Selecting β-glucosidases to support cellulases in cellulose saccharification

Hele Teugjas,Priit Väljamäe

doi:10.1186/1754-6834-6-105

Abstract

BackgroundEnzyme end-product inhibition is a major challenge in the hydrolysis of lignocellulose at a high dry matter consistency. β-glucosidases (BGs) hydrolyze cellobiose into two molecules of glucose, thereby relieving the product inhibition of cellobiohydrolases (CBHs). However, BG inhibition by glucose will eventually lead to the accumulation of cellobiose and the inhibition of CBHs. Therefore, the kinetic properties of candidate BGs must meet the requirements determined by both the kinetic properties of CBHs and the set-up of the hydrolysis process.ResultsThe kinetics of cellobiose hydrolysis and glucose inhibition of thermostable BGs from Acremonium thermophilum (AtBG3) and Thermoascus aurantiacus (TaBG3) was studied and compared to Aspergillus sp. BG purified from Novozyme®188 (N188BG). The most efficient cellobiose hydrolysis was achieved with TaBG3, followed by AtBG3 and N188BG, whereas the enzyme most sensitive to glucose inhibition was AtBG3, followed by TaBG3 and N188BG. The use of higher temperatures had an advantage in both increasing the catalytic efficiency and relieving the product inhibition of the enzymes. Our data, together with data from a literature survey, revealed a trade-off between the strength of glucose inhibition and the affinity for cellobiose; therefore, glucose-tolerant BGs tend to have low specificity constants for cellobiose hydrolysis. However, although a high specificity constant is always an advantage, in separate hydrolysis and fermentation, the priority may be given to a higher tolerance to glucose inhibition.ConclusionsThe specificity constant for cellobiose hydrolysis and the inhibition constant for glucose are the most important kinetic parameters in selecting BGs to support cellulases in cellulose hydrolysis.

Highlights

Enzyme end-product inhibition is a major challenge in the hydrolysis of lignocellulose at a high dry matter consistency. β-glucosidases (BGs) hydrolyze cellobiose into two molecules of glucose, thereby relieving the product inhibition of cellobiohydrolases (CBHs)
Kinetics of cellobiose hydrolysis The hydrolysis of cellobiose by by β-glucosidases (BGs), AtBG3, TaBG3 and BG purified from Novozyme®188 (N188BG) was monitored by measuring the initial rates of glucose formation
The use of higher temperatures had the advantage of both increasing the catalytic efficiency and relieving the glucose inhibition of BGs

Summary

Introduction

Enzyme end-product inhibition is a major challenge in the hydrolysis of lignocellulose at a high dry matter consistency. β-glucosidases (BGs) hydrolyze cellobiose into two molecules of glucose, thereby relieving the product inhibition of cellobiohydrolases (CBHs). Enzyme end-product inhibition is a major challenge in the hydrolysis of lignocellulose at a high dry matter consistency. Β-glucosidases (BGs) hydrolyze cellobiose into two molecules of glucose, thereby relieving the product inhibition of cellobiohydrolases (CBHs). BG inhibition by glucose will eventually lead to the accumulation of cellobiose and the inhibition of CBHs. the kinetic properties of candidate BGs must meet the requirements determined by both the kinetic properties of CBHs and the set-up of the hydrolysis process. The major components of fungal cellulase systems are cellobiohydrolases (CBHs), exo-acting enzymes that processively release consecutive cellobiose units from cellulose chain ends. Which may be attacked by water (hydrolysis) or by a hydroxyl group of the substrate (transglycosylation). In addition to the substrate, attack by other nucleophiles, such as alcohols, can lead to transglycosylation [9]. Transglycosylation is under kinetic control, meaning that all cellobiose and transglycosylation products will eventually be hydrolyzed to glucose

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