Two carbon atoms leave in the form of CO2 in TCA cycle are derived from ?
Ans. is 'b' i.e., Acetyl CoA TCA cycle consists of sequential reactions. It begins with condensation of four carbon oxaloacetate molecule with 2 carbon acetyl CoA molecule to form six carbon citrate molecule. In subsequent reaction two carbon atoms are lost in the form of CO2 at two different reactions. A series of modifications occur in the remaining four carbon atoms to ultimately form oxaloacetate. Thus the last substrate (oxaloacetate) of one cycle is ready for use as a substrate in the next cycle. In this way, there is no net generation of OAA, or of any of the cycle intermediates. So, two carbon atoms enter the cycle as acetyl CoA (and condense with oxaloacetate) and two carbons leave in the form of two molecules of CO2. Thus, TCA cycle basically involves oxidation of acetyl CoA to CO2 and as such there is no net consumption or regeneration of oxaloacetate or any of the other cycle intermediates.
Oxaloacetate
Acetyl CoA
Succinyl CoA
fumerate