to a thiol ester; the hydrogens removed in this oxidation are transferred to NAD+. The thiol ester then undergoes phosphorolysis; inorganic phosphate (Pi) is added, forming 1,3-bisphosphoglycerate, and the SH group is reconstituted.
In the next reaction, catalyzed by phosphoglycerate kinase, phosphate is transferred from 1,3-bisphosphoglycerate onto ADP, forming ATP (substrate-level phosphorylation) and 3-phosphoglycerate. Since two molecules of triose phosphate are formed per molecule of glucose, two molecules of ATP are generated at this stage per molecule of glucose undergoing glycolysis. The toxicity of arsenic is due to competition of arsenate with inorganic phosphate (Pi) in the above reactions to give 1-arseno-3-phosphoglycerate, which hydrolyzes spontaneously to give 3-phosphoglycerate plus heat, without generating ATP. 3-Phosphoglycerate is isomerized to 2-phosphoglycerate by phosphoglycerate mutase.
It is likely that 2,3-bisphosphoglycerate (diphosphoglycerate; DPG) is an intermediate in this reaction. The subsequent step is catalyzed by enolase and involves a dehydration, forming phosphoenolpyruvate. Enolase is inhibited by fluoride. To prevent glycolysis in the estimation of glucose, blood is collected in tubes containing fluoride. The enzyme is also dependent on the presence of either Mg2+ or Mn2+.. The phosphate of phosphoenolpyruvate is transferred to ADP by pyruvate kinase to generate, at this stage, two molecules of ATP per molecule of glucose oxidized. The product of the enzyme-catalyzed reaction, enolpyruvate, undergoes spontaneous (nonenzymic) isomerization to pyruvate and so is not available to undergo the reverse reaction. The pyruvate kinase reaction is thus also irreversible under physiologic conditions.
The redox state of the tissue now determines which of two pathways is followed. Under anaerobic conditions, the reoxidation of NADH through the respiratory chain to oxygen is prevented. Pyruvate is reduced by the NADH to lactate, the reaction being catalyzed by lactate dehydrogenase. Several tissue-specific isoenzymes of this enzyme have been described and have clinical significance. The reoxidation of NADH via lactate formation allows glycolysis to proceed in the absence of oxygen by regenerating sufficient NAD+ for another cycle of the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. Under aerobic conditions, pyruvate is taken up into mitochondria and after conversion to acetyl-CoA is oxidized to CO2 by the citric acid cycle. The reducing equivalents from the NADH + H+ formed in glycolysis are taken up into mitochondria for oxidation via one of the two shuttles.
Tissues That Function Under Hypoxic Circumstances Tend to Produce Lactate
This is true of skeletal muscle, particularly the white fibers, where the rate of work output—and therefore the need for ATP formation—may exceed the rate at which oxygen can be taken up and utilized. Glycolysis in erythrocytes, even under aerobic conditions, always terminates in lactate, because the subsequent reactions of pyruvate are mitochondrial, and erythrocytes lack mitochondria. Other tissues that normally derive much of their energy from glycolysis and produce lactate include brain, gastrointestinal tract, renal medulla, retina, and skin. The liver, kidneys, and heart usually take up lactate and oxidize it but will produce it under hypoxic conditions.
Glycolysis Is Regulated at Three Steps Involving Nonequilibrium Reactions
Although most of the reactions of glycolysis are reversible, three are markedly exergonic and must therefore be considered physiologically irreversible. These reactions, catalyzed by hexokinase (and glucokinase), phosphofructokinase, and pyruvate kinase, are the major sites of regulation of glycolysis. Cells that are capable of reversing the glycolytic pathway (gluconeogenesis) have different enzymes that catalyze reactions which effectively reverse these irreversible reactions.
In Erythrocytes, the First Site in Glycolysis for ATP Generation May Be Bypassed
In the erythrocytes of many mammals, the reaction catalyzed by phosphoglycerate kinase may be bypassed by a process that effectively dissipates as heat the free energy associated with the high-energy phosphate of 1,3-bisphosphoglycerate. Bisphosphoglycerate mutase catalyzes the conversion of 1,3-bisphosphoglycerate to 2,3-bisphosphoglycerate, which is converted to 3-phosphoglycerate by 2,3-bisphosphoglycerate phosphatase (and possibly also phosphoglycerate mutase). This alternative pathway involves no net yield of ATP from glycolysis. However, it does serve to provide 2,3-bisphosphoglycerate, which binds to hemoglobin, decreasing its affinity for oxygen and so making oxygen more readily available to tissues.
