Cytochromes serve as electron carriers in a wide variety of biochemical processes including respiration, photosynthesis, nitrite reduction and surface reduction. Because cytochromes have a key role in biological processes, it is important to understand the factors which control both the rates and specificities of their electron transfer. We will address two questions in this area: How does the protein control its redox potential (and, thus, the driving force for electron transfer) and, what are the role of electrostatic interactions in determining the rate of electron transfer? Four factors have been proposed to control the redox potential of the heme, but have not yet been tested in detail: hydrogen bonding or deprotonation of the axial histidine NH proton, hydrogen bonding or deprotonation of one of the propionic acid side chains, changes in the axial ligand-heme bond angles and distances, and the distribution of charged groups around the heme. We propose to synthesize chelated model hemes to test each of these aspects of heme chemistry. In addition, we propose a series of experiments to delineate some of the factors which control the NMR spectra of cytochromes, and heme proteins in general. Much structural information has been obtained from NMR. A clearer understanding of the factors which affect heme protein NMR will make this an even more powerful tool. Electrostatic interactions may be a major determinant of the rate of electron transfer in heme proteins. If so, electron self-exchange rate constants (cyta+3+cytb+3=cyta+2+cytb+3) will be a function of the distribution of charges on the surface of the heme. This distribution can be altered by functionalizing a cytochrome with a wide variety of reagents. We will measure the rate of electron self-exchange of cytochromes functionalized at specific amino acid residues. Our long term goal in both the model and the protein experiments is to elucidate the factors which govern heme protein biochemistry and spectroscopy.
