Q.17 Which one of the following coenzymes is utilised by alanine racemase for the
conversion of L-Alanine to D-Alanine?
(A) Pyridoxal phosphate
(B) Thiamine pyrophosphate
(C) Tetrahydrofolate
(D) Flavin mononucleotide
Alanine racemase relies on pyridoxal phosphate to interconvert L-alanine and D-alanine, a vital process in bacterial cell wall synthesis. The correct answer is (A) Pyridoxal phosphate.
Correct Answer
Pyridoxal phosphate (PLP), the active form of vitamin B6, serves as the coenzyme for alanine racemase. This enzyme catalyzes the reversible racemization of L-alanine to D-alanine, essential for peptidoglycan biosynthesis in bacteria. PLP forms a Schiff base with the substrate alanine, facilitating alpha-proton abstraction and stereochemical inversion.
Option Explanations
(A) Pyridoxal Phosphate
Alanine racemase is a PLP-dependent enzyme that positions key residues like Tyr265 and Lys39 to enable proton exchange across the PLP-bound alanine. Structural studies confirm PLP’s role in both forward (L- to D-alanine) and reverse reactions.
(B) Thiamine Pyrophosphate
Thiamine pyrophosphate (TPP) functions in decarboxylation and aldehyde transfer reactions, such as those catalyzed by pyruvate decarboxylase or transketolase. It does not participate in racemization of amino acids like alanine.
(C) Tetrahydrofolate
Tetrahydrofolate (THF) acts as a carrier for one-carbon units in folate-dependent reactions, including thymidylate synthesis and methionine metabolism. It plays no role in alanine racemization.
(D) Flavin Mononucleotide
Flavin mononucleotide (FMN) supports redox reactions as a prosthetic group in enzymes like flavin-dependent oxidases. Racemization by alanine racemase involves no electron transfer, so FMN is not required.
