Resonance Raman Spectra of Ferrochelatase Reveal Porphyrin Distortion upon Metal Binding

Ferrochelatase catalyzes Fe 2+ insertion into porphyrins, and is inhibited by Hg 2+ . Resonance Raman spectra of mesoporphyrin IX show that binding to ferrochelatase restricts the conformation of the propionate side chains, but does not perturb the ring conformation. However, a pronounced perturbation is seen in the ternary complex with Hg 2+ . Several additional RR bands are activated, including some arising from IR-active vibrations, establishing loss of an effective symmetry center. Out-of-plane modes appear in the low frequency region. The strongest of these bands, γ 5 and γ 6, correspond to pyrrole tilting vibrations, which are in the same symmetry class as a doming distortion of the porphyrin. All four pyrrole N atoms are pointing toward the same side of the porphyrin plane, a geometry expected to facilitate Fe 2+ insertion. This distortion is proposed to result from occupation of a metal-binding site, proximate to the prophyrin, which promotes insertion of Fe 2+, while occupation by Hg 2+ is inhibitory.