Harper's Biochemistry Chapter 17 - Glycolysis and Oxidation to Pyruvate.PDF

C H A P T E R Glycolysis & the Oxidation of Pyruvate Owen P. McGuinness, PhD 17 OBJ E C TI VE S Describe the pathway of glycolysis and its control, and explai...

C H A P T E R Glycolysis & the Oxidation of Pyruvate Owen P. McGuinness, PhD 17 OBJ E C TI VE S Describe the pathway of glycolysis and its control, and explain how glycolysis can operate under anaerobic conditions. After studying this chapter, Describe the reaction of pyruvate dehydrogenase and its regulation. you should be able to: Explain how inhibition of pyruvate oxidation leads to lactic acidosis. BIOMEDICAL IMPORTANCE is the most common and is due to impaired tissue perfusion or hypoxia (eg, sepsis and hypovolemia). Type B is due to the Most tissues have at least some requirement for glucose. impaired ability to metabolize lactate (eg, liver disease, thia- The brain is predominantly reliant on glucose for its energy mine deficiency). Thiamin (vitamin B1) deficiency impairs the needs, except in prolonged fasting where ketone bodies can activity of pyruvate dehydrogenase. meet about 20% of its energy needs. Glycolysis is the main pathway through which cells metabolize glucose and other carbohydrates. It occurs in the cytosol of cells and can func- tion either aerobically or anaerobically depending on the GLYCOLYSIS CAN FUNCTION availability of oxygen and the activity of the electron trans- UNDER ANAEROBIC CONDITIONS port chain (and hence of the presence of mitochondria). Erythrocytes, which lack mitochondria, are completely reli- Early in the investigations of glycolysis, it was realized that ant on glucose as their metabolic fuel, and metabolize it by fermentation in yeast was similar to the breakdown of gly- anaerobic glycolysis. cogen in muscle. When a muscle contracts under anaerobic The ability of glycolysis to provide ATP in the absence of conditions, glycogen disappears and lactate appears. When oxygen allows skeletal muscle to perform at very high levels of oxygen is made available, aerobic recovery takes place and work output when oxygen supply is insufficient, and it allows lactate is no longer produced. If muscle contraction occurs tissues to survive anoxic episodes. However, heart muscle, under aerobic conditions, lactate does not accumulate as which is adapted for aerobic performance, has relatively low pyruvate and NADH, the products of glycolysis. Pyruvate and glycolytic activity and poor survival under conditions of isch- NADH can enter the mitochondria where they are oxidized emia. Diseases in which enzymes of glycolysis (eg, pyruvate further to CO2, H2O, and NAD (Figure 17–1). When oxy- kinase) are deficient are mainly seen as hemolytic anemias gen is in short supply, mitochondrial reoxidation of NADH or, if the defect affects skeletal muscle (eg, phosphofructo- formed during glycolysis is impaired. As there is a limited kinase), as fatigue. In fast-growing cancer cells, glycolysis supply of NAD to sustain glycolysis the NADH has to be proceeds at a high rate, forming large amounts of pyruvate, reoxidized. The NADH is reoxidized by reducing pyruvate to which is reduced to lactate and released. The lactate is used lactate. This allows NAD to be available for the early steps for gluconeogenesis in the liver (see Chapter 19) and com- in the pathway, so permitting glycolysis to continue. While bined with the marked increase in hepatic protein synthesis glycolysis can occur under anaerobic conditions, this has a secondary to the increased delivery of amino acids from the price. It limits the amount of ATP formed per mole of glucose catabolic muscle contributes to the hypermetabolism seen in oxidized, so that much more glucose must be metabolized cancer cachexia. Lactic acidosis can be of two types. Type A under anaerobic than aerobic conditions to supply the same quantity of ATP to supply cellular work (Table 17–1). In yeast and some other microorganisms, pyruvate formed in anaero- This was adapted from chapter in 30th edition by David A. Bender, bic glycolysis is not reduced to lactate, but is decarboxylated PhD, & Peter A. Mayes, PhD, DSc and reduced to form ethanol. 