question_answer 5)

Give the schematic representation of an overall view of Krebs' cycle. 

Answer:

Tricarboxylic Acid Cycle (i)        The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid. (ii)       The reaction is catalysed by the enzyme citrate synthase and a molecule of Co-A is released. (iii)      Citrate is then isomerised to isocitrate.   (iv)     It is followed by two successive steps of decarboxylation, leading to the formation of-xx-ketoglutaric acid and then succinyl Co-A. (v)      During the conversion of succinyl Co-A to succinic acid, a molecule of GTP is synthesized. This is a substrate level phosphorylation. (vi)     In a coupled reaction, GTP is converted to GDP with the simultaneous synthesis of ATP from ADP. (vii)    NAD+ is reduced to NADH+H++ and FAD+ is reduced to FADH2. The continued oxidation of acetic acid via the TCA cycle requires the continued replenishment of oxaloacetic acid, the first member of the cycle. In addition, it also requires regeneration of NAD+ and FAD+ from NADH2 and FADH2 respectively.