Question

Biochemistry: Heme is a tetrapyrrole, derived from how many molecules of aminolevulinic acid (ALA)? Each ALA is formed by condensation of glycine + succinyl CoA. What vitamin is cofactor of the condensing enzyme? What fragment does glycine contribute to the ALA?

Answer 1

• Pyridoxal phosphate, the active form of vitamin B6 is used as the cofactor for ALA synthase which catalyzes condensation of glycine and succinyl CoA to give aminolevulinic acid (ALA).
• Glycine contribute the amino group to ALA. The enzyme ALA synthase removes carboxyl group of glycine molecule and CoA from succinyl CoA, during condensation reaction. So the fragment contributed by glycine to the ALA is H₂N-CH₂-.
• In heme synthesis two molecules of ALA join to form one molecule of porphobilinogen. Four molecules of porphobilinogen join to form a linear tetrapyrrole called hydroxymethylbilane. This undergo few more steps to form a cyclic tetrapyrrole called heme. So based on this totally eight molecules of ALA will be needed for the formation of first tetrapyrrole intermediate in the heme synthesis.