The Structure of the Heme Crevice of Ferric Cytochrome c Alkylated at Methionine-80

Abstract

Abstract The kinetics of a heme-linked ionization of pK 5.5 in ferric carboxymethyl- and carboxamidomethyl-cytochrome c (alkcyt c) were followed by a series of pH jump experiments in a stopped flow apparatus. The results were consistent with a conformation change following deprotonation of a histidine residue. It is suggested that the heme-linked ionization leading to the formation of a low spin hemoprotein belongs to histidine-18, coordinating the heme iron of alk-cyt c. The extent of absorbance changes following pH jump from pH 4.0 decreased above pH 6.0. This finding was attributed to the coordination of a sixth ligand which increases the rate of the conformational change beyond the capabilities of the stopped-flow technique. A pK of 7.25 was estimated for this event. A similar pK was calculated from the pH dependence of the association constants of alk-cyt c with imidazole.