38
Aggregation, Protein
The hydrolysis of ATP by GroEL is used
only to induce conformational changes
of the chaperone, which permits the re-
lease of the folded protein. The molecular
chaperones, by their transient association
through
hydrophobic
interactions
with
nascent,
stress-destabilized,
or
translo-
cated proteins, have a role in preventing
improper folding and subsequent aggre-
gation. They do not interact with folded
proteins. They do not carry information
capable of directing a protein to assume a
structure different from that dictated by its
amino acid sequence. Therefore, molec-
ular chaperones assist the folding in the
cells without violation of the AnFnsen pos-
tulate. They increase the yield but not the
rate of folding reactions; in this respect
they do not act as catalysts. ±urthermore,
the majority of newly synthesized polypep-
tide chains in both bacterial and eukaryotic
cells fold spontaneously without the assis-
tance of molecular chaperones.
Many proteins from prokaryotic and
eukaryotic organisms are produced with
an amino-terminal propeptide, which is
removed by limited proteolysis during
the activation process. Several of these
propeptides consist of a long polypeptide
chain; for example, there are 174 amino
acids in the propeptide of pro-
α
-lytic pro-
tease, 91 in that of procarboxypeptidase
Y, and 77 in that of prosubtilisin. Several
studies have shown that the propeptide
is required for proper folding of these
proteins. The mature enzymes are not
able to refold correctly. They seem to
have kinetic stability only, whereas the
proenzymes have thermodynamic stabil-
ity. Since propeptides perform the function
of mediating protein folding, they have
been classiFed as intramolecular chaper-
ones. However, this terminology is not
appropriate since the nascent protein is
thep
roenzyme
,no
ttheenzymetha
thas
undergone proteolytic cleavage. Thus, it
is not surprising that the proenzyme re-
folds spontaneously, whereas the mature
protein does not. Indeed, the information
is contained in the totality of the proen-
zyme sequence.
Two
other
classes
of
proteins
play
the role of helpers during protein fold-
ing
in vivo
: protein disulFde isomerases
(PDIs) and peptidyl–prolyl cis – trans iso-
merases. Protein disulFde isomerase is an
abundant component of the lumen of the
endoplasmic reticulum in secretory cells.
The enzyme was discovered independently
in 1963 by two research groups: in rat
and ox by AnFnsen and coworkers, and
in chicken and pigeon pancreas by Straub
and coworkers. Proteins destined to be se-
creted enter the endoplasmic reticulum in
an unfolded state. In this environment,
the folding process is associated with the
formation of disulFde bonds, which is
catalyzed by PDI through thiol–disulFde
interchange. The Frst PDI cDNA was se-
quenced in 1985 by Edman et al. It displays
sequence homologies implying a multido-
main architecture. PDI consists of four
structural domains arranged in the order
a, b, b
0
,a
0
,w
i
thth
eb
0
and a
0
domains
being connected by a linker region. ±ur-
thermore, it possesses an acidic C-terminal
extension. The a and a
0
domains contain
the active site motif – W-C-G-H-C-. They
display signiFcant sequence identity to
thioredoxin, a small cytoplasmic protein
involved in several redox functions, and
they have a similar active site sequence.
Recombinants of the a and b domains
have been obtained and studied by high-
resolution NMR. The a domain has the
same overall fold as thioredoxin, an
α/β
fold with a central core made up of a
Fve-stranded
β
-sheet surrounded by four
helices. As in thioredoxin, the active site
is located at
the
N-terminus of helix
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