Tricarboxylic Acid Cycle 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 carbon dioxide (CO2), as shown in Figure 1.

This oxidation provides energy for the production of the majority of ATP in most animals, including humans. Because the TCA cycle occurs totally in mitochondria, it is in close proximity to the electron transport chain ([ETC] ), which oxidizes the reduced coenzymes nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) produced by the cycle. The TCA cycle is an aerobic pathway, because oxygen (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 (from the Greek for “filling up”) reactions.

The TCA cycle also provides intermediates for a number of important anabolic reactions, such as glucose formation from the carbon skeletons of some amino acids and the synthesis of some amino acids  and heme . Therefore, this cycle should not be viewed as a closed system but, instead, as an open one with compounds entering and
leaving as required.

Figure .1 The tricarboxylic acid cycle shown as a part of the essential pathways of energy metabolism.