Opening image: Chaperonin-60. View of monomer from inside the central cavity looking outward. The cpn60 subunit is tilted backward to provide a view of the base of the equatorial domain. This highly textured surface forms the interface between the two GroEL rings.

Each cpn60 subunit is organized into three domains: apical (red), middle (green), and equatorial (blue).

The tip of the equatorial domain is marked by the magenta N-terminal Ala residue. Toggle spacefilled model.

In the absence of ATP or ADP, each large subunit exists in the "closed" state shown here. The ATP binding site lies at the junction between the middle and equatorial domains. Binding of ATP causes the apical domain (red) to twist upwards.

Animate the allosteric transition to open conformation. [Note: The crystal structure was determined for the ADP·Mg²⁺ complex, not ATP.]

1. The central cavity doubles in volume.
2. Upward rotation of the apical domain creates a hydrophilic surface within the central cavity, releasing the misfolded protein from the grip of the hydrophobic binding sites.
3. Each GroEL subunit is now in the "open" or "L" conformation.
4. In the L conformation the hydrophobic segments create a docking site for the cpn10 heptamer.
5. The GroES lid binds to the top of the cavity, preventing escape of the entrapped protein substrate.

ATP hydrolysis occurs during the catalytic cycle, leading to dissociation of the lid and escape of the protein. Thus, the entrapped non-native protein substrate has 15 seconds to finish folding within the nonpolar cavity. Finally, dissociation of ADP occurs and the GroEL subunit returns to the "closed" conformation at end of the catalytic cycle. Animate the transition from "open" to "closed".

If we show all the intermediate conformations, we note most of the protein motion occurs in the apical domain. Note the relative rigidity of the equatorial domain, especially near the base and one side of the active site.

Toggle allosteric transition with spacefill models. Can you find the ADP?

During ATP binding, the GroEL subunit forms new noncovalent interactions with the ATP molecule, actually entrapping the ATP (or ADP) molecule. This is truly an excellent example of induced fit, where the substrate instructs the enzyme how to change its conformation as the substrate·enzyme complex forms.

Highlight hydrophobic patches that alternate as protein subunit binding sites or docking sites for the GroES lid.

The allosteric transition is cooperative, causing all seven cpn60 subunits to switch from the closed to open conformation upon ATP binding. This widens the entrance to the central cavity and, as the next tutorial shows, allows docking of the cpn10 cap at the rim, which traps the misfolded substrate inside a hydrophilic cage.