Oxidation of Other Substrates by the TCA Cycle

The TCA cycle, strictly speaking, has only one input fuel— acetyl-CoA. Catabolism of carbohydrates and fats leads to the production of acetyl-CoA, so that the TCA cycle is ideally suited to serve as the major oxidative sequence in the catabolism of those types of compounds. However, degradation of the amino acids that result from the hydrolysis of protein produces a number of intermediates, among which are a-ketoglutarate, succinyl-CoA, and oxaloacetate (chapter 22). a-Ketoglutarate and succinyl-CoA, can be oxidized 

The TCA cycle, showing some of the branchpoint pathways (in red) that either drain or replenish the TCA intermediates. 

The problem of how to oxidize oxaloacetate is solved by the action of phosphoenolpyruvate carboxykinase, which we discussed in connection with gluconeogenesis (see equa- 

In addition to the amino acids that are converted to intermediates of the TCA cycle, others are converted to pyruvate or acetyl-CoA and thus enter the cycle in the usual way. In fact, all 20 of the protein amino acids are metabo- 

Major regulatory sites of the TCA cycle, with activators ( ) and inhibitors ( ) of specific reactions shown in red. 

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Oxidation of Other Substrates by the TCA Cycle The TCA Cycle Activity Is Regulated at Metabolic Branchpoints... 

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