Conversion of biomass to biofuels through sugar platform: A review of enzymatic hydrolysis highlighting the trade-off between product and substrate inhibitions

Specific combinations of enzymes depolymerize lignocellulosic biomass into respective monomers following various complex kinetic pathways. Conversion of biomass to biofuel through sugar platform is substantiated with specific kinetics controlling enzymatic hydrolysis in presence of unavoidable inhibitors. Some amount of lignin, still present in the delignified biomass broth acts as a substrate inhibitor for the specific group of cellulolytic enzymes during hydrolysis. Considering state-of-the-art research works, biomass to enzyme ratio is highlighted as one of the key factors in biomass hydrolysis, followed by time and agitation speed. Correlations between various kinetic parameters and different process variables are analysed in-depth using experimental outcomes as well as model predictions. Role of process parameters on hydrolysis outcomes, highlighting specific inhibition kinetics, deactivation of enzymes, and effect of mechanical agitation, is discussed thoroughly. Fractal kinetics and optimum product inhibition are also reviewed on a comparative basis. In order to understand the role of specific oligosaccharides on inhibition, the effects of mobility of sugar molecules, enzymatic trans-glycosylation, modes of hydrolysis (batch or fed-batch) on sugar yield are elaborated with respect to monomeric sugar supplements. Most research works have strong evidences to support that addition of other monosaccharides (excluding the end products) like glucose, galactose, mannose, fructose, of similar concentration, lowers the glucose yield to different degrees. Supplements of monomeric sugars, except xylose, increase the Michaelis-Menten [MM] constant, showing a reduced affinity of enzymes towards the substrate, through the competitive mode of inhibition. Xylose, however, exhibits a non-competitive type inhibition, through a strong affinity for the enzyme, enzyme-substrate, and enzyme-cellobiose complex. Most of the researchers focus on the removal of glucose as a primary end product in order to abate inhibition, except some periodically removed cellobiose. Cellobiohydrolase alone can generate cellobiose much more than endoglucanase alone. The synergistic action of different cellulolytic enzymes is found beneficial in terms of sugar generation. Product inhibition remains a crucial factor that needs to be overcome in order to attain a higher concentration of specific mono sugars.