Because the citric acid cycle is a source of biosynthetic intermediates, as well as a route for generating metabolic energy, regulation of the cycle is somewhat more complex than if it were solely an energy-generating pathway. The citric acid cycle is regulated in two primary ways, by controlling the entry of fuel into the cycle and by controlling key reactions within the cycle (Figure 14.16).

1. The entry of fuel into the cycle: Substrate level regulation occurs by limiting amounts of acetyl-CoA (see here for regulation of pyruvate oxidation), oxaloacetate, citrate, and NAD⁺/NADH. The latter is the most important and common substrate-level regulation. NAD⁺ is an essential substrate for three enzymes in the cycle as well as the pyruvate dehydrogenase complex.

Conditions which reduce NAD⁺ concentration include lack of oxygen, because the electron transport system, which converts NADH back to NAD⁺ usually depends upon oxygen. Oxygen shortage may occur during heavy exercise when the blood is unable to deliver oxygen as fast as it is needed in muscle cells.

2. Key reactions within the cycle: Movement of materials through the citric acid cycle is also regulated allosterically. Enzymes suspceptible to allosteric regulation include isocitrate dehydrogenase and -ketoglutarate dehydrogenase:

Isocitrate dehydrogenase: Activated by ADP, inhibited by NADH (apart from substrate level regulation of NAD⁺) . Phosphorylation of one serine residue in the enzyme prevents binding of isocitrate.

-Ketoglutarate dehydrogenase: Inhibited by succinyl-CoA and NADH.

To summarize, the citric acid cycle is responsive to the energy state of the cell, through allosteric activation of isocitrate dehydrogenase by ADP; to the redox state of the cell, through flux rate limitation caused when intramitochondrial [NAD⁺] decreases; and to the availability of energy-rich compounds, through inhibition of relevant enzymes by acetyl-CoA or succinyl-CoA.