500
Chaperones, Molecular
T
T
D
D
D
D
T
T
T
T
or
4
1
2
3
Fig. 3
The proposed reaction cycle of GroEL. GroEL is shown in cross-section, and
GroES is shown as a Flled ellipse. The complex at the apex of the cycle consists of
substrate protein encapsidated in the top (cis) ring of the GroEL complex, which has the
ATP (shown as T) bound, and is capped by GroES. The Frst reaction is the slow
hydrolysis of ATP to ADP (reaction 1), which has the effect of weakening the
GroES-GroEL interaction. During the time that this reaction occurs, folding of the
prote
inmaytakep
lacew
ithinthecav
ity.Inreaction2,ATPandanothermo
lecu
leof
substrate protein bind to the bottom (trans) ring. This binding results in the release of
GroES and ADP from the cis ring (reaction 3), and also the release of the bound
substrate, which may now be either folded or still in a partially folded state. The new
complex is now essentially the same as the starting one, as can be seen by flipping the
complex (step 4) except that the former cis ring is the new trans ring, and vice versa.
The Group II proteins lack a functional
GroES cochaperone, but in its place they
possess a large helical protrusion at the
top of the apical domain that forms a
lid to the complex in a way similar to
that formed by GroES. It is clear that
large domain movements occur in the
ATP-binding and hydrolysis cycle with the
Group II proteins, but their relationship
to the ability of this protein complex to
promote the folding of proteins is not yet
understood.
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