163 164 SECTION IV Metabolism of Carbohydrates Glucose Glycogen C6 (C6 ) n GLYCOLYSIS CONSTITUTES THE MAIN PATHWAY OF GLUCOSE UTILIZATION The overall equation for glycolysis from glucose to lactate is Hexose phosphates as follows: C6 Glucose + 2 ADP + 2 Pi → 2 Lactate + 2 ATP + 2 H2O All of the enzymes of glycolysis (Figure 17–2) are cytosolic. The general mechanism for the generation of ATP dur- ing glycolysis rearranges a phosphorylated monosaccharide glucose-6-phosphate to phosphorylated compounds with a Triose phosphate Triose phosphate high potential to transfer their phosphate groups. These C3 C3 triosephosphates transfer their phosphate to ADP to form NAD + H2 O ATP. This process is called substrate-level phosphorylation, as the phosphate is directly donated to ATP from an intermediate O2 NADH 1/2O in the pathway. This pathway uses ADP and produces ATP. + H+ 2 As ADP is limiting, it is required that the generated ATP be CO2 Pyruvate Lactate used to perform some metabolic work to regenerate the ADP + H2O C3 C3 to sustain glycolysis. After being transported across the plasma membrane by facilitated glucose transporters glucose enters glycoly- FIGURE 17–1 Summary of glycolysis. , blocked under sis by phosphorylation to glucose-6-phosphate, catalyzed by anaerobic conditions or by absence of mitochondria containing key hexokinase, using ATP as the phosphate donor. Under physi- respiratory enzymes, as in erythrocytes. ologic conditions, the phosphorylation of glucose to glucose- 6-phosphate can be regarded as irreversible. Hexokinase is inhibited allosterically by its product, glucose-6-phosphate. TABLE 17–1 ATP Formation in the Catabolism of Glucose Pathway Reaction Catalyzed by Method of ATP Formation ATP per mol of Glucose Glycolysis Glyceraldehyde-3-phosphate dehydrogenase Respiratory chain oxidation of 2 NADH 5a Phosphoglycerate kinase Substrate-level phosphorylation 2 Pyruvate kinase Substrate-level phosphorylation 2 9 Consumption of ATP for reactions of hexokinase and phosphofructokinase −2 Net 7 Citric acid cycle Pyruvate dehydrogenase Respiratory chain oxidation of 2 NADH 5 Isocitrate dehydrogenase Respiratory chain oxidation of 2 NADH 5 α-Ketoglutarate dehydrogenase Respiratory chain oxidation of 2 NADH 5 Succinate thiokinase Substrate-level phosphorylation 2 Succinate dehydrogenase Respiratory chain oxidation of 2 FADH2 3 Malate dehydrogenase Respiratory chain oxidation of 2 NADH 5 Net 25 Total per mol of glucose under aerobic conditions 32 Total per mol of glucose under anaerobic conditions 2 a This assumes that NADH formed in glycolysis is transported into mitochondria by the malate shuttle (see Figure 13–13). If the glycerophosphate shuttle is used, then only 1.5 ATP will be formed per mol of NADH. There is a considerable advantage in using glycogen rather than glucose for anaerobic glycolysis in muscle, since the product of glycogen phosphorylase is glucose-1-phosphate (see Figure 18–1), which is interconvertible with glucose-6-phosphate. This saves the ATP that would otherwise be used by hexokinase, increasing the net yield of ATP from 2 to 3 per glucose. (Note: The initial formation of glycogen requires ATP so in a net sense glucose first going to glycogen and then coming back out for oxidation via pyruvate is the same. It’s just that you have more ATP at a time where ATP is in short supply.  CHAPTER 17 Glycolysis & the Oxidation of Pyruvate 165 Glycogen Glucose-1-phosphate Hexokinase, glucokinase Phosphohexose Phosphofructokinase CH2O P HC O HC O isomerase CH2OH CH2O P C O HC OH ATP ADP HC OH C O ATP ADP C O CH2OH HO CH HO CH HO CH HO CH Dihydroxyacetone HC OH HC OH HC OH phosphate HC OH Aldolase Triose HC OH HC OH HC OH H PO HC OH H3PO4 3 4 phosphate CH2OH CH2O P CH2O P Fructose CH2O P isomerase *Glucose 6-phosphatase bisphosphatase CH2O P Glucose Glucose-6-phosphate Fructose-6-phosphate Fructose 1,6-bisphosphate HC OH HC O Glyceraldehyde- 3-phosphate 2 × 3 carbon sugar molecules per glucose H3PO4 NAD+ Glyceraldehyde- 3-phosphate dehydrogenase NADH Lactate dehydrogenase Pyruvate kinase Enolase Phosphoglyceromutase