Abstract
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 β-glucosidases. Rice Os4BGlu18 β-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 β-glucosidase, the structure of Os4BGlu18 was determined by X-ray crystallography. Crystals of Os4BGlu18 and its complex with δ-gluconolactone diffracted to 1.7 and 2.1 Å resolution, respectively. Two protein molecules were found in the asymmetric unit of the P212121 space group of their isomorphous crystals. The Os4BGlu18 structure exhibited the typical (β/α)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 β-glucosidases. Molecular docking studies of the Os4BGlu18 structure with monolignol substrate ligands placed the glycone in a similar position to the δ-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 β-glucosides. Thus, the structural basis of substrate recognition and hydrolysis by monolignol β-glucosidases was elucidated.
Highlights
Lignin has biological functions of strengthening and waterproofing plant cell walls, which differ in mechanical properties in specific cell types [1]
It is a natural polyphenolic polymer composed of the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, which are derived from oxidative polymerization of the corresponding monolignols, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, respectively [2]
It is well-known that monolignols are synthesized via the phenylalanine pathway and either directly secreted to the cell wall or glucosylated by uridine diphosphate glucosyltransferase to generate monolignol glucosides, including p-coumaryl alcohol glucoside, coniferin, and syringin [3,4,5,6]
Summary
Lignin has biological functions of strengthening and waterproofing plant cell walls, which differ in mechanical properties in specific cell types [1] It is a natural polyphenolic polymer composed of the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, which are derived from oxidative polymerization of the corresponding monolignols, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, respectively [2]. It is well-known that monolignols are synthesized via the phenylalanine pathway and either directly secreted to the cell wall or glucosylated by uridine diphosphate glucosyltransferase to generate monolignol glucosides, including p-coumaryl alcohol glucoside, coniferin, and syringin [3,4,5,6]. Monolignol β-glucosidase structure and can be found at the following: https://pdbj.org/ mine/summary/7D6A and https://pdbj.org/mine/ summary/7D6B
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