Endoglucanase Peripheral Loops Facilitate Complexation
of Glucan Chains on Cellulose via Adaptive Coupling to the Emergent
Substrate Structures
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Abstract
We examine how the catalytic domain
of a glycoside hydrolase family
7 endoglucanase catalytic domain (Cel7B CD) facilitates complexation
of cellulose chains from a crystal surface. With direct relevance
to the science of biofuel production, this problem also represents
a model system of biopolymer processing by proteins in Nature. Interactions
of Cel7B CD with a cellulose microfibril along different paths of
complexation are characterized by mapping the atomistic fluctuations
recorded in free-energy simulations onto the parameters of a coarse-grain
model. The resulting patterns of protein–biopolymer couplings
also uncover the sequence signatures of the enzyme in peeling off
glucan chains from the microfibril substrate. We show that the semiopen
active site of Cel7B CD exhibits similar barriers and free energies
of complexation over two distinct routes; namely, scooping of a chain
into the active-site cleft and threading from the chain end into the
channel. On the other hand, the complexation energetics strongly depends
on the surface packing of the targeted chain and the resulting interaction
sites with the enzyme. A revealed principle is that Cel7B CD facilitates
cellulose deconstruction via adaptive coupling to the emergent substrate.
The flexible, peripheral segments of the protein outside of the active-site
cleft are able to accommodate the varying features of cellulose along
the simulated paths of complexation. The general strategy of linking
physics-based molecular interactions to protein sequence could also
be helpful in elucidating how other protein machines process biopolymers