Question

Consider the first reaction of the TCA cycle. What is true about compound A?
$Acetyl \, CoA + OAA + H_{2}O\xrightarrow[Synthase]{Citrare} A + CoA$
A. First product of TCA cycle
B. Tricarboxylic acid and six-carbon compound
C. It undergoes a reorganization in the presence of enzyme aconitase to form cis-aconitase
D. All of these

Answer 1

The release of stored energy through the oxidation of Acetyl CoA aerobically through a series of chemical reactions is called the TCA cycle.

Complete answer:
TCA cycle or tricarboxylic acid cycle or the Kreb cycle or the citric acid cycle- is the second stage of the aerobic cellular respiration by which living organisms obtain the stored energy of the Acetyl CoA enzyme. The Acetyl CoA is formed by the oxidative decarboxylation of the Pyruvate from glycolysis.
The TCA cycle occurs in the- mitochondria of the cells, and it is an eight-step reaction that involves a series of- redox, decarboxylation, hydration, condensation, and dehydration reactions. The products of the TCA cycle are- carbon dioxide, NADH, FADH2, and ATP.
The first step of the TCA cycle is the synthesis of citrate from the Acetyl CoA. The citrate synthase enzyme binds with the oxaloacetate and reacts with the Acetyl CoA in the presence of water- to produce a common citric acid called Citrate. The Coenzyme -A is also released in this process. This reaction is a type of condensation reaction.

Let us analyze the options to find the correct answer.
In the given equation, product A is the 6 carbon compound- Citrate.

Option A: As the condensation of Acetyl CoA- is the first step of the TCA cycle, the product formed is the first product of the TCA cycle. Therefore, this is the correct option.
Option B: Citrate is a tricarboxylic acid as it contains three carboxyl functional groups (-COOH) in its structure. It is a 6 carbon compound formed by the condensation of 2 carbon compounds Acetyl CoA, and a 4 carbon compound oxaloacetate. Therefore, this is the correct option.
Option C: In the presence of the enzyme Aconitase and iron, citrate undergoes isomerization to form- cis- aconitase. This cis-aconitase is further isomerized to form isocitrate.
Therefore, this is the correct option.
Option D: Since all the above options are true. Therefore, All of these are the correct options.

Hence, the correct answer is option (D).

Note: The citrate formed in the first step of the citric acid cycle is isomerized in the presence of the enzyme aconitase to form- isocitrate in the second step of the cycle. It is a two-step process. In the first step- water is removed from the citrate molecule, and the intermediate cis-aconitase is formed. The water molecule is again added to the intermediate compound, and isocitrate is formed as the final product.