Mechanisms of redox-coupled proton transfer in proteins: role of the proximal proline in reactions of the [3Fe-4S] cluster in Azotobacter vinelandii ferredoxin I.

TitleMechanisms of redox-coupled proton transfer in proteins: role of the proximal proline in reactions of the [3Fe-4S] cluster in Azotobacter vinelandii ferredoxin I.
Publication TypeJournal Article
Year of Publication2003
AuthorsCamba, R, Jung, Y-S, Hunsicker-Wang, LM, Burgess, BK, C Stout, D, Hirst, J, Armstrong, FA
JournalBiochemistry
Volume42
Issue36
Pagination10589-99
Date Published2003 Sep 16
ISSN0006-2960
KeywordsAmino Acid Substitution, Aspartic Acid, Azotobacter vinelandii, Computer Simulation, Electrochemistry, Ferredoxins, Hydrogen Bonding, Hydrogen-Ion Concentration, Iron-Sulfur Proteins, Kinetics, Models, Molecular, Oxidation-Reduction, Proline, Protein Binding, Protons, Thermodynamics
Abstract

The 7Fe ferredoxin from Azotobacter vinelandii (AvFdI) contains a [3Fe-4S](+/0) cluster that binds a single proton in its reduced level. Although the cluster is buried, and therefore inaccessible to solvent, proton transfer from solvent to the cluster is fast. The kinetics and energetics of the coupled electron-proton transfer reaction at the cluster have been analyzed in detail by protein-film voltammetry, to reveal that proton transfer is mediated by the mobile carboxylate of an adjacent surface residue, aspartate-15, the pK of which is sensitive to the charge on the cluster. This paper examines the role of a nearby proline residue, proline-50, in proton transfer and its coupling to electron transfer. In the P50A and P50G mutants, a water molecule has entered the cluster binding region; it is hydrogen bonded to the backbone amide of residue-50 and to the Asp-15 carboxylate, and it is approximately 4 A from the closest sulfur atom of the cluster. Despite the water molecule linking the cluster more directly to the solvent, proton transfer is not accelerated. A detailed analysis reveals that Asp-15 remains a central part of the mechanism. However, the electrostatic coupling between cluster and carboxylate is almost completely quenched, so that cluster reduction no longer induces such a favorable shift in the carboxylate pK, and protonation of the base no longer induces a significant shift in the pK of the cluster. The electrostatic coupling is crucial for maintaining the efficiency of proton transfer both to and from the cluster, over a range of pH values.

DOI10.1021/bi035021v
Alternate JournalBiochemistry
Citation Key10.1021/bi035021v
PubMed ID12962482
Grant ListGM35342 / GM / NIGMS NIH HHS / United States
GM48495 / GM / NIGMS NIH HHS / United States