Heme-modification studies of myoglobin: II. Ligand binding characteristics of ferric and ferrous myoglobins containing unnatural hemes

Abstract

The oxygenation characteristics have been measured for synthetic myoglobins, which contain meso-, deutero- and hematohemes and protoheme monomethyl and dimethyl esters. The 2,4-modified oxymyoglobins showed a lower α band than β band. The effect of the modification of carboxyl groups on the spectra is very small. The oxygen affinity is increased in the order of deutero-, meso- and protomyoglobins. Hemato- and protoheme monomethyl and diemthyl ester myoglobins showed the same oxygen affinity as native protomyoglobin. The pK value of the acid alkaline transition of metmyoglobin were found to increase in order of meso>deutero>proto>hemato>protoheme monomethyl ester> protoheme dimethyl ester. The dissociation constants of metmyoglobin azide complex was decreased by modifications of carboxyl groups at 6 and 7 positions of the heme ring. On the other hand, the modification of side chains at positions 2 and 4 did not affect the dissociation constant of metmyoglobin azide. It is found that there is the opposite relationship between ferric and ferrous states in the ligand binding properties with hememodifications; the modification of carboxyl groups at 6,7 positions affects the ligation in ferrie state but not ferrous state, while that of 2,4 positions affects the ligation in ferrous state but not in ferric state.