Phosphoglycerate CH3 CH2 CH2OH kinase CH2O P CH3 CH2O P C O C O P HC O P HC OH HC OH HO C O + COOH ATP COOH COOH COOH COO P COOH NAD NADH ADP ATP ADP Lactate Pyruvate Phosphoenolpyruvate 2-Phosphoglycerate 3-Phosphoglycerate Bisphosphoglycerate FIGURE 17–2 The pathway of glycolysis. ( P , —PO32–; Pi, HOPO32–; , inhibition.) Carbons 1–3 of fructose bisphosphate form dihydroxy- acetone phosphate, and carbons 4–6 form glyceraldehyde-3-phosphate. Glucose-6-phosphatase is expressed only in the liver, kidney, and pancreatic islet; it is not expressed in other tissues. In muscle and adipose tissue, the transport of glucose is In the liver transport activity is not regulated and a dif- stimulated by insulin. In muscle glucose is used for glycoly- ferent isozyme of hexokinase is expressed (glucokinase). sis (or glycogen synthesis; see Chapter 18). In adipose tissue, Glucokinase has a Km higher than the normal plasma con- it is used for lipogenesis (see Chapter 23). The hexokinase centration of glucose and it is not inhibited by its product expressed in most tissues has a high affinity (low Km) for glucose-6-phosphate. Because of the relatively high constitu- glucose and is allosterically inhibited by its product glucose- tive glucose transport activity and the low affinity of glucoki- 6-phosphate. When transport activity is low, transport acts nase for glucose, the intracellular glucose in the liver is very as a barrier to glucose uptake. Thus transport activity is an similar to plasma glucose. The liver can be both a consumer important determinant of the overall rate of glycolysis. The and producer of glucose (feasting vs fasting; see Chapter 14). combined effect of a low transport activity and the high affinity In contrast to most tissues the liver also expresses glucose- of hexokinase for glucose keeps the intracellular glucose very 6-phosphatase, which allows the liver in the fasting state to low. Thus, the concentration of plasma glucose and the cellu- dephosphorylate glucose-6-phosphate generated by the liver lar ATP demand are the primary determinants of glycolysis in and release glucose. In the fed state the function of glucokinase an aerobic environment. In the presence of insulin transport in the liver is to remove glucose from the hepatic portal blood activity increases, thus lowering the barrier to glucose entry. following a meal; this limits the quantity of glucose available If, for example, transport activity increased 10-fold glycolysis to peripheral tissues. Glucokinase in the fasted setting is found will not increase 10-fold because the consequent increase in inactive in the nucleus bound to a glucokinase regulatory glucose-6-phosphate would serve as a brake on hexokinase protein. In response to signals during a meal it leaves the to limit the overall rate of glycolysis. This effectively shifts nucleus and resides in the cytosol where it is active. Because of the control of glycolysis to pathways (eg, pyruvate oxidation) the high capacity of glucokinase to phosphorylate glucose downstream of hexokinase. In muscle, hexokinase is found and the increase in glucose in the portal vein during a meal, it bound to the outer mitochondrial membrane. As hexokinase provides more glucose-6-phosphate than is required for liver requires ATP, it creates a coupling between hexokinase activity glucose oxidation, thus a large fraction is used for glycogen and mitochondrial ATP generation. synthesis and a smaller amount is used for lipogenesis. 166 SECTION IV Metabolism of Carbohydrates Glucokinase is also found in pancreatic islet β cells, where Glycolysis continues with the oxidation of glyceraldehyde- it functions to detect changes in concentrations of glucose in 3-phosphate to 1,3-bisphosphoglycerate and the formation of the systemic circulation. When glucose is increased more glu- NADH. The enzyme catalyzing this oxidation, glyceraldehyde- cose is phosphorylated by glucokinase, increasing glycolysis, 3-phosphate dehydrogenase, is NAD dependent. Structur- and leading to increased formation of ATP. The increase in ally, it consists of four identical polypeptides (monomers) ATP leads to closure