title: Methionine ligand lability in bacterial monoheme cytochromes c: an electrochemical study.
abstract: The direct electrochemical analysis of adsorbed redox active proteins has proven to be a powerful technique in biophysical chemistry, frequently making use of the electrode material pyrolytic "edge-plane" graphite. However, many heme-bearing proteins such as cytochromes c have been also examined systematically at alkanethiol-modified gold surfaces, and previously we reported the characterization of the redox properties of a series of bacterial cytochromes c in a side-by-side comparison of carbon and gold electrode materials. In our prior findings, we reported an unanticipated, low potential (E(m) ∼ -100 mV vs SHE) redox couple that could be analogously observed when a variety of monoheme cytochromes c are adsorbed onto carbon-based electrodes. Here we demonstrate that our prior phenomological data can be understood quantitatively in the loss of the methionine ligand of the heme iron, using the cytochrome c from  Hydrogenbacter thermophilum  as a model system. Through the comparison of wild-type protein with M61H and M61A mutants, in direct electrochemical analyses conducted as a function of temperature and exogenous ligand concentration, we are able to show that Met-ligated cytochromes c have a propensity to lose their Met ligand at graphite surfaces, and that energetics of this process (6.3 ± 0.2 kJ/mol) is similar to the energies associated with "foldons" of known protein folding pathways.
authors.affiliation: Department of Chemistry, Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215, United States.
authors.forename: Benjamin D
authors.initials: BD
authors.lastname: Levin
authors.forename: Mehmet
authors.initials: M
authors.lastname: Can
authors.forename: Sarah E J
authors.initials: SE
authors.lastname: Bowman
authors.forename: Kara L
authors.initials: KL
authors.lastname: Bren
authors.forename: Sean J
authors.initials: SJ
authors.lastname: Elliott
mesh_terms: D001419:Bacteria / Q000378:metabolism*; D045304:Cytochromes c / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D055664:Electrochemical Techniques; D004566:Electrodes; D006108:Graphite / Q000737:chemistry; D008024:Ligands; D008715:Methionine / Q000737:chemistry*; D009154:Mutation; D010084:Oxidation-Reduction; D017510:Protein Folding; D013696:Temperature; D013816:Thermodynamics
chemical_list: D008024:Ligands; D006108:Graphite; D045304:Cytochromes c; D008715:Methionine