Question

Which 5-carbon organic acid of TCA cycle is a key compound in a $N_2$ metabolism?
A) Citric acid
B) Fumaric acid
C) Iso-citrate
D) α keto-glutaric acid.

Answer 1

Hint:TCA cycle is also known as kreb cycle. It is an important process in respiration of aerobic organisms. In this reaction the oxidation of acetyl CoA takes place which is derived from fat, proteins and carbohydrates to produce NADH.

Complete answer:
Let us discuss the given options.
A) Citric acid – It is a 6-C compound which is produced in the first step from Acetyl CoA. It produces iso-citrate in the further step.
B) Fumaric acid- It is a 4-C compound which is produced from succinic acid. It further undergoes hydration to form Malic acid.
C) Iso-citrate- It is a 6-C compound which is produced from citrate by the process of isomerism.
D) α keto-glutaric acid- It is a 5-C compound which is produced from oxalo-succinate. This α keto glutaric acid acts as a nitrogen transporter (amino acid) in the urea cycle.

Thus the correct answer is option (D). α keto-glutaric acid is a 5-carbon organic acid of TCA cycle which is a key compound in a $N_2$ metabolism.

Note:Urea cycle takes place in Liver. It is a major excretory process. Urea (NH2)2CO is produced as a metabolite or the breakdown product of amino acids. The breakdown leads to the formation of ammonium ions. Excess ammonium ions are converted to urea.