of an ATP sensitive potassium channel, forming a tetramer. Four —SH groups are present on each causing membrane depolarization and opening of a voltage- polypeptide, derived from cysteine residues within the poly- gated calcium channel. The resultant influx of calcium ions peptide chain. One of the —SH groups is found at the active leads to fusion of the insulin secretory granules with the cell site of the enzyme (Figure 17–3). The substrate initially com- membrane and the release of insulin. bines with this —SH group, forming a thiohemiacetal that is Glucose-6-phosphate is an important compound at the junc- oxidized to a thiol ester; the hydrogens removed in this oxida- tion of several metabolic pathways: glycolysis, gluconeogenesis tion are transferred to NAD+. The thiol ester then undergoes (see Chapter 19), the pentose phosphate pathway (see Chapter 20), phosphorolysis; inorganic phosphate (Pi) is added, forming glycogenesis, and glycogenolysis (see Chapter 18). In glycolysis, it 1,3-bisphosphoglycerate and the free —SH group. is converted to fructose-6-phosphate by phosphohexose isomer- In the next reaction, catalyzed by phosphoglycerate kinase, ase, which involves an aldose–ketose isomerization. This reaction phosphate is transferred from 1,3-bisphosphoglycerate onto is followed by another phosphorylation catalyzed by the enzyme ADP, forming ATP (substrate-level phosphorylation) and phosphofructokinase (phosphofructokinase-1) forming fructose 3-phosphoglycerate. Since two molecules of triose phosphate 1,6-bisphosphate. The phosphofructokinase reaction is irrevers- are formed per molecule of glucose metabolized, 2× ATP are ible under physiologic conditions. Phosphofructokinase is both formed in this reaction per molecule of glucose undergoing inducible and subject to allosteric regulation, and has a major role glycolysis. The toxicity of arsenic is the result of competition in regulating the rate of glycolysis. Note up to this point in the path- of arsenate with inorganic phosphate (Pi) forming 1-arseno- way no ATP is generated; ATP is only being consumed. Fructose 3-phosphoglycerate, which undergoes spontaneous hydrolysis 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate to 3-phosphoglycerate without forming ATP. 3-Phosphoglycerate aldolase) into two triose phosphates, glyceraldehyde-3-phosphate is isomerized to 2-phosphoglycerate by phosphoglycerate mutase. and dihydroxyacetone phosphate, which are interconverted by It is likely that 2,3-bisphosphoglycerate (diphosphoglycerate, the enzyme phosphotriose isomerase. DPG) is an intermediate in this reaction. S Enz H C O H C OH H C OH NAD+ H C OH CH2 O P CH2 O P Glyceraldehyde-3-phosphate Enzyme–substrate complex HS Enz NAD+ Bound coenzyme Substrate oxidation O P by bound NAD+ C O H C OH Pi CH2 O P 1,3-Bisphosphoglycerate S Enz S Enz C O C O NAD+ NADH + H+ H C OH H C OH NADH + H+ NAD+ CH2 O P CH2 O P FIGURE 17–3 Mechanism of oxidation of glyceraldehyde-3-phosphate. (Enz, glyceraldehyde-3-phosphate dehydrogenase.) The enzyme is inhibited by the —SH poison iodoacetate, which is thus able to inhibit glycolysis. The NADH produced on the enzyme is not so firmly bound to the enzyme as is NAD+. Consequently, NADH is easily displaced by another molecule of NAD+.  CHAPTER 17 Glycolysis & the Oxidation of Pyruvate 167 Enolase catalyzes the next step. It involves dehydration, anaerobic glycolysis to support metabolism (eg, tumors, retina forming phosphoenolpyruvate. Enolase is inhibited by and renal medulla, skin). Lactate production is also increased fluoride. When blood samples are taken for measurement of in septic shock secondary to both impaired oxygen delivery glucose, the sample is placed into tubes containing fluoride to and shifts on metabolic capacity due to cellular injury. Other inhibit glycolysis and prevent the breakdown of the glucose tissues like the brain that normally derive much of their until the sample is analyzed. Enolase is also dependent on the energy from glucose oxidation produce some lactate (3-5% of presence of either Mg2+ or Mn2+ ions. total glycolytic flux) because of the high glycolytic rates. The The