A subject of continued interest is the impact of specific protein interactions with heme peripheral groups (vinyls, propionates, formyl, and farnesyl groups on heme a) and their effects on the function of the heme protein. In order to study a particular interaction, model heme systems must be quantified in aprotic solvents such as dichloromethane or benzene, with a ligand that can singularly interact with one of the heme peripheral groups. The low dielectric constants (εr) of these solvents facilitate the evaluation of hydrogen bonding and the electrostatic interactions between ligands and the heme peripheral groups. Using this approach, we carried out an electrochemical study on the effects of the ionization of the heme propionates (propionic acids) on the reduction potentials of heme b and heme a. We also measured the electrochemical contribution of the H-bond to the formyl group on the reduction potential of heme a. The performed electrochemical measurements show that ionization of the heme propionates lead to the largest redox change in low dielectric constant solvent conditions. The measured reduction potential changes for heme b and heme a were approximately -50 mV (±10 mV) per ionized propionate in dichloromethane (without electrolyte). Furthermore, the measured dissociation constant (Kd) between various H-bond donors and carbonyl-containing copper porphyrins show that the H-bond are significant in benzene and dichloromethane, but extremely weak in the presence of acetonitrile. Under low dielectric constant conditions such as dichloromethane (εr ≈ 8.5), the combined H-bond/protonation of the heme cofactor can lead to a redox change of over +200 mV. This study shows how the dielectric constant of the medium plays a role in modulating the electrochemical properties of heme a through H-bonding of its formyl group and protonation of the heme propionates which have been shown to be involved in the proton pump of cytochrome c oxidase (CcO). The implication is that the reduction of heme a (ferric-to-ferrous) in CcO can provide 4.5 kcal/mol for proton translocation through the mitochondrial membrane.

Heme peripheral groups interactions in proteins and the role of the dielectric constant of the medium

250th National Meeting (2015)

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