Glucuronoyl esterases (GEs) are alpha/beta serine hydrolases and a relatively new addition in the toolbox to reduce the recalcitrance of lignocellulose, the biggest obstacle in cost-effective utilization of this important renewable resource. While biochemical and structural characterization of GEs have progressed greatly recently, there have yet been no mechanistic studies shedding light onto the rate-limiting steps relevant for biomass conversion. The bacterial GE OtCE15A possesses a classical yet distinctive catalytic machinery, with easily identifiable catalytic Ser/His completed by two acidic residues (Glu and Asp) rather than one as in the classical triad, and an Arg side chain participating in the oxyanion hole. By QM/MM calculations, we identified deacylation as the decisive step in catalysis, and quantified the role of Asp, Glu and Arg, showing the latter to be particularly important. The results agree well with experimental and structural data. We further calculated the free-energy barrier of post-catalysis dissociation from a complex natural substrate, suggesting that in industrial settings non-catalytic processes may constitute the rate-limiting step, and pointing to future directions for enzyme engineering in biomass utilization.
Title: Structural and biochemical studies of the glucuronoyl esterase OtCE15A illuminate its interaction with lignocellulosic components Mazurkewich S, Poulsen JN, Lo Leggio L, Larsbrink J Ref: Journal of Biological Chemistry, 294:19978, 2019 : PubMed
Glucuronoyl esterases (GEs) catalyze the cleavage of ester linkages between lignin and glucuronic acid moieties on glucuronoxylan in plant biomass. As such, GEs represent promising biochemical tools in industrial processing of these chemically recalcitrant materials. However, details on how GEs interact and catalyze degradation of their natural substrates are sparse, calling for thorough enzyme structure-function studies. GEs belong to carbohydrate esterase family 15 (CE15), which is part of the larger alpha/beta hydrolase superfamily. We present here a structural and mechanistic investigation of the bacterial GE OtCE15A. GEs contain a Ser-His-Asp/Glu catalytic triad, but the location of the catalytic acid in GEs is known to be variable, and OtCE15A possesses two putative catalytic acidic residues in its active site. Through site-directed mutagenesis, we demonstrate here that these residues are functionally redundant, possibly indicating the evolutionary route toward new functionalities within the CE15 family. Structures determined with the bound products glucuronate and galacturonate, as well as a covalently bound intermediate, provided insights into the catalytic mechanism of CE15. A structure of OtCE15A with the glucuronoxylooligosaccharide 2(3)-(4-O-methyl-alpha-D-glucuronyl)-xylotriose (XUX) disclosed that the enzyme can indeed interact with polysaccharides from the plant cell wall, and an additional structure with the disaccharide xylobiose revealed an enzyme surface binding site that might indicate a mechanism by which the enzyme recognizes long glucuronoxylan chains. These results indicate that OtCE15A, and likely most CE15 family enzymes, can utilize glucuronoxylooligosaccharide esters and support the proposal that these enzymes are active on lignin-carbohydrate complexes in plant biomass.
