510
Chaperones, Molecular
chaperones discussed above, but it is not
signifcantly specifc about which cysteines
it oxidizes, so both correct and incorrect
isomers oF the same protein can be Formed.
The reduced DsbA must be reoxidized
For Further activity, a process that is
mediated by a membrane protein called
DsbB, which itselF interacts directly with
intermediates oF the oxidative electron
transport pathway to transFer the electrons
liberated in disulfde bond Formation to
quinones and ultimately to either oxygen
or, under anaerobic conditions, to other
electron acceptors.
The Formation oF incorrect disulfde-
bonded isomers by DsbA points to the
need For an isomerase protein that can
catalyze the breakage and reFormation oF
disulfde bonds in proteins until the correct
isomeric (and active) Form is reached. In
the
E. coli
periplasm, this role is perFormed
by the protein DsbC. Like protein disulfde
isomerase (PDI) (see below), this protein
has an innate chaperone activity, in that it
canrecognizeandbindunFo
ldedproteins
even when such proteins lack any cysteine
residues. It seems likely that DsbC acts
to bind incorrectly Folded proteins, and
then breaks the disulfdes iF they are
present. Disulfdes may then be reFormed
either by subsequent action oF DsbA, or
by spontaneous reoxidation oF the reduced
cysteines. In contrast to DsbA, thereFore,
DsbC must be maintained in a reduced
state, and this is mediated by another
membrane protein called DsbD. DsbD
is in turn maintained in a reduced state
by interacting with the cytosolic protein
thioredoxin, which itselF is kept reduced
by metabolically generated NADPH.
The site For disulfde bond Formation
and isomerization in eukaryotes is the
lumen oF the endoplasmic reticulum. As
is the case with bacteria, disulfde bond
Formation in newly imported proteins is an
enzyme-catalyzed process, the enzyme in
this case being protein disulfde isomerase.
PDI is by Far the most abundant chaperone
in the ER. As with bacteria, a membrane
protein
(Ero1p)
exists
to
restore
the
active (oxidized) state oF PDI aFter the
Formation oF disulfdes has taken place.
Reduced Ero1p in turn is reoxidized
directly by oxygen, though its activity is also
modulated by ±AD. In addition to acting
as an oxidase, PDI can shuFfle the disulfde
bonds in a protein aFter they have Formed
(hence its name), so it eFFectively combines
the roles oF DsbA and DsbC. As with DsbC,
it is capable oF showing chaperone activity,
and so presumably can recognize proteins
in which the disulfde bonds are not in
the correct position For complete Folding.
The catalytic site and chaperone site are
not the same and indeed are on diFFerent
domains oF the protein. This activity is
more important in eukaryotic cells than
in prokaryotic ones, as eukaryotic proteins
have much larger numbers oF disulfde
bonds on average, and hence the possibility
oF Forming incorrect disulfde bonds is
correspondingly higher.
5.1.2
Hsp33: A Chaperone with a Redox
Switch
An intriguing chaperone protein is Found
in
E. coli
, with homologs in many other
species, which is heat shock induced but
active only when oxidized. This protein,
Hsp33 or HslO, binds very tightly to
zinc ions when reduced, coordinating the
zinc with Four cysteine residues. Under
relatively mild oxidizing conditions, the
zinc is released and the cysteines Form
two disulfde bonds. This release causes a
substantial change in the conFormation oF
the protein, which signifcantly increases
the amount oF exposed hydrophobic area
oF the protein. In this state, Hsp33 acts as
a ‘‘holder’’ rather like the sHSPs, and has
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