Orientation of the heme vinyl groups in the hydrogen sulfide-binding hemoglobin I from Lucina pectinata

Abstract

Hemoglobin I (HbI) from the claim Lucina pectinata is a unique heme protein that binds and transfers hydrogen sulfide (H₂S) to a symbiotic bacteria. The metcyano, metaquo, carbon monoxy, oxy, and deoxy complexes of HbI were studied by resonance Raman (RR) spectroscopy, and the metcyano and carbon monoxy complexes were also studied by ¹H-NMR. The results indicate a unique orientation of the heme 2-vinyl group relative to other heme proteins. The RR spectra of the HbICO, metHbICN, metHbIH₂O, HbIO₂ and deoxyHbI heme derivatives show a band at 1621 cm⁻¹ and a shoulder at 1626 cm⁻¹, indicative of an out-of-plane position for one of the vinyls relative to the other one. Spin-lattice relaxation properties of protons in the metHbICN complex also suggest a unique orientation for the heme 2-vinyl group of HbI. The longitudinal relaxation time (T₁) for the 2-H_α, 2-H_βc, and H_βt protons are 120 ms, 115 ms, and 135 ms, respectively. The data from both techniques suggest an out-of-plane and trans-oriented 2-vinyl group, and an in-plane and cis-oriented 4-vinyl group for the low-spin complexes of HbI. These results imply that the electron withdrawing character of the out-of-plane vinyl group contributes to the stability of the heme Fe⁺³ oxidation state, facilitates the binding of the H₂S ligand, and promotes the stability of this ferric H₂S complex. © 1998 John Wiley & Sons, Inc. Biospectroscopy 4: 311–326, 1998