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Publication Abstract from PubMed

Agars are abundant polysaccharides from marine red algae and their chemical structure consists of alternating D- galactose and 3,6-anhydro-L-galactose residues, the latter of which is presumed to make the polymer recalcitrant to degradation by most terrestrial bacteria. Here we study a family 117 glycoside hydrolase (BpGH117) encoded within a recently discovered locus from the human gut bacterium Bacteroides plebeius. Consistent with this locus being involved in agarocolloid degradation, we show BpGH117 is an exo-acting 3,6-anhydro-alpha-(1,3)-L-galactosidase that removes the 3,6-anhydro-galactose from the non-reducing end of neoagaro-oligosaccharides. A Michaelis-complex of BpGH117 with neoagarobiose reveals the distortion of the constrained 3,6-anhydro-L-galactose into a conformation that favours catalysis. Furthermore, this complex, supported by analysis of site-directed mutants, provides evidence for and organization of the active site and positioning of the catalytic residues that is consistent with an inverting mechanism of catalysis and suggests that a histidine residue acts as the general acid. This latter feature differs from the vast majority of glycoside hydrolases, which use a carboxylic acid, highlighting the alternative strategies that enzymes may employ in catalyzing the cleavage of glycosidic bonds.

Analysis of a keystone enzyme in agar hydrolysis provides insight into polysaccharide degradation from red seaweeds.,Hehemann JH, Smyth L, Yadav A, Vocadlo DJ, Boraston AB J Biol Chem. 2012 Mar 5. PMID:22393053^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.