Gene Expression and Biochemical Characterization of a GH77 4-α-Glucanotransferase CcGtase From Corallococcus sp. EGB

The enzyme 4-alpha-glucanotransferase catalyzes the intramolecular and intermolecular transglycosylation of alpha-1,4-glucan, which plays an important role in starch metabolism and starch modification. In this study, the gene encoding a 4-alpha-glucanotransferase from Corallococcus sp. EGB (ccGtase) is cloned and expressed in Escherichia coli BL21 (DE3). CcGtase contains all the conserved amino acid residues of glycoside hydrolase family 77 (GH77), which form the active site environment of the enzymes in the alpha-amylase family, while sequence alignments show that 4-alpha-glucanotransferases from myxobacteria constitute an independent group along with the GH77 glucanotransferases. Product analysis shows that CcGtase is able to convert malto-oligosaccharides and starch into linear maltooligosaccharides with a low degree of polymerization (DP < 12) instead of cycloamylose. The transfer action toward starch is only observed in the presence of glucose, and delayed production of maltose from the transfer reaction is also observed. During the reaction, no apparent hydrolytic activity is observed. Based on the low sequence identity (9-35%) and unique characteristics, CcGtase can be regarded as a new member of the GH77 family.