title: Spectroscopic evidence of the role of an axial ligand histidinate in the mechanism of adrenal cytochrome b561.
abstract: Adrenal cytochrome b(561) (AdCytb) is the prototype of a widespread protein family that specializes in delivering electrons donated by ascorbic acid for different processes in eukaryotic cells. AdCytb transports redox equivalents from cytoplasmic ascorbate across the membranes of chromaffin granules to support norepinephrine synthesis within their matrix. The interaction of AdCytb with ascorbate is central to a proposed mechanism of AdCytb's function, and a histidine in the active site of AdCytb was suggested to bind cytoplasmic ascorbate and serve as the acceptor of the proton released during ascorbate oxidation. AdCytb contains high- and low-potential hemes but their orientation relative to the matrix and cytoplasmic interfaces of chromaffin granule membrane is disputed. Using a combination of three spectroscopic methods (UV-vis absorption, near-infrared magnetic circular dichroism, and electron paramagnetic resonance), we find that a histidine residue that serves as an axial ligand to the high-potential heme undergoes deprotonation with a pK of ~8.0 and is thus a good candidate for interaction with cytoplasmic ascorbate. The low-potential heme of AdCytb is found to have a pK of ~10.5, making it an unlikely candidate for accepting a proton at physiological pH. UV-vis spectroscopy reveals an additional proton acceptor group in AdCytb with a pK of ~6.5 that is not observed by the other two techniques; whether it plays a role in the mechanism of AdCytb is unknown. We incorporate these results into an updated mechanism of AdCytb reduction predicated on the high-potential heme's localization on the cytoplasmic interface of the chromaffin granule membrane.
authors.affiliation: Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77251, United States.
authors.forename: Giordano F Z
authors.initials: GF
authors.lastname: da Silva
authors.forename: Vladimir P
authors.initials: VP
authors.lastname: Shinkarev
authors.forename: Yury A
authors.initials: YA
authors.lastname: Kamensky
authors.forename: Graham
authors.initials: G
authors.lastname: Palmer
mesh_terms: D000311:Adrenal Glands / Q000378:metabolism; D001205:Ascorbic Acid / Q000378:metabolism; D002837:Chromaffin Granules / Q000378:metabolism; D045303:Cytochromes b / Q000737:chemistry / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme / Q000378:metabolism; D006639:Histidine / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction; D013057:Spectrum Analysis
chemical_list: D006418:Heme; D006639:Histidine; D045303:Cytochromes b; D001205:Ascorbic Acid