Hydration
Water molecules are bound to proteins internally and externally. Some
water molecules occasionally occupy small internal cavities in the protein structure,
and are hydrogen-bonded to peptide bonds and side chains of the protein and
often to a prosthetic group, or cofactor, within the protein. The protein
surface islarge and consists of a mosaic of polar and nonpolar amino acids, and
it binds a large number of water molecules, i.e., it is hydrated, from the
surrounding environment. As described in the previous section, water molecules
trapped in the interior of protein molecules are bound more tightly to
hydrogen-bonding donors and acceptors because of a lower dielectric constant.
Solvent around the protein surface clearly has a general role in
hydrating peptide and side chains but might be expected to be rather mobile and
non-specific in its interactions. Well-ordered water mole-cules can make
significant contributions to protein stability. One water molecule can
hydrogen-bond to two groups distant in the primary structure on a protein
molecule, acting as a bridge between these groups. Such a water molecule may be
highly restricted in motion, and can contribute to the stability, at least
locally, of the protein, since such tight binding may exist only when these
groups assume the proper configuration to accommodate a water molecule that is
present only in the native state of the protein. Such hydration can also
decrease the flexibility of the groups involved.
There is also evidence for solvation over hydro-phobic groups on the
protein surface. So-called hydrophobic hydration occurs because of the
unfa-vorable nature of the interaction between water molecules and hydrophobic
surfaces, resulting in the clustering of water molecules. Since this clustering
is energetically unfavorable, such hydrophobic hydra-tion does not contribute
to the protein stability. However, this hydrophobic hydration facilitates hy-drophobic
interaction. This unfavorable hydration is diminished as the various
hydrophobic groups come in contact either intramolecularly or intermolecularly,
leading to the folding of intrachain structures or to protein–protein
interactions.
Both the loosely and strongly bound water molecules can have an
important impact, not only on protein stability but also on protein function.
For example, certain enzymes function in non-aqueous solvent provided that a
small amount of water, just enough to cover the protein surface, is present.
Bound water can modulate the dynamics of surface groups, and such dynamics may
be critical for enzyme function. Dried enzymes are, in general, inactive and
become active after they absorb 0.2 g water per g protein. This amount of water
is only sufficient to cover surface polar groups, yet may give sufficient
flexibility for function.
Evidence that water bound to protein molecules has a different property
from bulk water can be demonstrated by the presence of non-freezable water. Thus,
when a protein solution is cooled below –40LC, a
fraction of water, ~0.3 g water/g protein, does
notfreeze and can be detected by high resolution NMR. Several other techniques
also detect a similar amount of bound water. This unfreezable water reflects the
unique property of bound water that prevents it from adopting an ice structure.
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