title: Thermodynamic investigation into the mechanisms of proton-coupled electron transfer events in heme protein maquettes.
abstract: To study the engineering requirements for proton pumping in energy-converting enzymes such as cytochrome c oxidase, the thermodynamics and mechanisms of proton-coupled electron transfer in two designed heme proteins are elucidated. Both heme protein maquettes chosen, heme b-[H10A24]2 and heme b-[delta7-His]2, are four-alpha-helix bundles that display pH-dependent heme midpoint potential modulations, or redox-Bohr effects. Detailed equilibrium binding studies of ferric and ferrous heme b with these maquettes allow the individual contributions of heme-protein association, iron-histidine ligation, and heme-protein electrostatics to be elucidated. These data demonstrate that the larger, less well-structured [H10A24]2 binds heme b in both oxidation states tighter than the smaller and more well-structured [Delta7-His]2 due to a stronger porphyrin-protein hydrophobic interaction. The 66 mV (1.5 kcal/mol) difference in their heme reduction potentials observed at pH 8.0 is due mostly to stabilization of ferrous heme in [H10A24]2 relative to [delta7-His]2. The data indicate that porphyrin-protein hydrophobic interactions and heme iron coordination are responsible for the Kd value of 37 nM for the heme b-[delta7-His]2 scaffold, while the affinity of heme b for [H10A24]2 is 20-fold tighter due to a combination of porphyrin-protein hydrophobic interactions, iron coordination, and electrostatic effects. The data also illustrate that the contribution of bis-His coordination to ferrous heme protein affinity is limited, <3.0 kcal/mol. The 1H+/1e- redox-Bohr effect of heme b-[H10A24]2 is due to the greater absolute stabilization of the ferric heme (4.1 kcal/mol) compared to the ferrous heme (1.4 kcal/mol) binding upon glutamic acid deprotonation, i.e., an electrostatic response mechanism. The 2H+/1e- redox-Bohr effect observed for heme b-[delta7-His]2 is due to histidine protonation and histidine dissociation of ferrous heme b upon reduction, i.e., a ligand loss mechanism. These results indicate that the contribution of porphyrin-protein hydrophobic interactions to heme affinity is critical to maintaining the heme bound in both oxidation states and eliciting an electrostatic response from these designed heme protein scaffolds.
authors.affiliation: Department of Chemistry, Columbia University, 3000 Broadway, MC 3121, New York, New York 10027, USA.
authors.forename: Amit R
authors.initials: AR
authors.lastname: Reddi
authors.forename: Charles J
authors.initials: CJ
authors.lastname: Reedy
authors.forename: Steven
authors.initials: S
authors.lastname: Mui
authors.forename: Brian R
authors.initials: BR
authors.lastname: Gibney
mesh_terms: D004579:Electron Transport; D006418:Heme / Q000737:chemistry / Q000378:metabolism; D006420:Hemeproteins / Q000737:chemistry* / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D008954:Models, Biological; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011522:Protons; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
chemical_list: D006420:Hemeproteins; D011522:Protons; D006418:Heme