Structural highlights
FunctionIPUA_ASPNG Hydrolyzes pullulan, a linear polymer which is composed of maltotriose units with alpha-1,6 glucosidic linkages, to produce isopanose (Glca1-4Glca1-6Glc). Publication Abstract from PubMedN-glycosylation has been shown to be important for the stability of some glycoproteins. Isopullulanase (IPU), a polysaccharide-hydrolyzing enzyme, is a highly N-glycosylated protein, and IPU deglycosylation results in a decrease in thermostability. To investigate the function of N-glycan in IPU, we focused on an N-glycosylated residue located in the vicinity of the active site, Asn448. The thermostabilities of three IPU variants, Y440A, N448A and S450A, were 0.5-8.4 degrees C lower than the wild-type enzyme. The crystal structure of endoglycosidase H (Endo H)-treated N448A variant was determined. There are four IPU molecules, Mol-A, B, C and D, in the asymmetric unit. The conformation of a loop composed of amino acid residues 435-455 in Mol-C was identical to wild-type IPU, whereas the conformations of this loop in Mol-A, Mol-B and Mol-D were different from each other. These results suggest that the Asn448 side chain is primarily important for the stability of IPU. Our results indicate that mutation of only N-glycosylated Asn residue may lead to incorrect conclusion for the evaluation of the function of N-glycan. Usually, the structures of N-glycosylation sites form an extended configuration in IPU; however, the Asn448 site had an atypical structure that lacked this configuration. The side chain of a glycosylated asparagine residue is important for the stability of isopullulanase.,Miyazaki T, Yashiro H, Nishikawa A, Tonozuka T J Biochem. 2015 Apr;157(4):225-34. doi: 10.1093/jb/mvu065. Epub 2014 Oct 29. PMID:25359784[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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