• α-amylase;
  • Aspergillus niger;
  • maltose;
  • Aspergillus oryzae;
  • TAKA-amylase.

Aspergillus nigerα-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. nigerα-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.