THE OXIDATION OF PYRUVATE TO ACETYL-CoA IS THE IRREVERSIBLE ROUTE FROM
GLYCOLYSIS TO THE CITRIC ACID CYCLE
Pyruvate, formed in the cytosol, is transported into the mitochondrion by a proton symporter. Inside the mitochondrion, pyruvate is oxidatively decarboxylated to acetyl-CoA by a multienzyme complex that is associated with the inner mitochondrial membrane. This pyruvate dehydrogenase complex is analogous to the α-ketoglutarate dehydrogenase complex of the citric acid cycle. Pyruvate is decarboxylated by the pyruvate dehydrogenase component of the enzyme complex to a hydroxyethyl derivative of the thiazole ring of enzyme-bound thiamin diphosphate, which in turn reacts with oxidized lipoamide, the prosthetic group of dihydrolipoyl transacetylase, to form acetyl lipoamide. Thiamin is vitamin B1, and in thiamin deficiency glucose metabolism is impaired and there is significant (and potentially life-threatening) lactic and pyruvic acidosis. Acetyl lipoamide reacts with coenzyme A to form acetyl-CoA and reduced lipoamide.
The cycle of reaction is completed when the reduced lipoamide is reoxidized by a flavoprotein, dihydrolipoyl dehydrogenase, containing FAD. Finally, the reduced flavoprotein is oxidized by NAD+, which in turn transfers reducing equivalents to the respiratory chain. The pyruvate dehydrogenase complex consists of a number of polypeptide chains of each of the three component enzymes, all organized in a regular spatial configuration.
Movement of the individual enzymes appears to be restricted, and the metabolic intermediates do not dissociate freely but remain bound to the enzymes.
Oxidation of Glucose Yields Up to 38 Mol of ATP Under Aerobic Conditions But Only 2 Mol When O2 Is Absent
When 1 mol of glucose is combusted in a calorimeter to CO2 and water, approximately 2870 kJ are liberated as heat. When oxidation occurs in the tissues, approximately 38 mol of ATP are generated per molecule of glucose oxidized to CO2 and water. In vivo, ∆G for the ATP synthase reaction has been calculated as approximately 51.6 kJ. It follows that the total energy captured in ATP per mole of glucose oxidized is 1961 kJ, or approximately 68% of the energy of combustion. Most of the ATP is formed by oxidative phosphorylation resulting from the reoxidation of reduced coenzymes by the respiratory chain. The remainder is formed by substratelevel phosphorylation.
Inhibition of Pyruvate Metabolism Leads to Lactic Acidosis
Arsenite and mercuric ions react with the SH groups of lipoic acid and inhibit pyruvate dehydrogenase, as does a dietary deficiency of thiamin, allowing pyruvate to accumulate. Nutritionally deprived alcoholics are thiamin-deficient and may develop potentially fatal pyruvic and lactic acidosis. Patients with inherited pyruvate dehydrogenase deficiency, which can be due to defects in one or more of the components of the enzyme complex, also present with lactic acidosis, particularly after a glucose load. Because of its dependence on glucose as a fuel, brain is a prominent tissue where these metabolic defects manifest themselves in neurologic disturbances. Inherited aldolase A deficiency and pyruvate kinase deficiency in erythrocytes cause hemolytic anemia. The exercise capacity of patients with muscle phosphofructokinase deficiency is low, particularly on high-carbohydrate diets. By providing an alternative lipid fuel, eg, during starvation, when blood free fatty acids and ketone bodies are increased, work capacity is improved.
The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism
The pentose phosphate pathway is an alternative route for the metabolism of glucose. It does not generate ATP but has two major functions: (1) The formation of NADPH for synthesis of fatty acids and steroids and (2) the synthesis of ribose for nucleotide and nucleic acid formation. Glucose, fructose, and galactose are the main hexoses absorbed from the gastrointestinal tract, derived principally from dietary starch, sucrose, and lactose, respectively. Fructose and galactose are converted to glucose, mainly in the liver.
Genetic deficiency of glucose 6-phosphate dehydrogenase, the first enzyme of the pentose phosphate pathway, is a major cause of hemolysis of red blood cells, resulting in hemolytic anemia and affecting approximately 100 million people worldwide. Glucuronic acid is synthesized from glucose via the uronic acid pathway, of major significance for the excretion of metabolites and foreign chemicals (xenobiotics) as glucuronides. A deficiency in the pathway leads to essential pentosuria. The lack of one enzyme of the pathway (gulonolactone oxidase) in primates and some other animals explains why ascorbic acid (vitamin C) is a dietary requirement for humans but not most other mammals. Deficiencies in the enzymes of fructose and galactose metabolism lead to essential fructosuria
and the galactosemias.
