Energy Produced by The TCA  Cycle

Four pairs of electrons are transferred during one turn of the TCA cycle: three pairs reducing three NAD+ to NADH and one pair reducing FAD to FADH2.

Oxidation of one NADH by the ETC leads to formation of three ATP, whereas oxidation of FADH2 produces two ATP . The total yield of ATP from the oxidation of one acetyl CoA is shown in Figure 1. Figure 2summarizes the reactions of the TCA cycle. [Note: The cycle does not involve the net consumption or production of intermediates. Two carbons entering as acetyl CoA are balanced by two CO2 exiting.]

Figure 1: Number of ATP molecules produced from the oxidation of one molecule of acetyl coenzyme A (CoA) using both substrate-level and oxidative phosphorylation. NAD(H) and FAD(H2) = nicotinamide and flavin adenine dinucleotides; GDP and GTP = guanosine di- and triphosphates; Pi = inorganic phosphate.

Figure 2: A. Production of reduced coenzymes, ATP, and carbon dioxide (CO2) in the tricarboxylic acid cycle. [Note: Guanosine triphosphate (GTP) and ATP are interconverted by nucleoside diphosphate kinase.] B. Inhibitors and activators of the cycle.