Bacterial ferrochelatase turns human: Tyr13 determines the apparent metal specificity of Bacillus subtilis ferrochelatase

Ferrochelatase catalyzes the insertion of Fe²⁺ into protoporphyrin IX. The enzymatic product heme (protoheme IX) is a well-known cofactor in a wide range of proteins. The insertion of metal ions other than Fe²⁺ occurs rarely in vivo, but all ferrochelatases that have been studied can insert Zn²⁺ at a good rate in vitro. Co²⁺, but not Cu²⁺, is known to be a good substrate of the mammalian and Saccharomyces cerevisiae ferrochelatases. In contrast, Cu²⁺, but not Co²⁺, has been found to be a good substrate of bacterial Bacillus subtilis ferrochelatase. It is not known how ferrochelatase discriminates between different metal ion substrates. Structural analysis of B. subtilis ferrochelatase has shown that Tyr13 is an indirect ligand of Fe²⁺ and a direct ligand of a copper mesoporphyrin product. A structure-based comparison revealed that Tyr13 aligns with a Met residue in the S. cerevisiae and human ferrochelatases. Tyr13 was changed to Met in the B. subtilis enzyme by site-directed mutagenesis. Enzymatic measurements showed that the modified enzyme inserted Co²⁺ at a higher rate than the wild-type B. subtilis ferrochelatase, but it had lost the ability to use Cu²⁺ as a substrate. Thus, the B. subtilis Tyr13Met ferrochelatase showed the same metal specificity as that of the ferrochelatases from S. cerevisiae and human.