THE PENTOSE PHOSPHATE PATHWAY GENERATES NADPH & RIBOSE PHOSPHATE
The pentose phosphate pathway (hexose monophosphate shunt) is a more complex pathway than glycolysis. Three molecules of glucose 6-phosphate give rise to three molecules of CO2 and three five-carbon sugars. These are rearranged to regenerate two molecules of glucose 6-phosphate and one molecule of the glycolytic intermediate, glyceraldehyde 3-phosphate. Since two molecules of glyceraldehyde 3-phosphate can regenerate glucose 6-phosphate, the pathway can account for the complete oxidation of glucose.
The Oxidative Phase Generates NADPH
Dehydrogenation of glucose 6-phosphate to 6-phosphogluconate occurs via the formation of 6-phosphogluconolactone, catalyzed by glucose-6-phosphate dehydrogenase, an NADP-dependent enzyme.
The hydrolysis of 6-phosphogluconolactone is accomplished by the enzyme gluconolactone hydrolase. A second oxidative step is catalyzed by 6-phosphogluconate dehydrogenase, which also requires NADP+ as hydrogen acceptor and involves decarboxylation followed by formation of the ketopentose, ribulose 5-phosphate.
The Nonoxidative Phase Generates Ribose Precursors
Ribulose 5-phosphate is the substrate for two enzymes. Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phosphate. Ribose 5-phosphate ketoisomerase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5-phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis. Transketolase transfers the two-carbon aldehyde carbon of an aldose sugar. It therefore effects the conversion of a ketose sugar into an aldose with two carbons less and simultaneously converts an aldose sugar into a ketose with two carbons more. The reaction requires Mg2+ and thiamin diphosphate (vitamin B1) as coenzyme. Thus, transketolase catalyzes the transfer of the two-carbon unit from xylulose 5-phosphate to ribose 5-phosphate, producing the seven-carbon ketose sedoheptulose 7-phosphate and the aldose glyceraldehyde 3-phosphate. Transaldolase allows the transfer of a three-carbon dihydroxyacetone moiety (carbons 1–3) from the ketose sedoheptulose 7-phosphate onto the aldose
glyceraldehyde 3-phosphate to form the ketose fructose 6-phosphate and the four-carbon aldose erythrose 4-phosphate. In a further reaction catalyzed by transketolase, xylulose 5-phosphate donates a two-carbon unit to erythrose 4-phosphate to form fructose 6-phosphate and glyceraldehyde 3-phosphate.
In order to oxidize glucose completely to CO2 via the pentose phosphate pathway, there must be enzymes present in the tissue to convert glyceraldehyde 3-phosphate to glucose 6-phosphate. This involves reversal of glycolysis and the gluconeogenic enzyme fructose 1,6- bisphosphatase. In tissues that lack this enzyme, glyceraldehyde 3-phosphate follows the normal pathway of glycolysis to pyruvate.
The Two Major Pathways for the Catabolism of Glucose Have Little in Common
Although glucose 6-phosphate is common to both pathways, the pentose phosphate pathway is markedly different from glycolysis. Oxidation utilizes NADP rather than NAD, and CO2, which is not produced in glycolysis, is a characteristic product. No ATP is generated in the pentose phosphate pathway, whereas ATP is a major product of glycolysis.
Reducing Equivalents Are Generated in Those Tissues Specializing in Reductive Syntheses
The pentose phosphate pathway is active in liver, adipose tissue, adrenal cortex, thyroid, erythrocytes, testis, and lactating mammary gland. Its activity is low in nonlactating mammary gland and skeletal muscle. Those tissues in which the pathway is active use NADPH in reductive syntheses, eg, of fatty acids, steroids, amino acids via glutamate dehydrogenase, and reduced glutathione. The synthesis of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase may also be induced by insulin during conditions associated with the “fed
state” , when lipogenesis increases.
Ribose Can Be Synthesized in Virtually All Tissues
Little or no ribose circulates in the bloodstream, so tissues must synthesize the ribose required for nucleotide and nucleic acid synthesis. The source of ribose 5-phosphate is the pentose phosphate pathway. Muscle has only low activity of glucose- 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Nevertheless, like most other tissues, it is capable of synthesizing ribose 5-phosphate by reversal of the nonoxidative phase of the pentose phosphate pathway utilizing fructose 6-phosphate. It is not necessary to have a completely functioning pentose phosphate pathway for a tissue to synthesize ribose phosphates.
