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Title
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Crystal structure of a free radical enzyme, galactose oxidase.
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Authors
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N.Ito,
S.E.Phillips,
K.D.Yadav,
P.F.Knowles.
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Ref.
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J Mol Biol, 1994,
238,
794-814.
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PubMed id
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Abstract
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The crystal structure of the copper-containing enzyme, galactose oxidase, has
been solved by multiple isomorphous replacement and refined to a resolution of
1.7 A. The X-ray structure reveals a unique polypeptide fold. The protein can be
divided into three domains, all of which consist almost entirely of
beta-strands. The structure of the second domain is particularly striking, 28
beta-strands arranged in a pseudo 7-fold symmetry. The copper site is on the
surface of the protein and extremely rich in aromatic side-chains. The copper
ion has two histidines, two tyrosines, and one external ligand in distorted
square pyramidal coordination. The presence of pyrroloquinoline quinone as a
covalently bound cofactor in GOase has been excluded. Instead, an unexpected
covalent linkage between Tyr272 and Cys228 has been observed, whose functional
role may relate to the presence of a tyrosine free radical at Tyr272. The
tyrosine free radical could be stabilized by delocalization to Cys228 and
stacking interactions with Trp290. A structural model for substrate binding is
proposed that offers an explanation for the substrate specificity of the enzyme
and many of the spectroscopic and enzymological data. Although the model lacks
direct confirmation at present, it should provide a stimulus for further
spectroscopic and crystallographic studies.
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