phosphate of phosphoenolpyruvate is transferred to liver, kidneys, oxidative skeletal muscle, and heart have very ADP in another substrate-level phosphorylation catalyzed by high oxidative capacity to oxidize multiple fuels. They nor- pyruvate kinase to form 2× ATP per molecule of glucose oxi- mally take up lactate and oxidize it, but can produce it under dized. The reaction of pyruvate kinase is essentially irrevers- hypoxic conditions. When lactate production is high, as in ible under physiologic conditions, partly because of the large vigorous exercise, septic shock, and cancer cachexia, lactate is free-energy change involved and partly because the immediate used in the liver for gluconeogenesis (see Chapter 19), leading product of the enzyme-catalyzed reaction is enolpyruvate, to an increase in metabolic rate to provide the ATP and GTP which undergoes spontaneous isomerization to pyruvate, so needed. This increase may contribute to the increase in meta- that the product of the reaction is not available to undergo the bolic rate after cessation of vigorous exercise. reverse reaction. Under some states lactate can be directly transported into The availability of oxygen determines which of the two path- the mitochondria to generate pyruvate and NADH. This allows ways pyruvate follows. Under anaerobic conditions, the NADH for the transfer of reducing equivalents (eg, NADH) from the cannot be reoxidized through the respiratory chain, and pyru- cytosol into the mitochondrion for the electron transport vate is reduced to lactate catalyzed by lactate dehydrogenase. chain in addition to the glycerophosphate (see Figure 13–12) This permits the oxidization of NADH to form NAD permit- and malate-aspartate (see Figure 13–13) shuttles. ting another molecule of glucose to undergo glycolysis. Under aerobic conditions, pyruvate is transported into the mitochon- dria and undergoes oxidative decarboxylation to acetyl-CoA then oxidation to CO2 in the citric acid cycle (see Chapter 16). GLYCOLYSIS IS REGULATED The reducing equivalents from the NADH formed in glycoly- AT THREE NONEQUILIBRIUM sis are taken up into mitochondria for oxidation via either the malate-aspartate shuttle or the glycerophosphate shuttle REACTIONS (see Chapter 13). Although most of the reactions of glycolysis are freely reversible, three are markedly exergonic and are considered to be physi- ologically irreversible. These reactions, catalyzed by hexokinase TISSUES THAT FUNCTION (and glucokinase), phosphofructokinase, and pyruvate kinase, are the major sites of regulation of glycolysis. Phosphofructo- UNDER HYPOXIC CONDITIONS kinase is significantly inhibited at normal intracellular con- OR HAVE INTRISICALLY HIGH centrations of ATP; as discussed in Chapter 19, this inhibition RATES OF GLUCOSE OXIDATION can be rapidly relieved by 5′ AMP that is formed as ADP begins to accumulate. It is due to a failure to rapidly convert ADP PRODUCE LACTATE to ATP, signaling the need for an increased rate of glycolysis. In organs where glucose is the major substrate for oxidation, Cells that are capable of gluconeogenesis (reversing the a variable fraction of the glucose uptake can be diverted to glycolytic pathway; see Chapter 19) have different enzymes lactate depending on the metabolic state of the tissue. In tis- that catalyze reactions to reverse these irreversible steps: sues or metabolic states where pyruvate oxidation is absent or glucose-6-phosphatase, fructose 1,6-bisphosphatase and, to impaired, pyruvate is diverted to lactate and released so as to reverse the reaction of pyruvate kinase, pyruvate carboxyl- recycle the NAD to support glycolysis and the generation of ase, and phosphoenolpyruvate carboxykinase. The reciprocal ATP to support cellular metabolic work. In erythrocytes pyru- regulation of phosphofructokinase in glycolysis and fruc- vate always terminates in lactate. They lack mitochondria and tose 1,6-bisphosphatase in gluconeogenesis is discussed in thus cannot oxidize pyruvate. This is true of some skeletal mus- Chapter 19. cle, particularly the white fibers. During states of high-work Fructose enters glycolysis by