Electron transfer parameters for Methemoglobin formation in mutant Hemoglobin α-chains

Abstract

AbstractHemoglobin mediated transport of dioxygen (O2) critically depends on the stability of the reduced (Fe2+) form of the Heme cofactors. Some protein mutations stabilize oxidized (Fe3+) state (Methemoglobin, Hb M) causing methemoglobinemia and can be lethal above 30 %. Majority of the analyses of factors influencing Hb oxidation are retrospective and give insights only for inner sphere mutations of Heme (His58, His87). Herein, we report the first all atom MD simulations on redox states and calculations of the Marcus ET parameters for the α-chain Hb oxidation and reduction rates for Hb M. The Hb (wild type), and most of the studied α-chain variants maintain globin structure except the Hb M Iwate (H87Y). Using linear response approximation we calculated average energy gaps (<ΔE>), total (λ), protein (λprot), solvent (λsolv) reorganization energies, and redox potentials (E°), and oxidation free energies (ΔG°). The total λ ranges from 0.685 – 0.730 eV in agreement with literature on Hb and similar Heme proteins. The mutants forming Hb M tend to lower the E° and thus stabilize the oxidized (Fe3+) state (e.g. the Hb Miyagi variant with K61E mutation). Solvent reorganization (λsolv 73 – 96 %) makes major contributions to λ, while protein reorganization (λprot) accounts for 27 – 30 % except for the Miyagi and J-Buda variants (λprot ∼ 4 %). Analysis of Heme-solvent H-bonding interactions among variants provide insights into the role of Lys61 residue in stabilizing Fe2+ state and ET parameters. The ET parameters provide valuable insights into the Hb oxidation to Hb M in agreement with the experimental data. Thus our methodology explains the effect of mutations on the structure, stability and Hb oxidation, and has potential for the prediction of methemoglobinemia.