A new cyclodextrin glycosyltransferase from an alkaliphilic Bacillus agaradhaerens isolate: purification and characterisation

A new cyclodextrin glycosyltransferase (CGTase) from an alkaliphilic B. agaradhaerens strain LS-3C, isolated from an Ethiopian soda lake, was purified up to 43-fold by starch adsorption with a yield of 50%. The enzyme was a monomer with an estimated molecular weight of 110 kDa, representing the largest Bacillus CGTase reported so far. The isoelectric point (pI) of the enzyme was 6.9. The CGTase was stable over a very wide pH range, 5.0–11.4, at 25°C and was most active at pH 9.0. The enzyme exhibited an optimum temperature of 55°C and was stable up to 40°C for at least 1 h. Thermal stability could be improved in the presence of substrate, CaCl2, and to a lesser extent, by the product. The enzyme activity was stimulated by CaCl2 but was strongly inhibited by CuCl2, FeCl2, HgCl2, Fe(ClO4)3, and N-bromosuccinimide. The enzyme produced mainly β-CD (89% of the total cyclodextrin amount) with only α-CD as a minor product. The maximal conversion of maltodextrin to cyclodextrins varied between 10–15% depending on substrate concentration. Cyclodextrin formation was reduced with increase in enzyme concentration beyond 5 U/g substrate, and was marginal at 30 U/g suggesting the predominance of competing hydrolytic reactions catalysed by the enzyme.