phosphorylation to fructose-1- output they mobilize muscle glycogen stores. Because they phosphate, and bypasses the main regulatory steps, resulting in lack glucose-6-phosphatase the glucose-6-phosphate mobi- formation of more pyruvate and acetyl-CoA than is required lized from glycogen traverses down the glycolytic pathway for ATP formation. In addition, increases in fructose-1-P in and combined with the relatively low rates of pyruvate oxi- the liver activate glucokinase (compete for binding of gluco- dation due to lower mitochondrial oxidative capacity, lactate kinase with the glucokinase regulatory protein), amplifying is released. The need for ATP formation may exceed the rate hepatic glucose uptake and increasing hepatic lipogenesis and at which oxygen is delivered. In those tissues they rely on predisposing to hepatic steatosis. 168 SECTION IV Metabolism of Carbohydrates H C O Glucose with the inner mitochondrial membrane. This pyruvate dehy- H C OH drogenase complex is analogous to the α-ketoglutarate dehydro- CH2 O genase complex of the citric acid cycle (see Chapter 16). Pyruvate P is decarboxylated by the pyruvate dehydrogenase component Glyceraldehyde-3-phosphate of the enzyme complex to a hydroxyethyl derivative of the thia- Pi NAD+ zole ring of enzyme-bound thiamin diphosphate, which in Glyceraldehyde 3-phosphate turn reacts with oxidized lipoamide, the prosthetic group of dehydrogenase dihydrolipoyl transacetylase, to form acetyl lipoamide NADH + H+ (Figure 17–5). In thiamin (vitamin B1; see Chapter 44) defi- O ciency, pyruvate oxidation is impaired, and there is significant C O P (and potentially life-threatening) lactic and pyruvic acidosis. Bisphosphoglycerate Acetyl lipoamide reacts with coenzyme A to form acetyl-CoA H C OH mutase and reduced lipoamide. The reaction is completed when the CH2 O P reduced lipoamide is reoxidized by a flavoprotein, dihydroli- poyl dehydrogenase, containing flavin adenine dinucleotide 1,3-Bisphosphoglycerate (FAD). Finally, the reduced flavoprotein is oxidized by NAD+, ADP COO– which in turn transfers reducing equivalents to the respiratory Phosphoglycerate H C O P chain. The overall reaction is: kinase CH2 O P Pyruvate + NAD+ + CoA → Acetyl-CoA + NADH + H+ + CO2 ATP 2,3-Bisphosphoglycerate The pyruvate dehydrogenase complex consists of a num- COO– ber of polypeptide chains of each of the three component H C OH Pi enzymes and the intermediates do not dissociate, but are 2,3-Bisphosphoglycerate channeled from one enzyme site to the next. This increases CH2 O P phosphatase the rate of reaction and prevents side reactions. 3-Phosphoglycerate Pyruvate Pyruvate Dehydrogenase Is FIGURE 17–4 The 2,3-bisphosphoglycerate pathway in Regulated by End-Product erythrocytes. Inhibition & Covalent Modification Pyruvate dehydrogenase is inhibited by its products, acetyl-CoA In Erythrocytes, the First Site of and NADH (Figure 17–6). It is also regulated by phosphory- ATP Formation in Glycolysis May Be lation (catalyzed by a kinase) of three serine residues on the pyruvate dehydrogenase component of the multienzyme com- Bypassed plex, resulting in decreased activity and by dephosphorylation In erythrocytes, the reaction catalyzed by phosphoglycerate (catalyzed by a phosphatase) that causes an increase in activity. kinase may be bypassed to some extent by the reaction of The kinase is activated by increases in the [ATP]/[ADP], bisphosphoglycerate mutase, which catalyzes the conversion [acetyl-CoA]/[CoA], and [NADH]/[NAD+] ratios. Thus, pyru- of 1,3-bisphosphoglycerate to 2,3-bisphosphoglycerate, fol- vate dehydrogenase, and therefore glycolysis, is inhibited both lowed by hydrolysis to 3-phosphoglycerate and Pi, catalyzed by when there is adequate ATP (and reduced coenzymes for 2,3-bisphosphoglycerate phosphatase (Figure 17–4). This path- ATP formation) available, and also when fatty acids are being way involves no net yield of ATP from glycolysis, but provides oxidized. In fasting, when nonesterified fatty acid concentra- 2,3-bisphosphoglycerate, which binds to hemoglobin, decreasing tions increase, there is a decrease in the proportion of the its affinity