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Enzyme Microheterogeneous Hydration and Stabilization in Supercritical Carbon Dioxide
journal contribution
posted on 2012-05-17, 00:00 authored by Rodrigo
L. Silveira, Julian Martínez, Munir S. Skaf, Leandro MartínezSupercritical carbon dioxide is a promising green-chemistry
solvent
for many enzyme-catalyzed chemical reactions, yet the striking stability
of some enzymes in such unconventional environments is not well understood.
Here, we investigate the stabilization of the Candida
antarctica Lipase B (CALB) in supercritical carbon
dioxide–water biphasic systems using molecular dynamics simulations.
The preservation of the enzyme structure and optimal activity depend
on the presence of small amounts of water in the supercritical dispersing
medium. When the protein is at least partially hydrated, water molecules
bind to specific sites on the enzyme surface and prevent carbon dioxide
from penetrating its catalytic core. Strikingly, water and supercritical
carbon dioxide cover the protein surface quite heterogeneously. In
the first solvation layer, the hydrophilic residues at the surface
of the protein are able to pin down patches of water, whereas carbon
dioxide solvates preferentially hydrophobic surface residues. In the
outer solvation shells, water molecules tend to cluster predominantly
on top of the larger water patches of the first solvation layer instead
of spreading evenly around the remainder of the protein surface. For
CALB, this exposes the substrate-binding region of the enzyme to carbon
dioxide, possibly facilitating diffusion of nonpolar substrates into
the catalytic funnel. Therefore, by means of microheterogeneous solvation,
enhanced accessibility of hydrophobic substrates to the active site can be
achieved, while preserving the functional structure of the enzyme.
Our results provide a molecular picture on the nature of the stability
of proteins in nonaqueous media.