Multiple equilibria of the Escherichia coli chaperonin GroES revealed by mass spectrometry

Nanospray time‐of‐flight mass spectrometry has been used to study the assembly of the heptamer of the Escherichia coli cochaperonin protein GroES, a system previously described as a monomer–heptamer equilibrium. In addition to the monomers and heptamers, we have found measurable amounts of dimers and hexamers, the presence of which suggests the following mechanism for heptamer assembly: 2 Monomers ↔ Dimer; 3 Dimers ↔ Hexamer; Hexamer + Monomer ↔ Heptamer. Equilibrium constants for each of these steps, and an overall constant for the Monomer ↔ Heptamer equilibrium, have been estimated from the data. These constants imply a standard free‐energy change, ΔG 0 , of about 9 kcal/mol for each contact surface formed between GroES subunits, except for the addition of the last subunit, where ΔG 0 = 6 kcal/mol. This lower value probably reflects the loss of entropy when the heptamer ring is formed. These experiments illustrate the advantages of electrospray mass spectrometry as a method of measuring all components of a multiple equilibrium system.