Defects in Fructose Metabolism Cause Disease
Lack of hepatic fructokinase causes essential fructosuria, and absence of hepatic aldolase B, which cleaves fructose 1-phosphate, leads to hereditary fructose intolerance. Diets low in fructose, sorbitol, and sucrose are beneficial for both conditions. One consequence of hereditary fructose intolerance and of another condition due to fructose-1,6-bisphosphatase deficiency is fructose-induced hypoglycemia despite the presence of high glycogen reserves. The accumulation of fructose 1-phosphate and fructose 1,6-bisphosphate allosterically inhibits the activity of liver phosphorylase. The sequestration of inorganic phosphate also leads to depletion of ATP and hyperuricemia.
Fructose & Sorbitol in the Lens Are Associated With Diabetic Cataract
Both fructose and sorbitol are found in the lens of the eye in increased concentrations in diabetes mellitus and may be involved in the pathogenesis of diabetic cataract. The sorbitol (polyol) pathway (not found in liver) is responsible for fructose formation from glucose and increases in activity as the glucose concentration rises in diabetes in those tissues that are not insulin-sensitive, ie, the lens, peripheral nerves, and renal glomeruli. Glucose is reduced to sorbitol by aldose reductase, followed by oxidation of sorbitol to fructose in the presence of NAD+ and sorbitol dehydrogenase (polyol dehydrogenase). Sorbitol does not diffuse through cell membranes easily and accumulates, causing osmotic damage. Simultaneously, myoinositol levels fall. Sorbitol accumulation, myoinositol depletion, and diabetic cataract can be prevented by aldose reductase inhibitors in diabetic rats, and promising results have been obtained in clinical trials. When sorbitol is administered intravenously, it is converted to fructose rather than to glucose. It is poorly absorbed in the small intestine, and much is fermented by colonic bacteria to short-chain fatty acids, CO2, and H2, leading to abdominal pain and diarrhea (sorbitol intolerance).
Enzyme Deficiencies in the Galactose Pathway Cause Galactosemia
Inability to metabolize galactose occurs in the galactosemias, which may be caused by inherited defects in galactokinase, uridyl transferase, or 4-epimerase, though a deficiency in uridyl transferase is the best known cause. The galactose concentration in the blood and in the eye is reduced by aldose reductase to galactitol, which accumulates, causing cataract. In uridyl transferase deficiency, galactose 1-phosphate accumulates and depletes the liver of inorganic phosphate. Ultimately, liver failure and mental deterioration result. As the epimerase is present in adequate amounts, the galactosemic individual can still form UDPGal from glucose, and normal growth and development can occur regardless of the galactose-free diets used to control the symptoms of the disease.
• Glycolysis is the cytosolic pathway of all mammalian cells for the metabolism of glucose (or glycogen) to pyruvate and lactate.
• It can function anaerobically by regenerating oxidized NAD+ (required in the glyceraldehyde-3-phosphate dehydrogenase reaction) by reducing pyruvate to lactate.
• Lactate is the end product of glycolysis under anaerobic conditions (eg, in exercising muscle) or when the metabolic machinery is absent for the further oxidation of pyruvate (eg, in erythrocytes).
• Glycolysis is regulated by three enzymes catalyzing nonequilibrium reactions: hexokinase, phosphofructokinase, and pyruvate kinase.
• In erythrocytes, the first site in glycolysis for generation of ATP may be bypassed, leading to the formation of 2,3-bisphosphoglycerate, which is important in decreasing the affinity of hemoglobin for O2.
• Pyruvate is oxidized to acetyl-CoA by a multienzyme complex, pyruvate dehydrogenase, that is dependent on the vitamin cofactor thiamin diphosphate.
• Conditions that involve an inability to metabolize pyruvate frequently lead to lactic acidosis.
The pentose phosphate pathway, present in the cytosol,can account for the complete oxidation of glucose, producing NADPH and CO2 but not ATP.
• The pathway has an oxidative phase, which is irreversible and generates NADPH; and a nonoxidative phase, which is reversible and provides ribose precursors for nucleotide synthesis. The complete pathway is present only in those tissues having a requirement for NADPH for reductive syntheses, eg, lipogenesis or steroidogenesis, whereas the nonoxidative phase is present in all cells requiring ribose.
• In erythrocytes, the pathway has a major function in preventing hemolysis by providing NADPH to maintain glutathione in the reduced state as the substrate for glutathione peroxidase.
• The uronic acid pathway is the source of glucuronic acid for conjugation of many endogenous and exogenous substances before excretion as glucuronides in urine and bile.
• Fructose bypasses the main regulatory step in glycolysis, catalyzed by phosphofructokinase, and stimulates fatty acid synthesis and hepatic triacylglycerol secretion.
• Galactose is synthesized from glucose in the lactating mammary gland and in other tissues where it is required for the synthesis of glycolipids, proteoglycans, and glycoproteins.