Mechanistic kinetic models of enzymatic cellulose hydrolysis—A review

Abstract

Bioconversion of lignocellulose forms the basis for renewable, advanced biofuels, and bioproducts. Mechanisms of hydrolysis of cellulose by cellulases have been actively studied for nearly 70 years with significant gains in understanding of the cellulolytic enzymes. Yet, a full mechanistic understanding of the hydrolysis reaction has been elusive. We present a review to highlight new insights gained since the most recent comprehensive review of cellulose hydrolysis kinetic models by Bansal et al. (2009) Biotechnol Adv 27:833-848. Recent models have taken a two-pronged approach to tackle the challenge of modeling the complex heterogeneous reaction-an enzyme-centric modeling approach centered on the molecularity of the cellulase-cellulose interactions to examine rate limiting elementary steps and a substrate-centric modeling approach aimed at capturing the limiting property of the insoluble cellulose substrate. Collectively, modeling results suggest that at the molecular-scale, how rapidly cellulases can bind productively (complexation) and release from cellulose (decomplexation) is limiting, while the overall hydrolysis rate is largely insensitive to the catalytic rate constant. The surface area of the insoluble substrate and the degrees of polymerization of the cellulose molecules in the reaction both limit initial hydrolysis rates only. Neither enzyme-centric models nor substrate-centric models can consistently capture hydrolysis time course at extended reaction times. Thus, questions of the true reaction limiting factors at extended reaction times and the role of complexation and decomplexation in rate limitation remain unresolved. Biotechnol. Bioeng. 2017;114: 1369-1385. © 2017 Wiley Periodicals, Inc.

The Role of Evolving Interfacial Substrate Properties on Heterogeneous Cellulose Hydrolysis Kinetics

“…. 3 Fraction of productive binding sites blocked by irreversibly bound Cel7A obtained experimentally ( Figure 5 and Equation 9).…”

“…For example, the enzymatic hydrolysis of insoluble cellulose has long been studied, yet understanding of the molecular origins of the rate slowdown of the enzymatic hydrolysis of cellulose remains incomplete. Product inhibition and enzyme instability only partially account for this slowdown 1 and the underlying mechanisms causing rate slowdown have been probed with numerous kinetic models [2][3] . Studies elucidating inherent enzyme properties contributing to rate limitations have pointed to the processivity 4-7 , on-rate [8][9][10] , or off -rate 5,7,[11][12][13][14] as inherent properties of cellulases that lead to hydrolysis slowdown, but these "enzyme-centric" models often fail to predict hydrolysis trends in their entirety 3 .…”

“…Further complicating our understanding of hydrolysis kinetics is the substantial effect of cellulose morphology on hydrolysis rates, and many studies conclude that rate loss is substrate limited 3,10,[26][27][28][29][30][31] . However, understanding of the role of inherent cellulose properties and enzymatic modification of the substrate in cellulose hydrolysis remains incomplete, with limited understanding of substrate properties affecting hydrolysis both initially and longer term.…”

Modeling the activity burst in the initial phase of cellulose hydrolysis by the processive cellobiohydrolase Cel7A

“…An important role in deciphering the mechanism of cellulose hydrolysis is played by mathematical modeling (Bansal, Hall, Realff, Lee, & Bommarius, ; Jeoh, Cardona, Karuna, Mudinoor, & Nill, ), including both the models based on ordinary differential equations (Bommarius et al, ; Griggs, Stickel, & Lischeske, ; Praestgaard et al, ) and on particle‐based simulations (Eibinger, Zahel, Ganner, Plank, & Nidetzky, ; Shang, Chang, & Chu, ; Shang & Chu, ; Warden, Little, & Haritos, ).…”

Structural and functional analyses of the cellulase transcription regulator CelR

“…Cellulases catalyze the hydrolysis of cellulose, a linear homopolymer of β‐1,4‐linked glucose, which is the primary structural polysaccharide of plant cell wall and the most abundant renewable biomass on the biosphere . Complete hydrolysis of cellulose yields a single easily fermentable sugar, and interest in lignocellulosic biomass as a renewable energy resource has spurred recent research into improving the activity of cellulases .…”