Lactobacillus plantarum S21 α-amylase possesses 475 amino acids at the C-terminal region identified as the starch-binding domain (SBD) and has been previously reported to play a role in raw starch degradation. To understand the specific roles of this SBD, cloning and expression of the complete (AmyL9) and C-terminally truncated (AmyL9ΔSBD) forms of α-amylase were conducted for enzyme purification and comparative characterization. AmyL9 and AmyL9ΔSBD were overproduced in Escherichia coli at approximately 10- and 20-times increased values of volumetric productivity when compared to α-amylase produced by the wild type, respectively. AmyL9ΔSBD was unable to hydrolyze raw starch and exhibited substrate specificity in a similar manner to that of AmyL9, but it was weakly active toward amylopectin and glycogen. The hydrolysis products obtained from the amylaceous substrates of both enzymes were the same. In addition, AmyL9ΔSBD showed comparatively higher Km values than AmyL9 when it reacted with starch and amylopectin, and lower values for other kinetic constants namely vmax, kcat, and kcat/Km. The results indicated that the C-terminal SBDs of L. plantarum S21 α-amylase contribute to not only substrate preference but also substrate affinity and the catalytic efficiency of the α-amylase without any changes in the degradation mechanisms of the enzyme.
Keywords: Alpha-amylase; C-terminal truncation; Lactobacillus; Raw starch; Saccharifying; Starch-binding domain.
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