5gqv
From Proteopedia
Crystal structure of branching enzyme from Cyanothece sp. ATCC 51142 in complex with maltohexaose
Structural highlights
Function[B1WPM8_CYAA5] Catalyzes the formation of the alpha-1,6-glucosidic linkages in glycogen by scission of a 1,4-alpha-linked oligosaccharide from growing alpha-1,4-glucan chains and the subsequent attachment of the oligosaccharide to the alpha-1,6 position.[HAMAP-Rule:MF_00685] Publication Abstract from PubMedBranching enzyme (BE) catalyzes the formation of alpha-1,6-glucosidic linkages in amylopectin and glycogen. The reaction products are variable, depending on the organism sources, and the mechanistic basis for these different outcomes is unclear. Although most cyanobacteria have only one BE isoform belonging to glycoside hydrolase family 13, Cyanothece sp. ATCC 51142 has three isoforms (BE1, BE2, and BE3) with distinct enzymatic properties, suggesting that investigations of these enzymes might provide unique insights into this system. Here, we report the crystal structure of ligand-free wild-type BE1 (residues 5-759 of 1-773) at 1.85 A resolution. The enzyme consists of four domains, including domain N, carbohydrate-binding module family 48 (CBM48), domain A containing the catalytic site, and domain C. The central domain A displays a (beta/alpha)8-barrel fold, whereas the other domains adopt beta-sandwich folds. Domain N was found in a new location at the back of the protein, forming hydrogen bonds and hydrophobic interactions with CBM48 and domain A. Site-directed mutational analysis identified a mutant (W610N) that bound maltoheptaose with sufficient affinity to enable structure determination at 2.30 A resolution. In this structure, maltoheptaose was bound in the active site cleft, allowing us to assign subsites -7 to -1. Moreover, seven oligosaccharide-binding sites were identified on the protein surface, and we postulated that two of these in domain A served as the entrance and exit of the donor/acceptor glucan chains, respectively. Based on these structures, we propose a substrate binding model explaining the mechanism of glycosylation/deglycosylation reactions catalyzed by BE. Bound Substrate in the Structure of Cyanobacterial Branching Enzyme Supports a New Mechanistic Model.,Hayashi M, Suzuki R, Colleoni C, Ball SG, Fujita N, Suzuki E J Biol Chem. 2017 Mar 31;292(13):5465-5475. doi: 10.1074/jbc.M116.755629. Epub, 2017 Feb 13. PMID:28193843[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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