1z9a
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(New page: 200px<br /><applet load="1z9a" size="450" color="white" frame="true" align="right" spinBox="true" caption="1z9a, resolution 2.40Å" /> '''Crystal Structure Of...)
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Revision as of 05:11, 21 November 2007
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Crystal Structure Of The Asn-309 To Asp Mutant Of Candida Tenuis Xylose Reductase (Akr2B5) Bound To Nad+
Overview
Little is known about how substrates bind to CtXR (Candida tenuis xylose, reductase; AKR2B5) and other members of the AKR (aldo-keto reductase), protein superfamily. Modelling of xylose into the active site of CtXR, suggested that Trp23, Asp50 and Asn309 are the main components of, pentose-specific substrate-binding recognition. Kinetic consequences of, site-directed substitutions of these residues are reported. The mutants, W23F and W23Y catalysed NADH-dependent reduction of xylose with only 4 and, 1% of the wild-type efficiency (kcat/K(m)) respectively, but improved the, wild-type selectivity for utilization of ketones, relative to xylose, by, factors of 156 and 471 respectively. Comparison of multiple sequence, alignment with reported specificities of AKR members emphasizes a, conserved role of Trp23 in determining aldehyde-versus-ketone substrate, selectivity. D50A showed 31 and 18% of the wild-type catalytic-centre, activities for xylose reduction and xylitol oxidation respectively, consistent with a decrease in the rates of the chemical steps caused by, the mutation, but no change in the apparent substrate binding constants, and the pattern of substrate specificities. The 30-fold preference of the, wild-type for D-galactose compared with 2-deoxy-D-galactose was lost, completely in N309A and N309D mutants. Comparison of the 2.4 A (1 A=0.1, nm) X-ray crystal structure of mutant N309D bound to NAD+ with the, previous structure of the wild-type holoenzyme reveals no major structural, perturbations. The results suggest that replacement of Asn309 with alanine, or aspartic acid disrupts the function of the original side chain in, donating a hydrogen atom for bonding with the substrate C-2(R) hydroxy, group, thus causing a loss of transition-state stabilization energy of 8-9, kJ/mol.
About this Structure
1Z9A is a Single protein structure of sequence from Candida tenuis with NAD as ligand. Full crystallographic information is available from OCA.
Reference
Probing the substrate binding site of Candida tenuis xylose reductase (AKR2B5) with site-directed mutagenesis., Kratzer R, Leitgeb S, Wilson DK, Nidetzky B, Biochem J. 2006 Jan 1;393(Pt 1):51-8. PMID:16336198
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