Journal
JOURNAL OF BIOLOGICAL CHEMISTRY
Volume 285, Issue 44, Pages 34134-34143Publisher
AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
DOI: 10.1074/jbc.M110.164251
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- Ministry of Agriculture, Forestry, and Fisheries of Japan
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Exo-1,5-alpha-L-arabinofuranosidases belonging to glycoside hydrolase family 43 have strict substrate specificity. These enzymes hydrolyze only the alpha-1,5-linkages of linear arabinan and arabino-oligosaccharides in an exo-acting manner. The enzyme from Streptomyces avermitilis contains a core catalytic domain belonging to glycoside hydrolase family 43 and a C-terminal arabinan binding module belonging to carbohydrate binding module family 42. We determined the crystal structure of intact exo-1,5-alpha-L-arabinofuranosidase. The catalytic module is composed of a 5-bladed beta-propeller topologically identical to the other family 43 enzymes. The arabinan binding module had three similar subdomains assembled against one another around a pseudo-3-fold axis, forming a beta-trefoil-fold. A sugar complex structure with alpha-1,5-L-arabinofuranotriose revealed three subsites in the catalytic domain, and a sugar complex structure with alpha-L-arabinofuranosyl azide revealed three arabinose-binding sites in the carbohydrate binding module. A mutagenesis study revealed that substrate specificity was regulated by residues Asn-159, Tyr-192, and Leu-289 located at the aglycon side of the substrate-binding pocket. The exo-acting manner of the enzyme was attributed to the strict pocket structure of subsite -1, formed by the flexible loop region Tyr-281-Arg-294 and the side chain of Tyr-40, which occupied the positions corresponding to the catalytic glycon cleft of GH43 endo-acting enzymes.
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