it may be that such conformations cannot be reached because of the
intricacies of the folding process. It is by no means certain that the native
conformation of a protein is the one of lowest free energy possible; the
conformation may just represent a local minimum.
There are some mechanisms thatenhance conformational stability:
Formation of 22 S 22 S 22 bridges between Cys residues adjacent in the
folded state; these would anyway prevent complete unfolding.
Other posttranslational modifications, notably glycosylation of
specific residues, presumably after folding of the peptide chain.
Formation of structures on a larger scale than that of a domain (e.g.,
‘‘domain pairing’’ or ‘‘subunit docking’’).
Binding of ligands mostly alters the conformation somewhat, but then
generally stabilizes it.
Complications. The discussion given above is to some extent an
oversimplification. The following complications may arise.
- The protein consists ofmore than one domainand these can unfold
independently of each other. A fairly simple example is in Figure 7.6b. In
other cases, deconvolution of the DSC diagram may be less straightforward.
In some proteins, two (or three) domains are so closely similar that they
unfold (almost) simultaneously. In such a case,DHobtained from a DSC
diagram would be about twice (or three times) the value derived from a
van’t Hoff plot. Serum albumin (molar mass 66 kDa), for example, consists
of three similar domains of about 190 residues. Mutual interaction of
domain unfolding may occur, but in most cases each domain unfolds just
like a small globular protein. - There may beintermediate stages between the native and the
unfolded state. In some cases a transient and not very stable ‘‘prefolded
state’’ is observed, which appears to be similar to that mentioned next. - Some proteins exhibit at some pH values, for instance, about three
units below the isoelectric pH, an intermediate conformation designated
‘‘molten globule state’’. Such a state appears to be characterized by
Being not fixed, the molecules showing a population of states;
A hydrodynamic size somewhat larger than in the native state, but far
smaller than in the unfolded conformation;
A large amount of secondary structure, but any tertiary structure
appears to be fluctuating and many residues are in contact with
water;
A conformational entropy that is almost the same as that in the
unfolded state, which seems difficult to reconcile with the
abundance of secondary structure;