Molecular cloning and biochemical characterization of a heat-stable type I pullulanase from Thermotoga neapolitana

The gene encoding a type I pullulanase from the hyperthermophilic anaerobic bacterium Thermotoga neapolitana (pulA) was cloned in Escherichia coli and sequenced. The pulA gene from T. neapolitana showed 91.5% pairwise amino acid identity with pulA from Thermotoga maritima and contained the four regions conserved in all amylolytic enzymes. pulA encodes a protein of 843 amino acids with a 19-residue signal peptide. The pulA gene was subcloned and overexpressed in E. coli under the control of the T7 promoter. The purified recombinant enzyme (rPulA) produced a 93-kDa protein with pullulanase activity. rPulA was optimally active at pH 5-7 and 80°C and had a half-life of 88 min at 80°C. rPulA hydrolyzed pullulan, producing maltotriose, and hydrolytic activities were also detected with amylopectin, starch, and glycogen, but not with amylose. This substrate specificity is typical of a type I pullulanase. Thin layer chromatography of the reaction products in the reaction with pullulan and aesculin showed that the enzyme had transglycosylation activity. Analysis of the transfer product using NMR and isoamylase treatment revealed it to be α-maltotriosyl-(1,6)-aesculin, suggesting that the enzyme transferred the maltotriosyl residue of pullulan to aesculin by forming α-1,6-glucosidic linkages. Our findings suggest that the pullulanase from T. neapolitana is the first thermostable type I pullulanase which has α-1,6-transferring activity.