53 Nordicom Review 35 (2014) Special Issue, pp. 53-63 Cosmopolitan Narratives Documentary and the Global ‘Other’

The plant enzyme hevamine has both chitinase and lysozyme activity. HPLC analysis of the products of the hydrolysis of chitopentaose shows that hevamine acts with retention of the configuration, despite the absence of a nucleophilic or stabilizing carboxylate. To analyze the stabilization of a putative oxocarbonium ion intermediate, the X-ray structure of hevamine complexed with the inhibitor allosamidin was determined at 1.85 Angstrom resolution. This structure supports the role of Glu127 as a proton donor. The allosamizoline group binds in the center of the active site, mimicking a reaction intermediate in which a positive charge at Cl is stabilized intramolecularly by the carbonyl oxygen of the N-acetyl group at C2

CRYSTAL-STRUCTURES OF HEVAMINE, A PLANT DEFENSE PROTEIN WITH CHITINASE AND LYSOZYME ACTIVITY, AND ITS COMPLEX WITH AN INHIBITOR

Background: Hevamine is a member of one of several families of plant chitinases and lysozymes that are important for plant defence against pathogenic bacteria and fungi. The enzyme can hydrolyze the linear polysaccharide chains of chitin and peptidoglycan. A full understanding of the structure/function relationships of chitinases might facilitate the production of transgenic plants with increased resistance towards a wide range of pathogens. Results: The crystal structure of hevamine has been determined to a resolution of 2.2 Angstrom, and refined to an R-factor of 0.169. The enzyme possesses a (beta alpha)(8)-barrel fold. An inhibitor binding study shows that the substrate-binding cleft is located at the carboxy-terminal end of the beta-barrel, near the conserved Glu127. Glu127 is in a position to act as the catalytic proton donor, but no residue that might stabilize a positively charged oxocarbonium ion intermediate was found. A likely mechanism of substrate hydrolysis is by direct attack of a water molecule on the C1 atom of the scissile bond, resulting in inversion of the configuration at C1. Conclusions: The structure of hevamine shows a completely new lysozyme/chitinase fold and represents a new class of polysaccharide-hydrolyzing (beta alpha)(8)-barrel enzymes. Because the residues conserved in the family to which hevamine belongs are important for maintaining the structure of the (beta alpha)(8)-barrel, all members of the family, including fungal, bacterial and insect chitinases, are likely to share this architecture. The crystal structure obtained provides a basis for protein engineering studies in this family of chitinases

X-RAY STUDIES REVEAL LANTHANIDE BINDING-SITES AT THE A/B5 INTERFACE OF ESCHERICHIA-COLI HEAT LABILE ENTEROTOXIN

The crystal structure determination of heat labile enterotoxin (LT) bound to two different lanthanide ions, erbium and samarium, revealed two distinct ion binding sites in the interface of the A subunit and the B pentamer of the toxin. One of the interface sites is conserved in the very similar cholera toxin sequence. These sites may be potential calcium binding sites. Erbium and samarium binding causes a change in the structure of LT: a rotation of the Al subunit of up to two degrees relative to the B pentamer