Resonance Raman spectra of ferrochelatase reveal porphyrin distortion upon metal binding

Ferrochelatase catalyzes Fe²⁺ insertion into porphyrins, and is inhibited by Hg²⁺. 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 Hg²⁺. Several additional RR bands are activated, including some arising from [R-active vibrations, establishing loss of an effective symmetry center. Out-of-plane modes appear in the low frequency region. The strongest of these bands, γ₅ and γ₆, 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 Fe²⁺ insertion. This distortion is proposed to result from occupation of a metal-binding site, proximate to the prophyrin, which promotes insertion of Fe²⁺, while occupation by Hg²⁺ is inhibitory.