Thermodynamic investigation into the mechanisms of proton-coupled electron transfer events in heme protein maquettes

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.