THE PH-DEPENDENCE OF INTRAMOLECULAR ELECTRON-TRANSFER IN AZURINS

Long range electron transfer (LRET) can be induced between the disulfi de radical anion, produced pulse radiolytically, and the copper(II) ce nter in the single blue copper protein, azurin. The rate constant of t his intramolecular process increases by one order of magnitude upon de creasing the pH from 8 to 4 in all azurins (wild types as well as sing le site mutants of Pseudomonas aeruginosa azurin) studied so far. In o rder to pursue the structural basis for the observed pH dependence we have extended our studies to a new mutant, Asp23Ala. In this derivativ e the aspartate residue is proximal to the electron donating cystine d isulfide bridge. However, LRET in this azurin was found to exhibit sim ilar pH dependence as all other wild type and single-site mutants with residues potentially being able to affect the electron transfer proce ss. A detailed consideration of the parameters that determine the effi ciency of this process leads to the suggestion that a pH induced chang e either in the electron transfer distance or in the electronic coupli ng may be the cause of this behavior.