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GATE 2025 BT Q.17: solution and answer

GATE 2025 · BTQ.17 · 1 mark · MCQ

Which one of the following coenzymes is utilised by alanine racemase for the conversion of L-Alanine to D-Alanine?

  1. (A)

    Pyridoxal phosphate

  2. (B)

    Thiamine pyrophosphate

  3. (C)

    Tetrahydrofolate

  4. (D)

    Flavin mononucleotide

See the question in the paper
GATE 2025 BT question 17Tap to open full size

Answer and solution

ANSWER

The coenzyme used by alanine racemase is pyridoxal phosphate, option (A).

Official answer: ASource: Official answer key, GATE 2025 BT (IIT Roorkee)

  1. Alanine racemase interconverts L-alanine and D-alanine. This needs the alpha-hydrogen to be removed and put back on the opposite face, so the reaction happens at the alpha-carbon of an amino acid.

  2. Reactions at the alpha-carbon of amino acids (racemisation, transamination, decarboxylation, beta-elimination) use pyridoxal phosphate (PLP), the active form of vitamin B6.

  3. Mechanism: the amino group of alanine forms a Schiff base (aldimine) with PLP. PLP acts as an electron sink, so the alpha-hydrogen is removed as a proton to give a resonance-stabilised quinonoid intermediate. Adding the proton back on the opposite face gives D-alanine.

  4. (A) Pyridoxal phosphate: correct, for the reasons above.

  5. (B) Thiamine pyrophosphate: carries active aldehyde groups (decarboxylation of alpha-keto acids, transketolase). It does not act on the alpha-carbon of amino acids, so it is wrong.

  6. (C) Tetrahydrofolate: carries one-carbon units (formyl, methylene, methyl), so it is wrong.

  7. (D) Flavin mononucleotide: a redox coenzyme for hydride or electron transfer. Racemisation involves no net oxidation or reduction, so it is wrong.

KEY IDEA

Pyridoxal phosphate (vitamin B6) is the coenzyme for reactions at the alpha-carbon of amino acids. It forms a Schiff base with the substrate and acts as an electron sink, which lets the alpha-proton leave and return on either face. This is how alanine racemase works.

WATCH OUT FOR

Picking a redox coenzyme such as flavin mononucleotide because the question mentions L- and D-forms, or picking thiamine pyrophosphate or tetrahydrofolate. Racemisation involves no net oxidation, and neither of those two acts on amino acid alpha-carbons.

From the GATE 2025 BT paper. Every question of this paper · Download the full paper · All 6 Enzymes questions, with notes

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