Question

In one Krebs’ cycle, decarboxylation takes place at _______ steps.
(a) Five
(b) Four
(c) Three
(d) Two

Answer 1

Hint: Decarboxylation is the process of removal of the carboxyl group from a compound with the release of a carbon dioxide molecule. So, the number of steps in which a $C{O}_2$ molecule is released after removal of a carboxyl from a compound is the number of decarboxylation reactions in one Krebs’ cycle.

Complete answer:
Krebs’ cycle is the central pathway for the metabolism of carbohydrates, fats, and proteins. It begins with a citric acid molecule which is a 6- carbon compound and ends at Oxaloacetate which is a 4-carbon compound which indicates the removal of two carboxyl groups.
These carboxyl groups are removed with the release of a $C{O}_2$ molecule in two different steps.
- The first oxidative decarboxylation takes place at the fourth step of the TCA cycle where isocitrate is converted to 5-carbon α-ketoglutarate, with the release of a pair of hydrogen atoms and a molecule of carbon dioxide.
- The second oxidative decarboxylation occurs at the fifth step of the Krebs’ cycle where a molecule of coenzyme-A reacts with the α-ketoglutarate to form a 4-carbon compound succinyl- coenzyme A and releasing carbon dioxide and a pair of hydrogen atoms.

So, the answer is, “Two”.

Note:Decarboxylation is the reaction in which a carboxyl group is removed and it generally refers to the reactions of carboxylic acids. The reverse of this process in which there is an addition of a $C{O}_2$ molecule to a compound is called carboxylation and it is the first chemical step in photosynthesis.