 The citric acid cycle is regulated mainly by the availability of key substrates , particularly the ratio of NAD + to NADH and the concentrations of calcium , inorganic phosphate , ATP , ADP , and AMP . <unk> – the molecule that gives its name to the cycle – is a feedback inhibitor of citrate synthase and also inhibits PFK , providing a direct link between the regulation of the citric acid cycle and glycolysis . 
