ELECTRON TRANSFER PROCESSES IN IRON AND COPPER PROTEINS

California Institute Of Technology, Pasadena CA

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Abstract

DESCRIPTION: Electron-transfer (ET) reactions, the simplest chemical processes, pervade biology. Photosynthesis, respiration, nitrogen fixation, drug metabolism, DNA synthesis, and immune response represent a small subset of the scores of biological processes in which ET reactions play pivotal roles. Any effort to understand the relationship between structure and function in redox enzymes rests on an understanding of long-range ET in proteins. Prior theoretical and experimental work indicates that different protein secondary-structure types mediate long-range ET with markedly different efficiencies. We will perform systematic measurements of the distance and deuterium isotope dependencies of ET rates on beta-sheet (azurin, amicyanin, soluble Cu(a) and alpha-helical (cytochrome b(562) proteins in order to define the long range ET efficiencies and the role of hydrogen bonds in these two structures. Theoretical models indicate that the efficiency of long-range ET depends on the relative energies of the electronic states of the redox sites and the protein matrix (polypeptide). There has never been a clear experimental demonstration of this so-called tunneling-energy dependence, yet the predicted effects can be quite significant. The role of tunneling energy in long-range ET will be evaluated in studies of the distance dependencies of ET rates in several proteins as functions of acceptor reduction potential. Metal complexes frequently serve as redox sites in electron-transfer proteins. Long-range ET depends on the efficiency of coupling these redox sites to the intervening polypeptide. Owing to varied metal-ligand interactions, this coupling can be highly anisotropic. The effects of these disparate ligand couplings will be examined in ET studies of several active-site mutants of the copper protein azurin. Many biological redox reactions are accompanied by significant changes in protein conformation. A common example in metalloproteins is the gain or loss of a ligand at the active site upon change in metal oxidation state. The kinetic consequences of a redox-coupled coordination change will be examined in ET studies of the copper protein amicyanin.