Proximal ligand tunes active site structure and reactivity in bacterial L. monocytogenes coproheme ferrochelatase

Ferrochelatases catalyze the insertion of ferrous iron into the porphyrin during the heme b biosynthesis pathway, which is fundamental for both prokaryotes and eukaryotes. Interestingly, in the active site of ferrochelatases, the proximal ligand coordinating the porphyrin iron of the product is not conserved, and its catalytic role is still unclear. Here we compare the L. monocytogenes bacterial coproporphyrin ferrochelatase native enzyme together with selected variants, where the proximal Tyr residue was replaced by a His (i.e. the most common ligand in heme proteins), a Met or a Phe (as in human and actinobacterial ferrochelatases, respectively), in their Fe(III), Fe(II) and Fe(II)-CO adduct forms. The study of the active site structure and the activity of the proteins in solution has been performed by UV-vis electronic absorption and resonance Raman spectroscopies, biochemical characterization, and classical MD simulations. All the mutations alter the H-bond interactions between the iron porphyrin propionate groups and the protein, and induce effects on the activity, depending on the polarity of the proximal ligand. The overall results confirm that the weak or non-existing coordination of the porphyrin iron by the proximal residue is essential for the binding of the substrate and the release of the final product.