Crystal structure of a monocotyledon (maize ZMGlu1) beta-glucosidase and amodel of its complex with p-nitrophenyl beta-D-thioglucoside

The maize a-glucosidase isoenzymes ZMGlu1 and ZMGlu2 hydrolyse the abundant natural substrate DIMBOAGlc (2-O-beta -D-glucopyranosyl-4-hydroxy-7-methox y-1,4-benzoxazin-3-one), whose aglycone DIMBOA (2,4-hydroxy-7-methoxy-1,4-b enzoxazin-3-one) is the major defence chemical protecting seedlings and you ng plant parts against herbivores and other pests. The two isoenzymes hydro lyse DIMBOAGlc with similar kinetics but differ from each other and their s orghum homologues with respect to specificity towards other substrates. To gain insights into the mechanism of substrate (i.e. aglycone) specificity b etween the two maize isoenzymes and their sorghum homologues, ZMGlu1 was pr oduced in Escherichia coli, purified, crystallized and its structure solved at 2.5 Angstrom resolution by X-ray crystallography. In addition, the comp lex of ZMGlu1 with the non-hydrolysable inhibitor p-nitrophenyl beta -D-thi oglucoside was crystallized and, based on the partial electron density, a m odel for the inhibitor molecule within the active site is proposed. The inh ibitor is located in a slot-like active site where its aromatic aglycone is held by stacking interactions with Trp-378. Whereas some of the atoms on t he non-reducing end of the glucose moiety can be modelled on the basis of t he electron density, most of the inhibitor atoms are highly disordered. Thi s is attributed to the requirement of the enzyme to accommodate two differe nt species, namely the substrate in its ground state and in its distorted c onformation, for catalysis.