Overview of Tricarboxylic Acid Cycle and Pyruvate Dehydrogenase Complex

Tricarboxylic Acid Cycle and Pyruvate Dehydrogenase Complex

OVERVIEW

The tricarboxylic acid cycle ([TCA cycle] also called the citric acid cycle or the Krebs cycle) plays several roles in metabolism. It is the final pathway where the oxidative catabolism of carbohydrates, amino acids, and fatty acids converge, their carbon skeletons being converted to CO₂ (Figure 9.1). This oxidation provides energy for the production of the majority of adenosine triphosphate (ATP) in most animals, including humans. The TCA cycle occurs totally in the mitochondria and is, therefore, in close proximity to the reactions of electron transport, which oxidize the reduced coenzymes (NADH and FADH₂) produced by the cycle. The TCA cycle is an aerobic pathway, because O₂ is required as the final electron acceptor. Reactions such as the catabolism of some amino acids generate intermediates of the cycle and are called anaplerotic (“filling up”) reactions. The TCA cycle also supplies intermediates for a number of important synthetic reactions. For example, the cycle functions in the formation of glucose from the carbon skeletons of some amino acids, and it provides building blocks for the synthesis of some amino acids and heme. Therefore, this cycle should not be viewed as a closed circle but, instead, as a traffic circle with compounds entering and leaving as required.

Figure 9.1 The tricarboxylic acid cycle shown as a part of the essential pathways of energy metabolism. (See Figure 8.2, for a more detailed view of the metabolic map.) CoA = coenzyme A.