Slowdown of Enzymatic Cellulose Conversion Emerges from Cellulase Mode of Action

TL;DR

This study explains why cellulose conversion slows down during hydrolysis and how enzyme behavior affects this slowdown, offering insights for improving biofuel production.

Contribution

The paper identifies system-specific slowdown mechanisms in cellulose conversion driven by enzyme mode of action and substrate nanomechanics.

Findings

Fungal cellulases cause slowdown by exposing a stiffer inner core of cellulose fibrils.

Cellulosomes stall degradation due to irreversible binding and low enzyme adsorption dynamics.

Substrate nanomechanics and enzyme adsorption dynamics interplay to dictate conversion efficiency.

Abstract

The problem of early
slowing of the conversion rate in
cellulose
hydrolysis, and the resulting large amounts of cellulase enzymes required
to achieve just useful conversion efficiencies, remains a major obstacle
in the development of cellulosic biofuels. While numerous studies
have implicated both substrate and enzyme factors, the underlying
mechanism of this rate limitation has remained unresolved. Here, using
bacterial cellulose as a well-defined model substrate, we demonstrate
that the reaction slowdown emerges from the specific mode of substrate
degradation imposed by the cellulolytic enzyme system. We introduce
nanomechanical mapping by time-lapse in situ atomic force microscopy
to characterize at nanometer spatial resolution the change in surface
material organization of cellulose due to enzymatic degradation. The
layer-by-layer ablation of surface material utilized by fungal cellulases
results in the gradual exposure of the nanomechanically stiffer (i.e.,
more densely organized and hence more resistant) inner core of the
cellulose fibrils, which leads to a rapid decline in the conversion
rate by these enzymes. Cellulases assembled into stable complexes
(the cellulosome) bind almost irreversibly to cellulose. Low dynamics
of their adsorption causes stalling of the cellulose degradation as
the portion of unproductively bound enzymes increases during the conversion.
Together, these findings reveal distinct, system-specific slowdown
mechanisms and uncover a functional interplay between substrate nanomechanics
and enzyme adsorption dynamics that dictates overall conversion efficiency.
By disentangling these coupled rate-limiting factors, this work establishes
previously unrecognized molecular design principles for engineering
cellulase systems capable of overcoming the intrinsic substrate recalcitrance
of crystalline cellulose.

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