CONTROL OF MYOGLOBIN ELECTRON-TRANSFER RATES BY THE DISTAL (NONBOUND)HISTIDINE RESIDUE

Changing the distal histidine (H64) of sperm-whale myoglobin into any of the following residues-valine, leucine, methionine, glycine, or phe nylalanine-causes a dramatic improvement in the reversibility (electro n-transfer kinetics) and reproducibility of the direct electrochemistr y. Cyclic voltammograms of native or wild-type recombinant myoglobin a re irreversible (in the electrochemical sense) and critically dependen t on the condition of the sample and electrode. By contrast, the H64 m utants display quasi-reversible electrochemistry much more typical of results obtained with true electron-transfer proteins. The difference in activity correlates sharply with alterations to the distal-pocket H -bonding network, which in the native protein comprises the H2O that i s coordinated to Fe(III), the N-epsilon of H64, and the ''lattice'' ex tending from arginine-45 to the heme periphery. It is proposed that th is H-bond network increases the electron-transfer activation energy by coupling the displacement of Fe(lII)-coordinated H2O to higher reorga nization requirements, including that of solvent H2O molecules near th e heme periphery. The poor reproducibility and extreme sensitivity of the electrochemical response to experimental conditions is rationalize d by the microscopic model for protein electrochemistry which predicts that the waveshape and potential positions for inherently irreversibl e (sluggish) systems will be critically dependent on the state of the electrode surface.