RESONANCE RAMAN-SPECTRA OF FERROCHELATASE REVEAL PORPHYRIN DISTORTIONUPON METAL-BINDING

Ferrochelatase catalyzes Fe2+ insertion into porphyrins, and is inhibi ted by Hg2+. Resonance Raman spectra of mesoporphyrin IX show that bin ding to ferrochelatase restricts the conformation of the propionate si dechains, but does not perturb the ring conformation. However, a prono unced perturbation is seen in the ternary complex with Hg2+. Several a dditional RR bands are activated, including some arising from IR-activ e vibrations, establishing loss of an effective symmetry Center. Out-o f-plane modes appear in the low frequency region. The strongest of the se bands, gamma(5) and gamma(6), correspond to pyrrole tilting vibrati ons, which are in the same symmetry class as a doming distortion of th e porphyrin. All four pyrrole N atoms are pointing toward the same sid e of the porphyrin plane, a geometry expected to facilitate Fe2+ inser tion. This distortion is proposed to result from occupation of a metal -binding site, proximate to the porphyrin which promotes insertion of Fe2+, while occupation by Hg2+ is inhibitory.