title: Fold versus sequence effects on the driving force for protein-mediated electron transfer.
abstract: Electron transport chains composed of electron transfer reactions mainly between proteins provide fast efficient flow of energy in a variety of metabolic pathways. Reduction potentials are essential characteristics of the proteins because they determine the driving forces for the electron transfers. As both polar and charged groups from the backbone and side chains define the electrostatic environment, both the fold and the sequence will contribute. However, although the role of a specific sequence may be determined by experimental mutagenesis studies of reduction potentials, understanding the role of the fold by experiment is much more difficult. Here, continuum electrostatics and density functional theory calculations are used to analyze reduction potentials in [4Fe-4S] proteins. A key feature is that multiple homologous proteins in three different folds are compared: six high potential iron-sulfur proteins, four bacterial ferredoxins, and four nitrogenase iron proteins. Calculated absolute reduction potentials are shown to be in quantitative agreement with electrochemical reduction potentials. Calculations further demonstrate that the contribution of the backbone is larger than that of the side chains and is consistent for homologous proteins but differs for nonhomologous proteins, indicating that the fold is the major protein factor determining the reduction potential, whereas the specific amino acid sequence tunes the reduction potential for a given fold. Moreover, the fold contribution is determined mainly by the proximity of the redox site to the protein surface and the orientation of the dipoles of backbone near the redox site.
authors.affiliation: Department of Chemistry, Georgetown University, Box 571227, Washington, District of Columbia 20057-1227, USA.
authors.forename: Bradley Scott
authors.initials: BS
authors.lastname: Perrin
authors.forename: Toshiko
authors.initials: T
authors.lastname: Ichiye
mesh_terms: D000465:Algorithms; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics; D018360:Crystallography, X-Ray; D004579:Electron Transport; D005288:Ferredoxins / Q000737:chemistry / Q000235:genetics; D007506:Iron-Sulfur Proteins / Q000737:chemistry / Q000235:genetics; D008958:Models, Molecular; D009154:Mutation; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry / Q000235:genetics; D011487:Protein Conformation; D017510:Protein Folding; D011506:Proteins / Q000737:chemistry* / Q000235:genetics; D055672:Static Electricity; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D005288:Ferredoxins; D007506:Iron-Sulfur Proteins; D011506:Proteins; D010088:Oxidoreductases; C029130:nitrogenase reductase