Ortwin Meyer, Lothar Gremer, Reinhold Ferner, Marion Ferner, Holger Dobbek, Manuel Gnida, Wolfram Meyer-Klaucke, Robert Huber
The Role of Se, Mo and Fe in the Structure and Function of Carbon Monoxide Dehydrogenase
CO dehydrogenase (EC 1.2.99.2) catalyzes the oxidation
of CO according to the following equation:
CO + H2O ? CO2 + 2 e? + 2 H+. It is a selenium-containing
molybdo-iron-sulfur-flavoenzyme, which has been
crystallized and structurally characterized in its oxidized
state from the aerobic CO utilizing bacteria
Oligotropha carboxidovorans and Hydrogenophaga
pseudoflava. Both CO dehydrogenase structures
show only minor differences, and the enzymes are
dimers of two heterotrimers. Each heterotrimer is
composed of a molybdoprotein, a flavoprotein, and an
iron-sulfur protein. CO oxidation takes place at the
molybdoprotein which contains a 1:1 mononuclear
complex of molybdopterin-cytosine dinucleotide and
a Mo-ion, along with a catalytically essential S-selanylcysteine.
The latter is appropriately positioned
in the SeMo-active site by a unique VAYRCSFR active
site loop. In H. pseudoflava the arginine preceeding
the cysteine in the active site loop is modified to a C?-hydroxy arginine residue which has no obvious function.
The substituents in the first coordination sphere
of the Mo-ion are the enedithiolate sulfur atoms of the
molybdopterin-cytosine dinucleotide, two oxo- and a
sulfido-group. Extended X-ray absorption fine structure
spectroscopy (EXAFS), along with the crystal
structure of CO dehydrogenase (23.2 U mg?1) at 1.85 ?
resolution, have identified a sulfur atom at 2.3 ? from
the Mo-ion. The sulfur reacts with cyanide yielding
thiocyanate. The corresponding inactive desulfo-CO
dehydrogenase shows a typical desulfo inhibited-type
of Mo-electron paramagnetic resonance (EPR) spectrum.
Structural changes at the SeMo-site during
catalysis are suggested by the Mo to Se distance of 3.7
? and the Mo-S-Se angle of 113° in the oxidized enzyme
which increase to 4.1 ?, and 121°, respectively,
in the reduced enzyme. The intramolecular electron
transport chain in CO dehydrogenase involves the
following prosthetic groups and minimal distances:
CO ? [Mo of the molybdenum cofactor]?14.6??[2Fe-2S] I?12.4 ??[2Fe-2S] II?8.7 ??[FAD].
Biological Chemistry, Walter de Gruyter
Print ISSN: 1431-6730
Volume: 381, 09/2000
Pages: 865 - 876
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