1uuq: Difference between revisions

The enzymatic hydrolysis of the glycosidic bond is central to numerous, biological processes. Glycoside hydrolases, which catalyze these, reactions, are grouped into families based on primary sequence, similarities. One of the largest glycoside hydrolase families is glycoside, hydrolase family 5 (GH5), which contains primarily endo-acting enzymes, that hydrolyze beta-mannans and beta-glucans. Here we report the cloning, characterization, and three-dimensional structure of the Cellvibrio mixtus, GH5 beta-mannosidase (CmMan5A). This enzyme releases mannose from the, nonreducing end of mannooligosaccharides and polysaccharides, an activity, not previously observed in this enzyme family. CmMan5A contains a single, glycone (-1) and two aglycone (+1 and +2) sugar-binding subsites. The -1, subsite displays absolute specificity for mannose, whereas the +1 subsite, does not accommodate galactosyl side chains but will bind weakly to, glucose. The +2 subsite is able to bind to decorated mannose residues., CmMan5A displays similar activity against crystalline and amorphous, mannans, a property rarely attributed to glycoside hydrolases. The 1.5 A, crystal structure reveals that CmMan5A adopts a (beta/alpha)(8) barrel, fold, and superimposition with GH5 endo-mannanases shows that dramatic, differences in the length of three loops modify the active center, accessibility and thus modulate the specificity from endo to exo. The most, striking and significant difference is the extended loop between strand, beta8 and helix alpha8 comprising residues 378-412. This insertion forms a, "double" steric barrier, formed by two short beta-strands that function to, "block" the substrate binding cleft at the edge of the -1 subsite forming, the "exo" active center topology of CmMan5A.

1UUQ is a [http://en.wikipedia.org/wiki/Single_protein Single protein] structure of sequence from [http://en.wikipedia.org/wiki/Cellvibrio_mixtus Cellvibrio mixtus] with SO4 and GOL as [http://en.wikipedia.org/wiki/ligands ligands]. Structure known Active Site: AC1. Full crystallographic information is available from [http://ispc.weizmann.ac.il/oca-bin/ocashort?id=1UUQ OCA].  

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