Biochemical and molecular characterization of recombinant acidic and thermostable raw-starch hydrolysing α-amylase from an extreme thermophile Geobacillus thermoleovorans

A gene encoding acidic, thermostable and raw starch hydrolysing α-amylase was cloned from an extreme thermophile Geobacillus thermoleovorans and expressed. The ORF of 1650 bp encodes a 515 amino acid protein (Gt-amy) with a signal peptide of 34 amino acids at the N-terminus. Seven conserved sequences of GH-13 family have been found in its sequence. The specific enzyme activity of recombinant Gt-amy is 1723 U mg−1 protein with a molecular mass of 59 kDa. It is optimally active at pH 5.0 and 80 °C with t1/2 values of 283, 184 and 56 min at 70, 80 and 90 °C, respectively. The activation energy required for its temperature deactivation is 84.96 kJ mol−1. Ca2+ strongly inhibits Gt-amy at 10 mM concentration, and inhibition kinetics with Ca2+ reveals that inhibition occurs as a result of binding to a lower affinity secondary Ca2+ binding site in the active centre in a mixed-type inhibition manner. The Km and kcat of the Gt-amy are 0.315 mg mL−1 and 2.62 × 103 s−1, respectively. Gt-amy is Ca2+-independent at the concentration used in industrial starch saccharification, and hydrolyses raw corn and wheat starches efficiently, and thus, is applicable in starch saccharification at the industrial sub-gelatinization temperatures.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Expressed a gene encoding acidic thermostable α-amylase from G. thermoleovorans. ► This is the first ever reported Ca2+-inhibited acidic α-amylase. ► Biochemical characterization of the recombinant enzyme (Gt-amy). ► Gt-amy is Ca2+-independent at the concentration used in starch saccharification. ► Gt-amy saccharifies raw starches.