Question

censider the first reaction of ICA cycle. $$ \text { Acety } \mathrm{CoA}+\mathrm{OAA}+\mathrm{H}_{2} \mathrm{O} \underset{\text { synthase }}{\text { Citrate }}{\text { synthase }}+\mathrm{CoA} $$ What is true about compound \(A\) ? (a) First product of TCA cycle (10) Tricarboxylic acid and six carbon compound (d) It undergoes reorganisation in the presence of enzyme aconitase to form cis-aconitate (d) \(\mathrm{Al}\) of these

Short answer

Compound A is Citrate, and all the given statements (a), (b), and (c) are correct, so the answer is (d) All of these.

Step-by-step solution

Identify Compound A

In the given reaction, the compound formed by the reaction between Acetyl CoA, OAA (Oxaloacetate), and water, catalyzed by the enzyme Citrate Synthase, is Citrate. Therefore, compound A must be Citrate.

Examine the Options

Analyze each of the given options in relation to Citrate. (a) As Citrate is the first product formed in the TCA cycle, this statement is true. (b) Citrate is a tricarboxylic acid because it has three carboxyl groups and it is a six carbon compound, so this statement is also true. (c) Citrate undergoes isomerization in the presence of the enzyme aconitase to form cis-aconitate, which is an intermediate step in the TCA cycle, making this statement true as well.

Select the Correct Answer

Since all the given statements about Citrate (compound A) provided in options (a), (b), and (c) are correct, the answer must be (d) All of these.

Key concepts

Citrate Synthase

Citrate Synthase is an important enzyme in the TCA cycle that catalyzes the condensation of acetyl-CoA and oxaloacetate to form citrate. This reaction is significant as it marks the entry of the two-carbon unit from acetyl-CoA into the cycle, which will undergo a series of transformations to produce energy.

In essence, Citrate Synthase kickstarts the energy-generating spiral of the TCA cycle, making it the pace-setting step. Its activity is tightly regulated by feedback inhibition, where high concentrations of ATP or NADH, indicating ample energy supply, inhibit the enzyme, preventing excessive energy production.

Oxaloacetate

Oxaloacetate (OAA) plays a dual role in the TCA cycle. On one hand, it is the four-carbon molecule that combines with acetyl-CoA to form citrate, thus initiating the cycle. On the other hand, it is also the final product of the TCA cycle, regenerated after a series of biochemical reactions.

As a crucial metabolic hub, OAA is not only involved in the TCA cycle but also interfaces with other pathways such as gluconeogenesis and amino acid synthesis. The availability of OAA is critical for the continuation of the TCA cycle; without it, acetyl-CoA cannot enter the cycle, leading to a halt in energy production.

Aconitase

Aconitase is an enzyme that mediates the second step in the TCA cycle. Once citrate is formed by citrate synthase, aconitase facilitates its transformation into isocitrate through the intermediate compound cis-aconitate.

While this might seem like a simple rearrangement, the isomerization of citrate to isocitrate is crucial for the following steps in the cycle. Isocitrate can then be further processed to produce energy-rich molecules, while citrate cannot. Interestingly, aconitase also plays a role as an iron-sulfur protein, which is integral to its structural and functional properties.

Tricarboxylic Acid

The Tricarboxylic Acid (TCA) cycle, also known as the Krebs cycle or citric acid cycle, is a cornerstone of cellular metabolism. It takes place in the mitochondria of cells and is foundational for the production of ATP, NADH, and FADH₂, which are essential for energy transfer within a cell.

This cycle involves eight sequential enzyme-mediated reactions that convert acetyl-CoA into carbon dioxide and water, with the concomitant generation of energy. Every turn of the TCA cycle not only produces chemical energy but also generates precursors for various other biosynthetic pathways, highlighting its central role in metabolism.