Ketoglutarate Dehydrogenase Is Negatively Regulated by NADH

This chapter is mainly concerned with the contribution of the tricarboxylic acid cycle to carbohydrate metabolism. The TCA cycle is the main source of electrons for oxidative phosphorylation, and thereby the major energetic sequence in the metabolism of aerobic cells or organisms. It serves as the main distribution center of metabo- 

The TCA cycle begins with acetyl-CoA, which is obtained either by oxidative decarboxylation of pyruvate available from glycolysis or by oxidative cleavage of fatty acids. 

The acetyl-CoA transfers its acetyl group to oxaloace-tate, thereby generating citrate. In a cyclic series of reactions, the citrate is subjected to two successive decarboxylations and four oxidative events, leaving a four-carbon compound malate from which the starting oxaloacetate is regenerated. 

Only a single ATP is directly generated by a turn of the TCA cycle. Most of the energy produced by the cycle is stored in the form of reduced coenzyme molecules, NADH and FADH2. Reoxidation of these compounds (see chapter 14) liberates a large amount of free energy, which is captured in the form of ATP. 

Some of the main biosynthetic pathways begin with intermediates in the TCA cycle. When intermediates in the cycle are used as starting materials for biosynthesis, they must be replenished to keep the cycle operating. When carbohydrates are being metabolized, TCA cycle intermediates are replenished by production of oxaloacetate from pyruvate. 

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