Glycolysis dehydrogenase-catalyzed reactions

Pyruvate dehydrogenase Catalyzes reactions connecting glycolysis to the Krebs cycle... 

Generally, NAD-linked dehydrogenases catalyze ox-idoreduction reactions in the oxidative pathways of metabolism, particularly in glycolysis, in the citric acid cycle, and in the respiratory chain of mitochondria. NADP-linked dehydrogenases are found characteristically in reductive syntheses, as in the extramitochon-drial pathway of fatty acid synthesis and steroid synthesis—and also in the pentose phosphate pathway. 

Glycolysis requires NAD+ for reaction 6 in which glyceraldehyde-3-phosphate dehydrogenase catalyzes the oxidation of glyceraldehyde-3-phosphate. NAD" is reduced. Two ATP per glucose... 

Under anaerobic conditions, NADH produced in glycolysis builds up. This results in a reduction in the amount of NAD+ available to support continuation of glycolysis. Organisms have two pathways for regenerating NAD+ under anaerobic conditions. Animal cells and lactic acid bacteria use the process of lactic acid fermentation. Yeast convert pyruvate to acetaldehyde in a reaction catalyzed by the enzyme pyruvate decarboxylase. This is followed by reduction of acetaldehyde to ethanol catalyzed by alcohol dehydrogenase. The reaction uses NADH and releases NAD+, which is subsequently used in glycolysis. 

Fig. 22.9. Lactate dehydrogenase reaction. Pyruvate, which may be produced by glycolysis, is reduced to lactate. The reaction, which occurs in the cytosol, requires NADH and is catalyzed by lactate dehydrogenase. This reaction is readily reversible.

EXAMPLE 13.5 Pyruvate dehydrogenase catalyzes an irreversible step in carbohydrate oxidation. In glycolysis, prior to the conversion of pyruvate to acetyl-CoA, the intermediates can all be converted back into glucose, provided that the three kinase reactions of glycolysis are bypassed this is achieved by the special enzymes of gluconeogenesis. In starvation, the conservation of glucose through the inhibition of the oxidation of pyruvate is important for survival. 

Initial data in the study of isozymes were received in the investigation of the isozymes of lactate dehydrogenase (LDH) whose genetic control had been sufficiently studied. This enzyme plays and important role in glycolysis, and catalyzes the reversible reaction. 

We know that anaerobic glycolysis of glncose yields pyruvate and/or lactate, interconvertable metabolites. Pyruvate is converted into acetyl-SCoAin the following reaction, catalyzed by the pyruvate dehydrogenase complex ... 

Thus far we have discussed the two most important enzymes regulating the supplies of acetyl-CoA and oxaloacetate for the TCA cycle. This leaves us with the three enzymes, all within the cycle, that regulate the activity of the cycle. The first of these, citrate synthase, catalyzes the formation of citrate from acetyl-CoA and oxaloacetate. Regulation, by energy charge and other parameters, of the rate of glycolysis and of the pyruvate dehydrogenase reaction play... 

Problem 6.2 illustrates the use of equation 6.2-1 by applying it to four net reactions that represent the oxidation of glucose to carbon dioxide and water (1) the net reaction for glycolysis, (2) the net reaction catalyzed by the pyruvate dehydrogenase complex, (3) the net reaction for the citric acid cycle, and (4) the net reaction for oxidative phosphorylation. The v in equation 6.2-1 is the apparent stoichiometric number matrix for these four reactions. The net reaction is... 

The connection between TCA and glycolysis is made through pyruvate. The pyruvate produced in glycolysis is transported from the cytosol across the mitochondrial membranes to the inner mitochondrial matrix. There, it reacts with coenzyme A (CoA), forming CO2 and the intermediate acetyl-CoA. This reaction, catalyzed by pyruvate dehydrogenase, is highly exergonic and essentially irreversible. 

No, because an important omission is inorganic phosphate. Even if phosphate were added to the incubation mixture, pyruvate would be produced only in an amount equivalent to that of the NAD+ present. Glycolysis requires NAD+ for Step 6, the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. 

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