Structure of Bacillus halmapalus alpha-amylase crystallized with and without the substrate analogue acarbose and maltose

Recombinant Bacillus halmapalus alpha-amylase (BHA) was studied in two different crystal forms. The first crystal form was obtained by crystallisation of BHA at room temperature in the presence of acarbose and maltose - data was collected at cryogenic temperatures to a resolution of 1.9 Å. It was found that the crystal belonged to the space group P212121 and had the unit cell dimensions: a = 47.0 Å, b = 73.5 Å and c = 151.1 Å. A maltose molecule was observed and found to bind to BHA and previous reports of the binding of a nonasaccharide were confirmed. The second crystal form was obtained by pH-induced crystallisation of BHA in a MES-HEPES-Boric acid-buffer (MHB-buffer) at 30oC - the solubility of BHA in MHB has a retrograde temperature dependency, and crystallisation of BHA was only possible by raising the temperature to at least 25oC. Data was collected at cryogenic temperatures to a resolution of 2.0 Å. The crystal belonged to the space group P212121 and had the unit cell dimensions: a = 38.6 Å, b = 59.0 Å and c = 209.8 Å. The structure was solved using molecular replacement. The maltose binding site is described and the two structures are compared. No significant changes were seen in the structure upon binding of the substrates.