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Publication Abstract from PubMed

Pullulanases are glycoside hydrolase family 13 (GH13) enzymes that target alpha1,6 glucosidic linkages within starch and aid in the degradation of the alpha1,4- and alpha1,6- linked glucans pullulan, glycogen and amylopectin. The human gut bacterium Ruminococcus bromii synthesizes two extracellular pullulanases, Amy10 and Amy12, that are incorporated into the multiprotein amylosome complex that enables the digestion of granular resistant starch from the diet. Here we provide a comparative biochemical analysis of these pullulanases and the x-ray crystal structures of the wild type and the nucleophile mutant D392A of Amy12 complexed with maltoheptaose and 6(3)-alpha-D glucosyl-maltotriose. While Amy10 displays higher catalytic efficiency on pullulan and cleaves only alpha1,6 linkages, Amy12 has some activity on alpha1,4 linkages suggesting that these enzymes are not redundant within the amylosome. Our structures of Amy12 include a mucin-binding protein (MucBP) domain that follows the C-domain of the GH13 fold, an atypical feature of these enzymes. The wild type Amy12 structure with maltoheptaose captured two oligosaccharides in the active site arranged as expected following catalysis of an alpha1,6 branch point in amylopectin. The nucleophile mutant D392A complexed with maltoheptaose or 6(3)-alpha-D glucosyl-maltotriose captured beta-glucose at the reducing end in the -1 subsite, facilitated by the truncation of the active site aspartate and stabilized by stacking with Y279. The core interface between the co-crystallized ligands and Amy12 occurs within the -2 through + 1 subsites, which may allow for flexible recognition of alpha1,6 linkages within a variety of starch structures.

Structure and substrate recognition by the Ruminococcus bromii amylosome pullulanases.,Cockburn DW, Kibler R, Brown HA, Duvall R, Morais S, Bayer E, Koropatkin NM J Struct Biol. 2021 Jun 27;213(3):107765. doi: 10.1016/j.jsb.2021.107765. PMID:34186214^[1]

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