PROTON NMR-STUDY OF THE HEME COMPLEX OF HEMOPEXIN

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 labeling experiments indicate that the methyls in heme positions 1-, 3-, and 8- are resolved downfield from the protein envelope of resonances; the 5-methyl may lie in the -5 to +12 ppm region. Two-dimensional nuclear Overhauser effect spectroscopy 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.