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'''X-RAY CRYSTALLOGRAPHIC STRUCTURES OF D-XYLOSE ISOMERASE-SUBSTRATE COMPLEXES POSITION THE SUBSTRATE AND PROVIDE EVIDENCE FOR METAL MOVEMENT DURING CATALYSIS''' | '''X-RAY CRYSTALLOGRAPHIC STRUCTURES OF D-XYLOSE ISOMERASE-SUBSTRATE COMPLEXES POSITION THE SUBSTRATE AND PROVIDE EVIDENCE FOR METAL MOVEMENT DURING CATALYSIS''' | ||
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[[Category: Petsko, G A.]] | [[Category: Petsko, G A.]] | ||
[[Category: Ringe, D.]] | [[Category: Ringe, D.]] | ||
''Page seeded by [http://oca.weizmann.ac.il/oca OCA ] on Sat May 3 15:39:39 2008'' | |||
''Page seeded by [http://oca.weizmann.ac.il/oca OCA ] on | |||
Revision as of 15:39, 3 May 2008
X-RAY CRYSTALLOGRAPHIC STRUCTURES OF D-XYLOSE ISOMERASE-SUBSTRATE COMPLEXES POSITION THE SUBSTRATE AND PROVIDE EVIDENCE FOR METAL MOVEMENT DURING CATALYSIS
OverviewOverview
The X-ray crystallographic structures of the metal-activated enzyme xylose isomerase from Streptomyces olivochromogenes with the substrates D-glucose, 3-O-methyl-D-glucose and in the absence of substrate were determined to 1.96-, 2.19-, and 1.81-A resolution and refined to R-factors of 16.6%, 15.9%, and 16.1%, respectively. Xylose isomerase catalyzes the interconversion between glucose and fructose (xylose and xylulose under physiological conditions) by utilizing two metal cofactors to promote a hydride shift; the metals are bridged by a glutamate residue. This puts xylose isomerase in the small but rapidly growing family of enzymes with a bridged bimetallic active site, in which both metals are involved in the chemical transformation. The substrate 3-O-methylglucose was chosen in order to position the glucose molecule in the observed electron density unambiguously. Of the two essential magnesium ions per active site, Mg-2 was observed to occupy two alternate positions, separated by 1.8 A, in the substrate-soaked structures. The deduced movement was not observed in the structure without substrate present and is attributed to a step following substrate binding but prior to isomerization. The substrates glucose and 3-O-methylglucose are observed in their linear extended forms and make identical interactions with the enzyme by forming ligands to Mg-1 through O2 and O4 and by forming hydrogen bonds with His53 through O5 and Lys182 through O1. Mg-2 has a water ligand that is interpreted in the crystal structure in the absence of substrate as a hydroxide ion and in the presence of substrate as a water molecule. This hydroxide ion may act as a base to deprotonate the glucose O2 and subsequently protonate the product fructose O1 concomitant with hydride transfer. Calculations of the solvent-accessible surface of possible dimers, with and without the alpha-helical C-terminal domain, suggest that the tetramer is the active form of this xylose isomerase.
About this StructureAbout this Structure
1XYB is a Single protein structure of sequence from Streptomyces olivochromogenes. Full crystallographic information is available from OCA.
ReferenceReference
X-ray crystallographic structures of D-xylose isomerase-substrate complexes position the substrate and provide evidence for metal movement during catalysis., Lavie A, Allen KN, Petsko GA, Ringe D, Biochemistry. 1994 May 10;33(18):5469-80. PMID:8180169 Page seeded by OCA on Sat May 3 15:39:39 2008