Structural Analysis Of Rice Os4bglu18 Monolignol Beta-Glucosidase

Monolignol glucosides are storage forms of monolignols, which are polymerized to lignin to strengthen plant cell walls. The conversion of monolignol glucosides to monolignols is catalyzed by monolignol beta -glucosidases. Rice Os4BGlu18 beta -glucosidase catalyzes hydrolysis of the monolignol glucosides, coniferin, syringin, and p-coumaryl alcohol glucoside more efficiently than other natural substrates. To understand more clearly the basis for substrate specificity of a monolignol beta -glucosidase, the structure of Os4BGlu18 was determined by X-ray crystallography. Crystals of Os4BGlu18 and its complex with delta -gluconolactone diffracted to 1.7 and 2.1 angstrom resolution, respectively. Two protein molecules were found in the asymmetric unit of the P2(1)2(1)2(1) space group of their isomorphous crystals. The Os4BGlu18 structure exhibited the typical (beta/alpha)(8) TIM barrel of glycoside hydrolase family 1 (GH1), but the four variable loops and two disulfide bonds appeared significantly different from other known structures of GH1 beta -glucosidases. Molecular docking studies of the Os4BGlu18 structure with monolignol substrate ligands placed the glycone in a similar position to the delta -gluconolactone in the complex structure and revealed the interactions between protein and ligands. Molecular docking, multiple sequence alignment, and homology modeling identified amino acid residues at the aglycone-binding site involved in substrate specificity for monolignol beta -glucosides. Thus, the structural basis of substrate recognition and hydrolysis by monolignol beta -glucosidases was elucidated.