The overall goal of this research project is to investigate the role of cytochrome b and the iron-sulfur protein ISP of the cytochrome bc1 complex in the electron transfer and proton releasing reactions which occur at the quinol-oxidizing site in the complex. The importance of an in-depth understanding of mitochondrial bioenergetics has been strengthened by the increasing evidence that defects in the electron transfer chain may contribute to human degenerative diseases and to the aging process. Specifically, we will investigate the mechanism(s) by which proton pumping and electron transfer appear to be uncoupled using yeast cytochrome b mutants which retain significant electron transfer activity but cannot grow on respiratory substrates. Proton pumping and electron transfer activity will be assayed in mitochondria and in bc1 complexes isolated from wild type yeast and from strains containing mutations in amino acids located near or in helix cd of cytochrome b. The fluorescent properties of NCD-4, the analogue of DCCD that binds to aspartate-160 located in helix cd of yeast cytochrome b, will be used to probe possible changes in the cd helix in the mutants in which proton pumping and electron transfer appear to be uncoupled. The suggestion that acidic amino acids localized in extra-membranous, yet hydrophobic, helix cd of cytochrome b are involved in proton pumping will be examined by site-directed mutagenesis of acidic amino acids in helix cd of R sphaeroides. The ISP locate in the cytochrome bc1 complex also participates in the oxidation of ubiquinol. Site directed mutagenesis of amino acids located in extra-membranous regions o the ISP will be. performed to determine whether charged amino acid residues ar required for efficient assembly of the ISP into the bc1 complex and for enzymatic activity.
