Transcript GroEL and GroES - ETH - D-INFK - TI

GroEL and GroES
By: Jim, Alan, John
Background
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Proteins fold spontaneously to
their native states, based on
information in their amino acid
sequence
Sometimes proteins fail to fold
and need help
Cells have developed
molecules that catalyse protein
folding called chaperones.
Molecular chaperones
supervise the state of newly
formed proteins, hold them to
the proper pathway of folding,
and keep them from improper
influences that might lead to
incorrect assembly.
Background
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“Chaperones act
catalytically to speed
up the process of
protein folding by
lowering the
activation barrier
between misfolded
and native states”
(Lesk 297)
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Chaperones
themselves contain
no information about
particular folding
patterns, rather they
anneal misfolded
proteins and allow
them to find their
native state.
GroEL-GroES
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GroEL-GroES contains 2 products of the GroE operon
 GroEL—L for large
 GroES—S for small
The active complex contains 14 copies of GroEL and 7
copies of GroES
In the presence of a nucleotide, GroEL and GroES form a
symmetrical
GroES7-GroEL14-GroES7 complex
GroEL Structure
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14 GroEL form 2 seven-fold
rings, packed back to back
Each ring surrounds an open
cavity to receive unfolded
proteins
The cavity is closed at the
bottom (with a wall between
the two rings) so the protein
cannot pass internally between
cavities
The 2 rings communicate with
allosteric structural changes
GroES Structure
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GroES subunits form another 7-membered ring
that caps one of the GroEL rings
The GroEL ring capped by GroES is called the cis
ring and the non-capped ring is the trans ring
GroEL-GroES Complex
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“Formation of the GroELGroES complex requires a
large and remarkable
conformational change in the
cis GroEL ring, changing the
interior surface of the cavity
from hydrophobic to
hydrophilic, and breaking the
symmetry between the two
GroEL rings.
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ATP is bound and broken
down to ADP+P for the
energy to make this change
GroEL-GroES Complex
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The enclosed cavity is the site
of protein folding
Misfolded proteins in the cavity
are given about 20 seconds to
refold. After 20 seconds, they
are expelled either refolded
successfully, or given another
chance to enter the same or a
different chaperon complex to
try again
A misfolded protein will be kept
in “solitary confinement” until it
has reformed correctly.
Swinging or Hinging Motion
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In order to do it’s work,
GroEL undergoes
conformation changes to its
domains: twisting and
bending along multiple axes
The apical domain “swings”
about 60º in a hinge motion,
hiding hydrophobic residues
inside the cavity.
A second hinge in the
equatorial domain swings to
lock in ATP and expose a
critical residue for
hydrolysis.
Rotational Motion
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The most important motion of GroEL is
the rotational motion of the apical
domain in order to expose a different
bounded surface to the inner GroEL
tube.
In its unbound form, the residues that
line the interior of the GroEL tube are
hydrophobic, or not willing to combine
with water molecules. In the bounded
form, they readily accept water
molecules.
The hydrophobic residues that formed
the lining become part of the intersubunit contacts, while the hydrophilic
residue surfaces are exposed.
Why is the Apical Domain Rotation
Important?
As stated previously, GroEL attempts to
allow misfolded proteins to refold
themselves.
 “The characteristic of misfolded proteins,
that renders them subject to non-specific
aggregation, is the surface exposure of
hydrophobic residues that are buried in the
native state.” (Lesk, 299)
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No, Really, Why is that Rotation So
Great?
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Proteins with this negative property bind to the
open form of GroEL. (Recall that in the open,
unbounded form, GroEL’s cavity is also
hydrophobic.) Then once the misfolded protein is
within GroEL’s inner tube, GroEL’s inner residues
become hydrophilic (having water affinity),
coaxing the protein to refold itself correctly.
The GroEL-GroES
Operational Cycle
•The operational cycle of the complex GroEL-GroES can best
be described through individual points.
•When capped by GroES, the GroEL rings have two different
states. The first is open, where the ring is available for the
reception of misfolded proteins. The second is closed, and
actually contains a misfolded protein.
When seen separate from GroES, the GroEL ring is in an open
state, allowing for the entry of proteins. The inside of the Gro-EL
ring has a flexible hydrophobic lining. This allows for the binding
of misfolded proteins, through hydrophobic and van der Waals
interactions. Throughout these processes, however, it is possible
for the protein to become unfolded partially, when in an incorrect
state.
With the binding of ATP and GroES, the cap (GroES) is ready for the ring (GroEL),
however, there must first be a conformational change in the cis ring. The ending result is
a closed cavity where the “substrate protein” can refold, once it is released from the
apical domains. It must also be taken away from potential aggregation partners.
When the cis ring undergoes the conformational change, it more than doubles the
available volume of the cavity. This allows for larger unfolding/refolding transition states
to exist.
The inside of the GroEL cavity also changes from hydrophobic to hydrophilic. This peels
the bound misfolded protein off of the surface and unfolds it even further.
“The burial of the original interior GroEL surface in intersubunit contacts within the
GroEL-GroES complex itself breaks the binding of the protein to the original hydrophobic
internal surface, leaving a macroclathrate complex.” This new complex (cavity) is
composed of a crystal lattice, housing the protein in the cavity.
Finally, hydrolysis of ATP in the cis ring allows for the weakening of the structure.
Binding of the ATP in the trans ring causes the disassembly of the cis assembly,
allowing for the release of the GroES and substrate protein. In the end, the ring is left
in original state.
It needs to be noted that through the process, there is a necessity of seven or
fourteen ATP molecules. In the end, the cost is much larger than the energy required
to unfold a protein. However, it needs to be stated that it this cost is infinitely smaller
than the death of a cell, and is also quite smaller than the synthesis of the
polypeptide chain.
GroEL-GroES
“The interaction
between GroEL and
GroES is necessary
for certain proteins to
fold under otherwise
non-permissive
conditions” (Lorimer
720).
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