Characterization of 4-α-glucanotransferase from Synechocystis sp. PCC 6803 and its application to various corn starches

A putative 4-α-glucanotransferase (αGTase) gene from Synechocystis sp. PCC 6803 was identified being composed of 1505 nucleotides, and the overexpressed protein was purified with an affinity chromatography. The recombinant αGTase had about 57 kDa of molecular mass when judged by SDS-PAGE analysis. The optimum reaction condition of the αGTase was shown to be pH 7 at 45°C in 50 mm phosphate buffer. This enzyme displayed transglycosylating activity on various maltooligosaccharides, of which the smallest donor and acceptor molecules were determined to be maltose and glucose, respectively.Various corn starches consisting of different proportions of amylopectin and amylose were incubated with the recombinant αGTase. The change in molecular weight distribution of αGTase-modified starch was analyzed by HPSEC. The reaction pattern of αGTase showed substantial decrease in amylopectin and increase in the peak corresponding to cycloamylose (CA). The production yield of CA tended to increase from 5 to 30% along with the increase in the apparent amylose content in corn starch, which suggested that linear amylose chain would be preferred to produce CA in the αGTase treatment. The detectable minimum degree of polymerization (DP) of CA was shown to be 22 by MALDI-TOF-MS analysis. As another action mode of αGTase, the rearrangement of amylopectin branch-chain distribution occurred without hydrolysis to small oligosaccharides. After isoamylolysis, αGTase-treated starch displayed the increase in DP 4–9 and longer than DP 21 when the relative proportion of branch chains in amylopectin was determined by HPAEC.