Monoclinic crystal form of Aspergillus niger α-­amylase in complex with maltose at 1.8 Å resolution

Aspergillus niger α-amylase catalyses the hydrolysis of α-1,4-glucosidic bonds in starch. It shows 100% sequence identity to the A. oryzae homologue (also called TAKA-amylase), three crystal structures of which have been published to date. Two of them belong to the orthorhombic space group P2₁2₁2₁ with one molecule per asymmetric unit and one belongs to the monoclinic space group P2₁ with three molecules per asymmetric unit. Here, the purification, crystallization and structure determination of A. niger α-amylase crystallized in the monoclinic space group P2₁ with two molecules per asymmetric unit in complex with maltose at 1.8 Å resolution is reported. Furthermore, a novel 1.6 Å resolution orthorhombic crystal form (space group P2₁2₁2) of the native enzyme is presented. Four maltose molecules are observed in the maltose–α-amylase complex. Three of these occupy active-site subsites −2 and −1, +1 and +2 and the hitherto unobserved subsites +4 (Asp233, Gly234) and +5 (Asp235). The fourth maltose molecule binds at the distant binding sites d1 (Tyr382) and d2 (Trp385), also previously unobserved. Furthermore, it is shown that the active-site groove permits different binding modes of sugar units at subsites +1 and +2. This flexibility of the active-site cleft close to the catalytic centre might be needed for a productive binding of substrate chains and/or release of products.