A malto-oligosaccharide forming thermostable acidic α-amylase from Bacillus stercoris YSP18 isolated from sediment of Sikkim hot spring: heterologous expression and characterisation

The current study deals with cloning and expressing a maltogenic α-amylase gene from thermophilic Bacillus stercoris YSP18 (AmyYSP) in Escherichia coli BL21 (DE3). AmyYSP belongs to the GH13_20 subfamily of Glycoside Hydrolases and entails five conserved regions found in maltogenic α-amylases. As a monomer of 67 kDa, AmyYSP exhibits maximal activity at 80°C, pH 5.0 and retains > 75% residual activity at 70–100°C and pH 5.0–8.0. The kinetic and thermodynamic studies displayed that it has a high affinity for soluble starch (K_m = 1.54 ± 0.236 mgmL^− 1), exhibits a longer half-life (38.5 h at 80°C and 8.88 h at 100°C), and a higher E_aD value of 3824 ± 1.03 kJ mol^− 1. It was characterised as a Ca²⁺-independent α-amylase, resistant to various denaturing additives. It hydrolyses soluble starch and raw corn starch efficiently, liberating glucose, maltose, maltotriose and maltotetraose as the main products. The thermostable and acid-stable, maltooligosaccharide forming AmyYSP is a versatile enzyme with prerequisites for successful application in starch-saccharification industries.