Ab initio calculations of electron distributions in heme-CO models

The effect of water on the Fe(3+)/Fe(2+) reduction potential of heme

Hemeproteins can act as catalysts, oxygen carriers or electron conductors. The ferric/ferrous reduction potential E(m7) of iron in the center of the prosthetic group ranges from negative values for peroxidases to an extreme positive value for cytochrome a(3) with Hb and Mb in the middle [1]. Proteins exercise their influence on E(m7) in several ways: via substituents at the periphery of the chelate structure, via the proximal ligand, and via interaction with the surrounding medium, amino acid side chains, or polar solvents. Work on recombined proteins and 2,4-substituted free hemes documented that the first two effects are additive [2]. For the third effect, models of the dielectric media on a molecular level have been successfully applied [3-5]. E(m7) has also been empirically correlated to the degree of heme exposure to water [6-8]. The apoprotein/porphyrin and water/porphyrin interfaces are complementary since water molecules fill any empty space in the crevice and surround any pertinent part of heme outside the protein boundary. The present work links to this idea by a combination of statistical mechanics simulations and quantum mechanical calculations comparing heme in water with heme in an apolar environment. Our results show that polarization of the porphyrin pi-electron cloud by the field from water dipoles influences E(m7). The dominant effect of this and other determinates of iron electron availability is perturbations of delocalized electron density in the porphyrin chelate, reproduced by a model where the prosthetic group is treated as a disc of uniform electron density. The present work is also of interest since the interfacial energy constitutes the main barrier for heme-protein separation [9-11].

Spin and electron distributions in heme-cyanide models and hemeproteins

Proton NMR spectra of low-spin Fe(III) cyanoprotoheme as prosthetic group in a number of proteins are presented. The diagonally positioned 1-, 5- and 3-, 8-methyl groups obey shifts proportional to the Fe(III)/(II) reduction potential Em7, which indicates a pseudo-contact interaction. The correlation with Em7 is understandable if one postulates an enhanced rhombic distortion, dominating the Fe-methyl dipolar interactions. Hartree-Fock-Slater quantum chemical calculations show no significant changes of spin density as a function of the Fe-L5 distance, except at the iron atom and predominantly in the 3dxz and 3dyz orbitals. 4p orbitals, on the other hand, uphold most of the changes of electron density. We also observe a principal difference in the amino acid sequences in the heme-accommodating pocket of oxygen carriers and two-electron transmitters.