Why Kreb’s cycle is called a cycle?
The Kreb’s cycle is otherwise called as citric acid cycle or tri-carboxylic acid cycle (TCA cycle). The TCA cycle occurs in cytoplasm and mitochondria of the cell. Kreb’s cycle is a part of cellular respiration. The precursor of TCA cycle is known as citric acid. The citric acid is synthesized from oxaloacetate in the presence of acetyl Co A. The citric acid that is formed undergoes several changes in the presence of enzymes with the formation of many intermediate compounds. Since the citric acid after a number of steps once again is synthesized to carry out the same process. This repeated re-synthesis and breakdown of citric acid was called as citric acid cycle. The citric acid cycle releases carbon-di-oxide that is left into the environment.
Initially, fats are broken down into glycerol and fatty acids. Glycerol gives rise to pyruvic acid which is again converted to acetyl Co A. Acetyl CoA combines with oxaloacetate which is formed from pyruvate to form citric acid. Citric acid is the first compound formed in TCA cycle. Further many changes takes place in this cycle by releasing carbon-di-oxide at two places. The NAD+, GDP and FAD+ are the coenzymes that get reduced in each of the steps in the presence of the relevant enzymes.
The intermediate carbon compounds that are formed in citric acid cycle are cis-aconitic acid, iso-citric acid, alpha-ketoglutaric acid, succinic acid, fumaric acid, malic acid, and oxaloacetic acid. These intermediate compounds are formed with the help of certain corresponding enzymes that catalyze the corresponding steps. The energy compounds like NADH2, GTP and FADH2 are formed at various steps of this cycle. The energy given out is used as power of the cell.
As citric acid cycle keeps on synthesizing the citric acid from acetyl CoA and oxloacetic acid and continues to break down into cis-aconitic acid and other acids, this process is referred as cycle. The oxaloacetate is also formed from pyruvate through another separate process continuously. Glucose that is released into the cell from the diet will be used up for cell respiration here in TCA cycle by getting converted to oxaloacetic acid.