Question

Which one of the following coenzymes is required by the enzymes involved in the catalysis of transamination reactions?

• A. Lipoic acid
• B. Thiamine pyrophosphate
• C. Coenzyme Q10
• D. Pyridoxal phosphate

Hint:

Associate Transamination with Vitamin $B_6$ (Pyridoxine).
The "Aldehyde" group of PLP is the business end of the molecule that "traps" the nitrogen.
If you see "Schiff Base" or "Amino group transfer" in a question, the answer is almost always PLP.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
Transamination is a vital biochemical process in amino acid metabolism where an amino group ($-NH_2$) is transferred from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid.
This reaction is catalyzed by enzymes known as aminotransferases or transaminases, which require a specific derivative of Vitamin $B_6$ to function.

Step 2: Detailed Explanation:

• Role of Pyridoxal Phosphate (PLP): Pyridoxal phosphate, the active form of Vitamin $B_6$, serves as the essential prosthetic group for all aminotransferases.

• Chemical Mechanism: The catalysis involves the aldehyde group of the PLP molecule. In the resting state of the enzyme, PLP is covalently bound to the amino group of a specific Lysine residue in the enzyme's active site via an "internal Schiff base" (aldimine) linkage.

• The Ping-Pong Bi-Bi Mechanism: When a substrate amino acid enters the active site, its amino group replaces the Lysine amino group to form an "external Schiff base." The enzyme then facilitates the removal of the $\alpha$-proton and the subsequent hydrolysis of the bond, releasing a keto acid and leaving the amino group attached to the cofactor, now called pyridoxamine phosphate (PMP).

• Completion of the Cycle: In the second half of the reaction, a different keto acid enters the active site, reacts with the PMP to pick up the amino group, and is released as a new amino acid, regenerating the original PLP-enzyme complex.

• Metabolic Importance: Transamination reactions are crucial for the synthesis of non-essential amino acids and the first step in the catabolism of most amino acids, funneling nitrogen toward the urea cycle. Common examples include Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST).

• Analysis of Other Options:
- Lipoic acid is a cofactor for multi-enzyme complexes involved in oxidative decarboxylation (e.g., Pyruvate Dehydrogenase).
- Thiamine pyrophosphate (TPP) is derived from Vitamin $B_1$ and is involved in cleavage of bonds adjacent to carbonyl groups, such as decarboxylation of $\alpha$-keto acids.
- Coenzyme Q10 (Ubiquinone) is a lipid-soluble mobile electron carrier in the mitochondrial electron transport chain, essential for ATP production.

Step 3: Final Answer:
Pyridoxal phosphate (PLP) is the universal coenzyme for transamination reactions because of its unique ability to stabilize carbanion intermediates through its conjugated ring system.