The impact of N-terminal nonessential domains on the enzymological properties of the pullulanase from a marine Bacillus megaterium.

Objective: To determine the impact of the N-terminal nonessential domains on the enzymological properties of a pullulanase (BmP) from Bacillus megaterium strain P6.

Methods: The domains of BmP were identified by the conserved domain (CD) search online software. BmP was prepared by fermentation with the strain P6 and its N-terminal truncated form (BmNTP) was obtained by heterologous expression. Structure-property relations were analyzed by homology modeling.

Results: BmP showed a domain architecture consisting of CBM41a-CBM41b-X-CBM48-pulA. CBM41a-CBM41b-X was removed in BmNTP. In comparison with BmP, BmNTP was lower in pH stability, specific activity and optimum NaCl concentration, but higher in K value, thermostability, optimum pH and temperature, and activity enhancement by NaCl. Particularly, BmNTP was active over 0-35% (w/v) NaCl concentrations and enhanced 8.7 folds in specific activity (from 17.6 to 170 U/mg) in 10% NaCl. The solvent accessible surface area (SASA) of the catalytic triad was found to be correlated positively to the substrate affinity and negatively to the optimum temperature and activity enhancement by NaCl.

Conclusion: The impact of CBM41a-CBM41b-X on the pullulanase properties was extensive and distinguished from the previous reports. The decrease in SASA may be responsible for the enzymological changes.