1iew
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(New page: 200px<br /><applet load="1iew" size="450" color="white" frame="true" align="right" spinBox="true" caption="1iew, resolution 2.55Å" /> '''Crystal structure of...)
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Revision as of 19:46, 24 November 2007
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Crystal structure of barley beta-D-glucan glucohydrolase isoenzyme Exo1 in complex with 2-deoxy-2-fluoro-alpha-D-glucoside
Overview
BACKGROUND: Barley beta-D-glucan glucohydrolases represent family 3, glycoside hydrolases that catalyze the hydrolytic removal of nonreducing, glucosyl residues from beta-D-glucans and beta-D-glucooligosaccharides., After hydrolysis is completed, glucose remains bound in the active site., RESULTS: When conduritol B epoxide and 2', 4'-dinitrophenyl, 2-deoxy-2-fluoro-beta-D-glucopyranoside are diffused into enzyme crystals, they displace the bound glucose and form covalent glycosyl-enzyme, complexes through the Odelta1 of D285, which is thereby identified as the, catalytic nucleophile. A nonhydrolyzable S-glycosyl analog, 4(I), 4(III), 4(V)-S-trithiocellohexaose, also diffuses into the active site, and a, S-cellobioside moiety positions itself at the -1 and +1 subsites. The, glycosidic S atom of the S-cellobioside moiety forms a short contact (2.75, A) with the Oepsilon2 of E491, which is likely to be the catalytic, acid/base. The glucopyranosyl residues of the S-cellobioside moiety are, not distorted from the low-energy 4C(1) conformation, but the, glucopyranosyl ring at the +1 subsite is rotated and translated about the, linkage. CONCLUSIONS: X-ray crystallography is used to define the three, key intermediates during catalysis by beta-D-glucan glucohydrolase. Before, a new hydrolytic event begins, the bound product (glucose) from the, previous catalytic reaction is displaced by the incoming substrate, and a, new enzyme-substrate complex is formed. The second stage of the hydrolytic, pathway involves glycosidic bond cleavage, which proceeds through a, double-displacement reaction mechanism. The crystallographic analysis of, the S-cellobioside-enzyme complex with quantum mechanical modeling, suggests that the complex might mimic the oxonium intermediate rather than, the enzyme-substrate complex.
About this Structure
1IEW is a Single protein structure of sequence from Hordeum vulgare with NAG and G2F as ligands. Active as Glucan 1,3-beta-glucosidase, with EC number 3.2.1.58 Full crystallographic information is available from OCA.
Reference
Catalytic mechanisms and reaction intermediates along the hydrolytic pathway of a plant beta-D-glucan glucohydrolase., Hrmova M, Varghese JN, De Gori R, Smith BJ, Driguez H, Fincher GB, Structure. 2001 Nov;9(11):1005-16. PMID:11709165
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