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Title
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Involvement of a labile axial histidine in coupling electron and proton transfer in Methylophilus methylotrophus cytochrome c''.
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Authors
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H.S.Costa,
H.Santos,
D.L.Turner,
A.V.Xavier.
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Ref.
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Eur J Biochem, 1992,
208,
427-433.
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PubMed id
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Abstract
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Methylophilus methylotrophus cytochrome c'' is an unusual monohaem protein (15
kDa) undergoing a redox-linked spin-state transition [Santos, H. & Turner,
D. L. (1988) Biochim. Biophys. Acta 954, 277-286]. The midpoint redox potential
of cytochrome c" was measured over the pH range 4-10. The pH dependence of the
midpoint redox potential was interpreted in terms of a model that considers the
redox-state dependence of the ionization of two distinct and non-interacting
protonated groups in the protein. This analysis led to the following pKa values
within the pH range studied: pKa10 = 6.4, pKa1r = 5.4 and pKa2r = 8.1.
Proton-NMR spectroscopy was used to assist the characterization of the two
ionizing groups responsible for the observed redox-Bohr effect: the group
ionizing with a lower pKar was assigned to a haem propionic acid substituent and
the other to the axial histidine ligand which becomes detached upon reduction,
which has a pKa0 too low to be measured. It is shown that M. methylotrophus
cytochrome c" is able to couple electron and proton transfer in the
physiological pH range through a mechanism involving reversible change in the
haem-iron coordination. Possible implications for the physiological role of the
protein are discussed.
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