Protein Folding

Protein Folding - IV

Here is how these chaperonins work: [Lehninger, p. 144]

DnaJ and DnaK bind to stretches of hydrophobic residues along the unfolded or misfolded protein chain
The energy for repeated cycles of binding and release comes from hydrolysis of ATP
Both chaperones can release their bound protein to a third chaperone, GroEL

GroEL has been called a folding machine. It provides a chamber formed of flexible subunits that binds and then twists proteins that have fallen into non-native states.

An excellent review: Saibil and Ransom, Trends Biochem. Sci., 2002, 27, 627.

GroEL is a tetradecamer of identical subunits, each having molecular weight 57 kiloDaltons. In the unactivated, so-called T (tense) state, it looks like this:

GroEL (two subunits), Looking Down the Cylinder (1oel)	GroEL (two subunits), From the Side

GroEL binds denatured proteins very tightly because of the high affinity of its hydrophobic pocket for their hydrophobic side chains; further unfolding is facilitated.

Seven molecules of ATP bind to GroEL, initiating a conformational change that elongates the cylinder:

GroEL (full) + ATP, top view (1der)	GroEL (full) + ATP, side view (ATP in magenta/blue)

GroEL Showing the Multiple Chains

Here is another view of the conformational change, taken from the review mentioned above:

In (a) and (b), the surface of GroEL from cryo-electronmicroscopy is shown with three of the subunits docked in. The circle calls attention to a change in contacts among the domains as ATP binds.

Pictures (c) and (d) look down the GroEL cylinder; very substantial changes are visible in the shape of the interior hole and the exterior surface.

Again, the circle calls attention to the change in contacts between domains.

A second (assistant?) chaperone, GroES, now binds to the end of the GroEL cylinder proximate to the bound ATP, producing a hollow, bullet-shaped structure. This is the D (relaxed) state.

GroEL + GroES, Side View (1aon)	GroEL + GroES, Into the Cavity

The combination of ATP and GroES binding causes the cavity in GroEL to enlarge.

The conformational change also changes the lining of the cavity from hydrophobic to hydrophilic

Cryo-Electron Micrographs from Helen Saibil's group at Birkbeck College, London

The Gerstein group at Yale has prepared a number of "molecular movies" that also show these conformational changes.
The action of the chaperone is something like this:
- ATP binds in one chamber of GroEL
- Then the misfolded protein binds in that chamber; the GroES cap binds
- Hydrolysis of ATP provides the energy to change the interior residues and refold the captured protein.
- Binding of ATP in the other chamber causes a conformational change releasing the GroES and the refolded protein
- The cycle repeats
Effectively, the GroEL gives misfolded proteins a second, third, fourth... chance to get it right.
A simplified version of the scheme looks like this: [Lehninger, p. 145]

At each annealing, a proportion of the unfolded species will "get it right", leaving fewer wrongly folded molecules to be annealed in the next cycle.
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