The first and rate-limiting reaction of heme biosynthesis is catalysed by delta-aminolevulinic acid synthase (ALAS). Which vitamin-derived cofactor does this enzyme require?
- A Thiamine pyrophosphate
- B Tetrahydrobiopterin
- C Pyridoxal phosphate ✓
- D Methylcobalamin
Correct answer: C. Pyridoxal phosphate
Explanation
ALA synthase is a PLP-dependent enzyme: pyridoxal phosphate (vitamin C6) forms a Schiff base with glycine during the condensation reaction. This explains why isoniazid, which antagonises C6, can produce sideroblastic changes with ringed sideroblasts. Thiamine pyrophosphate serves oxidative decarboxylation reactions such as pyruvate dehydrogenase, not ALAS, and tetrahydrobiopterin functions in aromatic amino acid hydroxylations.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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