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Title
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Structure of cobalt carbonic anhydrase complexed with bicarbonate.
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Authors
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K.Håkansson,
A.Wehnert.
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Ref.
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J Mol Biol, 1992,
228,
1212-1218.
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PubMed id
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Abstract
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The three-dimensional structure of a complex between catalytically active
cobalt(II) substituted human carbonic anhydrase II and its substrate bicarbonate
was determined by X-ray crystallography (1.9 A). One water molecule and two
bicarbonate oxygen atoms are found at distances between 2.3 and 2.5 A from the
cobalt ion in addition to the three histidyl ligands contributed by the peptide
chain. The tetrahedral geometry around the metal ion in the native enzyme with a
single water molecule 2.0 A from the metal is therefore lost. The geometry is
difficult to classify but might best be described as distorted octahedral. The
structure is suggested to represent a water-bicarbonate exchange state relevant
also for native carbonic anhydrase, where the two unprotonized oxygen atoms of
the substrate are bound in a carboxylate binding site and the hydroxyl group is
free to move closer to the metal thereby replacing the metal-bound water
molecule. A reaction mechanism based on crystallographically determined
enzyme-ligand complexes is represented.
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