The citric acid cycle is a closed loop; thelast part of the pathway reforms the molecule used in the first step.
In the first step ofthe citric acid cycle, acetyl CoA joins with a four-carbon molecule, oxaloacetate,releasing the CoA group and forming a six-carbon molecule called citrate.Enzyme citrate synthase helps in proceeding of this step. Citrate furtherconverted to isocitrate with the help of enzyme aconitase. In this step firstthere is removal of water molecule followed by addition of water molecule. isocitrateis oxidized and releases a molecule of carbon dioxide, leaving behind afive-carbon molecule—?-ketoglutarate.
Also NAD+ is reduced to form NADH. The enzyme catalyzing this is isocitrate dehydrogenase, is important in regulating the speed of the citric acidcycle. ?-ketoglutarate that’s oxidized, reducing NAD+ to NADH and releasing a molecule of carbon dioxide in theprocess. The remaining four-carbon molecule picks up Coenzyme A, forming theunstable compound succinyl CoA. The enzyme catalyzing this step, ?-ketoglutarate dehydrogenase, is also importantin regulation of the citric acid cycle. the CoAsuccinyl CoA isreplaced by a phosphate group, which is then transferred to ADP tomake ATP.The four-carbon molecule produced in this step is called succinate.Enzyme succinate thiokinase play important role in this part of the reaction bymaking it easy for succinyl CoA to release CoA.
Succinate is oxidized, forminganother four-carbon molecule called fumarate. In this reaction, two hydrogenatoms—with their electrons—are transferred to FADproducing FADH2.The enzyme succinate dehydrogenase that carries out this step is embedded inthe inner membrane of the mitochondrion, so FADH2 cantransfer its electrons directly into the electron transport chain. Wateris added to the fumarate molecule in the presence of fumrase, converting itinto another four-carbon molecule called malate. In the last step of thecitric acid cycle, oxaloacetate the starting four-carbon compound isregenerated by oxidation of malate with the help of maltase hydrogenase enzyme.Another molecule of NAD+ isreduced to NADHin the process.