Crystal structure of yellow meal worm α-amylase at 1.64 Å resolution

The three-dimensional structure of the cl-amylase from Tenebrio molitor larvae (TMA) has been determined by molecular replacement techniques using diffraction data of a crystal of space group P2(1)2(1)2(1) (a = 51.24 Angstrom; b = 93.46 Angstrom; c =96.95 Angstrom). The structure has been refined to a crystallographic X-factor of 17.7% for 58,219 independent reflections in the 7.0 to 1.64 Angstrom resolution range, with root-mean-square deviations of 0.008 Angstrom for bond lengths and 1.482 degrees for bond angles. The final model comprises all 471 residues of TMA, 261 water molecules, one calcium cation and one chloride anion. The electron density confirms that the N-terminal glutamine residue has undergone a post-transitional modification resulting in a stable 5-oxo-proline residue. The X-ray structure of TMA provides the first three-dimensional model of an insect alpha-amylase. The monomeric enzyme exhibits an elongated share approximately 75 Angstrom x 46 Angstrom x 40 Angstrom and consists of three distinct domains, in line with models for a-amylases from microbial, plant and mammalian origin. However, the structure of TMA reflects in the substrate and inhibitor binding region a remarkable difference from mammalian cl-amylases: the lack of a highly flexible, glycine-rich loop, which has been proposed to be involved in a "trap-release' mechanism of substrate hydrolysis by mammalian a-amylases. The structural differences between a-amylases of various origins might explain the specificity of inhibitors directed exclusively against insect a-amylases. (C) 1998 Academic Press Limited.