for oxygen. This increases the efficiency with which enzyme in the active form, decreasing pyruvate oxidation. In hemoglobin delivers oxygen to tissues (see Chapter 6). adipose tissue, where glucose provides acetyl-CoA for lipo- genesis, the enzyme is activated in response to insulin. THE OXIDATION OF PYRUVATE TO ACETYL-COA IS THE CLINICAL ASPECTS IRREVERSIBLE ROUTE FROM Inhibition of Pyruvate Metabolism GLYCOLYSIS TO THE CITRIC Leads to Lactic Acidosis ACID CYCLE Arsenite and mercuric ions react with the —SH groups of Pyruvate is transported into the mitochondrion by a proton lipoic acid and inhibit pyruvate dehydrogenase, as does a symporter. It then undergoes oxidative decarboxylation to dietary deficiency of thiamin (see Chapter 44), allowing acetyl-CoA, catalyzed by a multienzyme complex that is associated pyruvate to accumulate. Many alcoholics are thiamin deficient  CHAPTER 17 Glycolysis & the Oxidation of Pyruvate 169 CH3 C O COO– Pyruvate Enzyme-bound CH3 thiamin diphosphate C O Pyruvate dehydrogenase SCoA CO2 CoASH Acetyl CoA Dihydrolipoyl CH3 CH3 transacetylase HC OH C O S SH Thiamin diphosphate S S SH SH Hydroxyethyl thiamin diphosphate Acetyl lipoamide Lipoamide Dihydrolipoyl Reduced lipoamide dehydrogenase FAD FADH2 NAD+ NADH FIGURE 17–5 Oxidative decarboxylation of pyruvate by the pyruvate dehydrogenase complex. Lipoic acid is joined by an amide link to a lysine residue of the transacetylase component of the enzyme complex. It forms a long flexible arm, allowing the lipoic acid prosthetic group to rotate sequentially between the active sites of each of the enzymes of the complex. (FAD, flavin adenine dinucleotide; NAD+, nicotin- amide adenine dinucleotide.) [ Acetyl-CoA ] [ NADH ] [ ATP ] [ CoA ] [ NAD+ ] [ ADP ] + + + – Dichloroacetate Acetyl-CoA – – Ca2+ PDH kinase Pyruvate NADH + H+ CO2 Mg 2+ ATP ADP – PDH – PDH-a PDH-b (Active dephospho-enzyme) (Inactive phospho-enzyme) P NAD+ CoA Pi H 2O Pyruvate PDH phosphatase + A B + Mg2+, Ca2+ Insulin (in adipose tissue) FIGURE 17–6 Regulation of pyruvate dehydrogenase (PDH). Arrows with wavy shafts indicate allosteric effects. (A) Regulation by end- product inhibition. (B) Regulation by interconversion of active and inactive forms. 170 SECTION IV Metabolism of Carbohydrates (both because of a poor diet and also because alcohol inhibits It can function anaerobically by regenerating oxidized NAD+ thiamin absorption) and may develop potentially fatal pyruvic (required in the glyceraldehyde-3-phosphate dehydrogenase and lactic acidosis. Patients with inherited pyruvate dehydro- reaction), by reducing pyruvate to lactate. genase deficiency, which can be the result of defects in one or Lactate is the end product of glycolysis under anaerobic more of the components of the enzyme complex, also present conditions (eg, in exercising muscle) and in erythrocytes, with lactic acidosis, particularly after a glucose load. Because where there are no mitochondria to permit further oxidation of of the dependence of the brain on glucose as a fuel, these met- pyruvate. abolic defects commonly cause neurologic disturbances. Glycolysis is regulated by glucose transport and by three Inherited aldolase A deficiency and pyruvate kinase defi- enzymes catalyzing nonequilibrium reactions: hexokinase, ciency in erythrocytes cause hemolytic anemia. The exercise phosphofructokinase, and pyruvate kinase. capacity of patients with muscle phosphofructokinase defi- In erythrocytes, the first site in glycolysis for generation ciency is low, particularly if they are on high-carbohydrate of ATP may be bypassed, leading to the formation of diets, which requires robust carbohydrate (pyruvate) oxidation. 2,3-bisphosphoglycerate, which is important in decreasing the affinity of hemoglobin for O2. Pyruvate is oxidized to acetyl-CoA by a multienzyme complex, SUMMARY pyruvate dehydrogenase, which is dependent on the vitamin- Glycolysis is in the cytosolic pathway in all mammalian cells derived cofactor thiamin diphosphate. for the metabolism of glucose (or glycogen) to pyruvate or Conditions that impair pyruvate metabolism frequently lead to lactate. lactic acidosis.