Structure of Bacillus amyloliquefaciens[alpha]-amylase at high resolution: implications for thermal stability

The crystal structure of Bacillus amyloliquefaciens[alpha]-amylase (BAA) at 1.4Å resolution revealed ambiguities in the thermal adaptation of homologous proteins in this family. The final model of BAA is composed of two molecules in a back-to-back orientation, which is likely to be a consequence of crystal packing. Despite a high degree of identity, comparison of the structure of BAA with those of other liquefying-type [alpha]-amylases indicated moderate discrepancies at the secondary-structural level. Moreover, a domain-displacement survey using anisotropic B-factor and domain-motion analyses implied a significant contribution of domain B to the total flexibility of BAA, while visual inspection of the structure superimposed with that of B. licheniformis[alpha]-amylase (BLA) indicated higher flexibility of the latter in the central domain A. Therefore, it is suggested that domain B may play an important role in liquefying [alpha]-amylases, as its rigidity offers a substantial improvement in thermostability in BLA compared with BAA.