Crystal structure of <i>Klebsiella pneumoniae</i> maltohexaose-producing α-amylase

The α-amylase from Klebsiella pneumoniae (KpAmy13), which belongs to glycoside hydrolase family 13 subfamily 19, produces maltohexaose as an initial product when acting on starch and has been characterized as a maltohexaose-producing α-amylase. The crystal structure of KpAmy13 was determined at a resolution of 1.9 Å, revealing the structures of all its domains: N, A, B and C. Domain N resembles the starch-binding domain known as carbohydrate-binding module family 69, found in α-glucan-related proteins. Although domain N does not conserve the starch-binding residues observed in other proteins, it has several hydrophobic residues on its surface, which might be involved in promoting catalysis. The catalytic cleft is located at the bottom of a circular depression. The domain N-truncated mutant of KpAmy13 in complex with maltohexaose showed that its non-reducing end glucose docks at subsite -6. The long and complex structure of domain B contributes to forming a cleft of the right size for six glucose moieties, demonstrating the exo-acting mechanism.