Question

Why is the citric acid cycle considered to be a “central metabolic hub” of the cell? Provide an
explanation with specific examples, including control and regulatory aspects of the citric acid
cycle. What happens when there are "defects" in the citric acid cycle? Provide examples

Answer 1

The citric acid cycle is considered as the" central metabolic hub of the cell" it is so because it is the final common pathway for the oxidation of fuel molecules usch as amino acids fatty acids carbohydrates.The reactions of citric acid cycle takes place in the mitochondria .The citric acid cycle whuch is also called krebs cycle TCA cycleincludes aseries of oxidation-reduction reactions happening in mitochondria that result in oxidation of acetyl group to 2 molecules of Carbon dioxide and reduce the coenzymes that that are reoxidized through the electron transport chain.

Here a 4 carbon compund condenses with a 2 carbon acetylunit to yield a six carbon tricarboxylic acid ie Citrate.An isomer of citrate is then oxidatively decarboxylated giving rise to a 5 carbon compound ie Alpha ketoglutarate which i turn is again oxidatively decarboxylated to yield a 4 carbon Succinate Oxaloacetate is then regenerated from succinate.Thus here 2 carbon atoms enter thecycle as an acetyl unit and 2 carbon atoms leave the cycle in the form of 2 molecules of carbon dioxide.Thus the overall function of citric acid cycle os to harvest high enrgy electrons from carbon fuels.

The regulation of TCA cycle is based upon the netry of substrates into the cycle as well as the key reactions of the cycle.Fuel enters the TCA cycle primarily as A cetyl co a which is generated from carbohydrates which in turn is the major control point.Regulations also occur at the level of enzymes involved in TCA cycle like citrate synthase,isocitrate dehydrogenase.Thus excess ATP depicts energy rich state of the cell tuhus TCA cycle is inhibbited at such states.

When there are defect in Citric acid cycle it may lead dirupt oxidation of 2 carbon compounsds also may lead to disruption in interconversion of metabolites arising from transamination and deamination of amino acids,Citric acid cycle also plays an important role in synthesis of non essential amino acids ,fatty acids,heme synthesis,purine and pyrimidine synthesis.In turn these meachanisms will also be affected .