Distant Electron Transfer in Proteins. The Role of Electrostatic Interactions and Histidine Residues in the Electron Transfer from Myoglobin to Cytochrome C

Abstract

The pH and ionic strength dependence of the rate of redox reaction between the ferro-Mb derivatives and ferri-Cyt C have been studied. Not every collision of the molecules was shown to result in electron transfer but only a contact between their active sites. The ‘active sites’ have been identified. Both are found to lie in planes parallel to the heme plane, being at 1.7 nm distance in Mb and 1.2 nm in Cyt C, so that the hemes in the electron transfer complex must be co-planar and about 2.9 nm apart. Electrostatic interactions between the charged groups in contact sites of Mb and Cyt C play a decisive role in the efficiency of the reaction. From 4 to 5 such groups, including cationic groups of His A10, His GH1 and Arg G19 of Mb, and anionic ones of Glu 66 and 69 of Cyt C, seem to be involved in these interactions. Unlike His A10, His GH1 must participate directly in electron transfer to Cyt C because binding Zn ion to this His inhibits the process. It is also found that some nonionized group of Mb, most likely ‘inner’ His B5 hydrogen-bonded to the surface His GH1, takes part in the electron transfer.