CRYSTAL-STRUCTURE OF YELLOW MEAL WORM ALPHA-AMYLASE AT 1.64 ANGSTROM RESOLUTION

The three-dimensional structure of the cl-amylase from Tenebrio molito r 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 i ndependent reflections in the 7.0 to 1.64 Angstrom resolution range, w ith root-mean-square deviations of 0.008 Angstrom for bond lengths and 1.482 degrees for bond angles. The final model comprises all 471 resi dues of TMA, 261 water molecules, one calcium cation and one chloride anion. The electron density confirms that the N-terminal glutamine res idue has undergone a post-transitional modification resulting in a sta ble 5-oxo-proline residue. The X-ray structure of TMA provides the fir st three-dimensional model of an insect alpha-amylase. The monomeric e nzyme exhibits an elongated share approximately 75 Angstrom x 46 Angst rom x 40 Angstrom and consists of three distinct domains, in line with models for a-amylases from microbial, plant and mammalian origin. How ever, the structure of TMA reflects in the substrate and inhibitor bin ding region a remarkable difference from mammalian cl-amylases: the la ck 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 o f various origins might explain the specificity of inhibitors directed exclusively against insect a-amylases. (C) 1998 Academic Press Limite d.