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Journal of Biochemistry Advance Access originally published online on September 28, 2007
Journal of Biochemistry 2007 142(5):553-560; doi:10.1093/jb/mvm181
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© 2007 The Japanese Biochemical Society

JB Review

Histidine Cycle Mechanism for the Concerted Proton/Electron Transfer from Ascorbate to the Cytosolic Haem b Centre of Cytochrome b561: A Unique Machinery for the Biological Transmembrane Electron Transfer

Nobuyuki Nakanishi1, Fusako Takeuchi2 and Motonari Tsubaki2,3,*

1Department of Molecular Science and Material Engineering, Graduate School of Science and Technology; 2Department of Chemistry, Graduate School of Science, Kobe University, Rokkodai-cho, Nada-ku, Kobe, Hyogo 657-8501; and 3CREST, JST, Honcho, Kawaguchi, Saitama 332-0012, Japan

*To whom correspondence should be addressed. Tel: +81-(0)78-803-6582, Fax: +81-(0)78-803-6582, E-mail: mtsubaki{at}kobe-u.ac.jp

Received June 26, 2007; Accepted August 18, 2007


  Abstract

Cytochromes b561 are a family of transmembrane proteins found in most eukaryotic cells and contain two haem b prosthetic groups per molecule being coordinated with four His residues from four different transmembrane {alpha}-helices. Although cytochromes b561 residing in the chromaffin vesicles has long been known to have a role for a neuroendocrine-specific transmembrane electron transfer from extravesicular ascorbate to intravesicular monodehydroascorbate radical to regenerate ascorbate, newly found members were apparently lacking in the sequence for putative ascorbate-binding site but exhibiting a transmembrane ferrireductase activity. We propose that cytochrome b561 has a specific mechanism to facilitate the concerted proton/electron transfer from ascorbate by exploiting a cycle of deprotonated and protonated states of the N{delta}1 atom of the axial His residue at the extravesicular haem center, as an initial step of the transmembrane electron transfer. This mechanism utilizes the well-known electrochemistry of ascorbate for a biological transmembrane electron transfer and might be operative for other type of electron transfer reactions from organic reductants.

Key Words: ascorbate, cytochrome b561, electron transfer, haem axial His residue, membrane protein

Abbreviations: AsA, ascorbate; MDA, monodehydroascorbate; DEPC, diethylpyrocarbonate; MALDI-TOF, matrix-assisted laser desorption/ionization-time of flight


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