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PDBsum entry 1s5m
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Contents |
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* Residue conservation analysis
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References listed in PDB file
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Key reference
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Title
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Xylose isomerase in substrate and inhibitor michaelis states: atomic resolution studies of a metal-Mediated hydride shift.
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Authors
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T.D.Fenn,
D.Ringe,
G.A.Petsko.
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Ref.
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Biochemistry, 2004,
43,
6464-6474.
[DOI no: ]
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PubMed id
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Abstract
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Xylose isomerase (E.C. 5.3.1.5) catalyzes the interconversion of aldose and
ketose sugars and has an absolute requirement for two divalent cations at its
active site to drive the hydride transfer rates of sugar isomerization. Evidence
suggests some degree of metal movement at the second metal site, although how
this movement may affect catalysis is unknown. The 0.95 A resolution structure
of the xylitol-inhibited enzyme presented here suggests three alternative
positions for the second metal ion, only one of which appears positioned in a
catalytically competent manner. To complete the reaction, an active site
hydroxyl species appears appropriately positioned for hydrogen transfer, as
evidenced by precise bonding distances. Conversely, the 0.98 A resolution
structure of the enzyme with glucose bound in the alpha-pyranose state only
shows one of the metal ion conformations at the second metal ion binding site,
suggesting that the linear form of the sugar is required to promote the second
and third metal ion conformations. The two structures suggest a strong degree of
conformational flexibility at the active site, which seems required for
catalysis and may explain the poor rate of turnover for this enzyme. Further,
the pyranose structure implies that His53 may act as the initial acid
responsible for ring opening of the sugar to the aldose form, an observation
that has been difficult to establish in previous studies. The glucose ring also
appears to display significant segmented disorder in a manner suggestive of ring
opening, perhaps lending insight into means of enzyme destabilization of the
ground state to promote catalysis. On the basis of these results, we propose a
modified version of the bridged bimetallic mechanism for hydride transfer in the
case of Streptomyces olivochromogenes xylose isomerase.
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