Non-essential amino acid carbon skeletons (for example aspartate and asparagine) are made from oxaloacetate. Furthermore, alpha-ketoacids formed from intermediates obtain their amino groups from glutamate in a transamination reaction. This results in glutamate being converted into alpha-ketoglutarate. Alpha- ketogluterate can be converted back into glutamine, proline and arginine. Aspartate and glutamine combined with nitrogen and carbon (from other sources) can be used to form the purine bases used by DNA, RNA, ATP, AMP, GTP, NAD, FAD, and CoA. Pyrimidines can be partly formed by aspartate (from oxaloacetate). Porphyrin carbon atoms mainly come from succinyl-CoA. These carbon skeletons are significant constituents of hemoproteins, like myoglobin, haemoglobin and a variety of cytochromes.
The cycle will be affected when conditions are changed and so it must be regulated. Regulation is largely determined by product inhibition and substrate availability. Regulation is also required to ensure that there is no overproduction of reduced coenzymes and so metabolic energy is not wasted. Defects in the citric acid cycle can contribute to the development of cancer. ADP is a substrate used to be converted into ATP, NADH will accumulate if ADP sources are in limitation. NADH build up inhibits various enzymes. Most dehydrogenation reactions in the citric acid cycle result in NADH. NADH inhibits pyruvate dehydrogenase, isocitrate dehydrogenase, alpha-ketogluarate dehydrogenase and citrate synthase. Acetyl-CoA inhibits pyruvate dehydrogenase and succinyl-CoA inhibits alpha-ketoglutarate dehydrgoenase and citrate synthase. ATP and NADH are the negative principal regulators of the cycle whilst the need for energy and carbon skeleton synthesis is the positive regulator.
ATP as a product of the anabolic pathway participates in end product inhibition. ATP inhibits the action of phosphofructokinase (an enzyme used in glycolysis). ATP levels do not change more than 10% (tested experimentally) between rest and vigorous exercise. Citrate also inhibits phosphofructokinase (feedback inhibition) preventing a constantly high rate of flow in the cycle, so citrate does not accumulate and so there is a decrease in substrate for the enzymes. ATP also inhibits citrate synthase and alpha-ketoglutarate dehydrogenase.
Hypoxia-inducible factors (HIF) participate in regulating oxygen homeostasis and are transcription factors that target angiogenesis, vascular remodelling, iron transport, apotheosis and glucose utilisation.There is a link between intermediates and the regulation of HIF. Prolyl 4- hydroxylases catalyse the reaction which allows HIF to be targeted for degradation. HIF instability has been found as fumerate and succinate have been identified as inhibitors of prolyl hydroxylases.
To synthesise Acetyl CoA from pyruvate, pyruvate dehydrogenase is needed. The pyruvate dehydrogenase complex must be regulated. The figures below demonstrate how products of the citric acid cycle regulate the enzyme complex.
Changes in the levels of calcium regulates the citric acid cycle. Calcium accumulation can activate pyruvate dehydrogenase phosphatase which then activates the pyruvate dehydrogenase complex. During exercise, ADP and pyruvate concentrations will rise as muscle contraction uses ATP, and glucose is converted into pyruvate to fulfil respiratory demands. ADP and pyruvate activate dehydrogenase by inhibition of kinases. Calcium levels rise through muscle contraction and so phosphatases are stimulated, enhancing dehydrogenase activity. This in turn means that more acetyl-CoA can be produced to enter the citric acid cycle for respiration or to produce carbon skeletons.