Thermostable and alkalophilic maltogenic amylase of Bacillus thermoalkalophilus ET2 in monomer-dimer equilibrium

A gene encoding a thermostable and alkalophilic maltogenic amylase (BTMA) was cloned from the thermophilic bacterium Bacillus thermoalkalophilus ET2. BTMA was composed of 588 amino acids with a predicted molecular mass of 68.8 kDa. The enzyme had an optimal temperature and pH of 70 degrees C and 8, respectively, the highest among maltogenic amylases reported so far. The Tin of BTMA at pH 8 was 76.7 degrees C with an enthalpy of 113.6 kJ mol(-1). Both hydrolysis and transglycosylation activities for various carbohydrates were evident. beta-Cyclodextrin (beta-CD) and soluble starch were hydrolyzed mainly to maltose, and pullulan to panose. Acarbose, a strong amylase inhibitor, was hydrolyzed by BTMA to glucose and acarviosine-glucose. The K-m and k(cat) values of BTMA for beta-CD hydrolysis were 0.128 mM and 165.8 s(-1) mM, respectively. The overall catalytic efficiency (k(cat)/K-m) of the enzyme was highest toward beta-CD. BTMA was present in a monomer-dimer equilibrium with a molar ratio of 54:46 in 50 mM glycine-NaOH buffer (pH 8.0). This equilibrium could be affected by KCl and enzyme concentrations. The multi-substrate specificity of the enzyme was modulated by the structural differences between monomeric and dimeric forms. Starch was hydrolyzed more readily when monomeric BTMA was prevalent, while the opposite was observed for beta-CD.