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chapter 13

Carbohydrate Metabolism I: Glycolysis and TCA Cycle

H

S-Acetylhydrolipoyl-enzym e

FIGURE 13-8

Steps involved in the decarboxylation and formation of the S-acetylhydrolipoyl enzyme of the dehydrogenase complex.

TPP = Thiamine pyrophosphate.

The structure of FAD that contains the vitamin riboflavin

is given in Figure 13-9. In the last step, catalyzed by di-

hydrolipoyl dehydrogenase, the hydrogens (or reducing

equivalents) are transferred to NAD+, with the formation

of NADH and the oxidized flavoprotein.

E

3

-FADH

2

+ NAD+ ^ E3-FAD + NADH + H+

Thus, the flow of reducing equivalents in the pyruvate

dehydrogenase complex is from pyruvate to lipoyl to

FAD to NAD+. Conversion of pyruvate to acetyl-CoA

requires four vitamins: thiamine, pantothenic acid, ri-

boflavin, and niacin. In contrast, in glycolysis, niacin

is the only vitamin used. NADH generated in this re-

action is oxidized to NAD+ in the electron transport