Proton NMR study of the heme complex of hemopexin

Abstract

Proton nuclear magnetic resonance spectroscopy of the complex of heme with hemopexin, a plasma protein with an exceptionally high affinity for heme, is reported. Characteristic spectra are shown for heme · hemopexin of cow, human, rabbit and rat. Each of these spectra demonstrate that the iron of heme bound by hemopexin is paramagnetic and low-spin. Rabbit heme · hemopexin, which exhibits the best signal-to-noise ratio, is studied in detail. Deuterium isotope labelling experiments indicate that the methyl in heme positions 1-, 3- and 8- are resolved downfield from the protein envelope of resonances; the 5-methyl may llie in the −5 to +12 ppm region. Two-dimensional nuclear Overhauser effect locates other protons of the heme periphery, including from the 2-vinyl. Strongly relaxed upfield resonances are identified and assigned to protons on the axial ligands. Cyanide interaction with heme · hemopexin produces an additional low-spin adduct.