NMR structure note: oxidized microsomal human cytochrome b5

Cytochrome b5 (cytb5) is a small membrane-bound hemoprotein present in all eukaryotic organisms. In most eukaryotic cells, cytb5 is attached to the cytosolic face of the endoplasmic reticulum and is described as the microsomal form (Mc). In vertebrates, two supplementary cytb5 can be found: soluble in the erythrocytes and membrane-bound attached to the internal face of the outer membrane in the mitochondria (OM) (Wang et al. 2007). Cytb5 can be divided into three domains: an N-terminal hydrosoluble globular domain that contains approximately 90 residues, a C-terminal hydrophobic domain about 25 residues long and a linker between the two above-mentioned domains. The hydrophilic domain contains the redox center, an iron protoporphyrin IX, ligated to the apoprotein by two histidyl residues. Mc cytb5 transfers electrons to various acceptors and is consequently involved in many different metabolic pathways. In the mixed function oxidase system, Mc cytb5 is a facultative electron donor to cytochromes P450 (Vergeres and Waskell 1995). Several mechanisms for the recognition between cytb5 and its physiological or artificial acceptors have been hypothesized. The implication of electrostatic interactions has been mostly studied with the cytochromes P450 as acceptors and is now widely accepted as a dominant factor (Schenkman et al. 1994). Nonetheless, several other factors may contribute positively to the recognition mechanism. First, the soluble domain of cytb5 lacking its C-terminal membrane domain cannot transfer electrons to membrane electron acceptors like cytochrome P450 (Dailey and Strittmatter 1978) while the full-length cytb5 can transfer electrons to both soluble and membranebound electron acceptor proteins (Vergeres and Waskell 1995). The linker is also recognized as an important factor controlling cytb5 to P450 interactions (Clarke et al. 2004). The three dimensional structures of the oxidized or reduced soluble domain of wild-type and mutant cytb5 from different species have been determined, either by X-ray or NMR. So far, no three dimensional structure of the full length protein has been reported, although some solid-state NMR data of the full-length rabbit cytb5 inserted in bicelles was Electronic supplementary material The online version of this article (doi:10.1007/s10858-010-9428-6) contains supplementary material, which is available to authorized users.