期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 106, 期 13, 页码 4969-4974出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0900995106
关键词
electron transfer; reduction potential; thermodynamics; DFT calculation; spectroscopy
资金
- National Science Foundation [CHE 0446304]
- National Institutes of Health [EB00326929]
A combination of spectroscopies and density functional theory calculations indicate that there are large temperature-dependent absorption spectral changes present in green nitrite reductases (NiRs) due to a thermodynamic equilibrium between a green and a blue type 1 (T1) copper site. The axial methionine (Met) ligand is unconstrained in the oxidized NiRs, which results in an enthalpically favored (Delta H approximate to 4.6 kcal/mol) Met-bound green copper site at low temperatures, and an entropically favored (T Delta S approximate to 4.5 kcal/mol, at room temperature) Met-elongated blue copper site at elevated temperatures. In contrast to the NiRs, the classic blue copper sites in plastocyanin and azurin show no temperature-dependent behavior, indicating that a single species is present at all temperatures. For these blue copper proteins, the polypeptide matrix opposes the gain in entropy that would be associated with the loss of the weak axial Met ligand at physiological temperatures by constraining its coordination to copper. The potential energy surfaces of Met binding indicate that it stabilizes the oxidized state more than the reduced state. This provides a mechanism to tune down the reduction potential of blue copper sites by >200 mV.
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