Enhancing the α-Cyclodextrin Specificity of Cyclodextrin Glycosyltransferase from Paenibacillus macerans by Mutagenesis Masking Subsite -7.

Cyclodextrin glycosyltransferases (CGTases) (EC 2.4.1.19) catalyze the conversion of starch or starch derivates into mixtures of α-, β-, and γ-cyclodextrins. Because time-consuming and expensive purification procedures hinder the widespread application of single-ingredient cyclodextrins, enzymes with enhanced specificity are needed. In this study, we tested the hypothesis that the α-cyclodextrin selectivity of Paenibacillus macerans α-CGTase could be augmented by masking subsite -7 of the active site, blocking the formation of larger cyclodextrins, particularly β-cyclodextrin. Five single mutants and three double mutants designed to remove hydrogen-bonding interactions between the enzyme and substrate at subsite -7 were constructed and characterized in detail. Although the rates of α-cyclodextrin formation varied only modestly, the rate of β-cyclodextrin formation decreased dramatically in these mutants. The increase in α-cyclodextrin selectivity was directly proportional to the increase in the ratio of their kcat values for α- and β-cyclodextrin formation. The R146A/D147P and R146P/D147A double mutants exhibited ratios of α-cyclodextrin to total cyclodextrin production of 75.1% and 76.1%, approximately one-fifth greater than that of the wild-type enzyme (63.2%), without loss of thermostability. Thus, these double mutants may be more suitable for the industrial production of α-cyclodextrin than the wild-type enzyme. The production of β-cyclodextrin by these mutants was almost identical to their production of γ-cyclodextrin, which was unaffected by the mutations in subsite -7, suggesting that subsite -7 was effectively blocked by these mutations. Further increases in α-cyclodextrin selectivity will require identification of the mechanism or mechanisms by which these small quantities of larger cyclodextrins are formed.