[Electron transfer in hemoproteins. VIII. Influence of ionic strength on the rate of reduction of ferricytochrome c by oxymyoglobin derivatives, chemically modified at histidine residues].

The influence of chemical modification of His residues in Mb on the rate of redox reaction in system MbO2--Cyt c has been studied at different ionic strengths and pH medium. The products of alkylation of all available His by bromacetate and iodacetamide, CM-Mb and CA-Mb, respectively, and myoglobin, modified by spin label 2,2', 6,6'-tetramethyl-4-bromoacetoxypiperidine-1-oxyl (SL) at His residue A10--Sl (His-A10)--Mb have been studied. It has been shown, that the character of the ionic strength dependence of reaction SL(His-A10)--MbO2 with Cyt c at pH 6.0 ann 7.0 is basically analogues to that, observed for intact protein. It means that only His-GH1 of two His residues, His-A10 and His-GH1, situated in the region of "active contact" of Mg with Cyt c molecule, participates in the interactions, essential for electron transfer. The interaction of the charge of this His with the negatively charged group of Cyt c is necessary, probably for the proper arrangement of other interactions in the active complex, because the deprotonation of His-GHl in the studied pH interval decreases the rate of the process by more than one order of magnitude. The rate of oxidation of MC-MbO2 and CA-MbO2 by ferricytochrome c, in contrast to intact protein, shows a weak dependence on the ionic strength and does not depend on the pH medium, throughout the range of ionic strengths from 0.005 to 1.0. The cause of the radical change in the ionic strength dependence is, probably, nearly entire disturbance of electrostatic interactions in the active complex due to chemical modification of His residues in the site of "active contact", and first of all, the His-CHl residue. The fact, that during alkylation of all available His in Mb the electron transfer persists in the system, points to that in the process of electron transfer to cytochrome c, uncharged group, most probably "inner" His-B5, participates. Based on the data on spatial structure and the obtained results, the positions of the charged groups in the site of "active contact" of Mb with Cyt c molecule are presented.