Initial rates of ATP hydrolysis by wild-type GroEL were measured as a function of ATP concentration from 0 to 0.8 mM. Two allosteric transitions are observed: one at relatively low ATP concentrations (⩽ 100μM) and the second at higher concentrations of ATP with respective midpoints of about 16 and 160μM. Two allosteric transitions were previously observed also in the case of the Arg- 196 → Ala GroEL mutant [Yifrach, O., & Horovitz, A. (1994) J. Mol. Biol. 243, 397-401], On the basis of these observations a mathematical model for nested cooperativity in ATP hydrolysis by GroEL is developed in which there are two levels of allostery: one within each ring and the second between rings. In the first level, each heptameric ring is in equilibrium between the T and R states, in accordance with the Monod-Wyman-Changeux (MWC) model of cooperativity [Monod et al. (1965) J. Mol. Biol. 12, 88-118], A second level of allostery is between the rings of the GroEL particle which undergoes sequential Koshland-Nemethy-Filmer (KNF)-type transitions from the TT state via the TR state to the RR state [Koshland et al. (1966) Biochemistry 5, 365-385]. Using our model, we estimate the values of the Hill coefficient for the negative cooperativity between rings in wild-type GroEL and the Arg-196 → Ala mutant to be 0.003 (±0.001) and 0.07 (±0.02), respectively. The inter-ring coupling free energies in wild-type GroEL and the Arg-196 - Ala mutant are -7.5 (±0.4) and -3.9 (±0.3) kcal mol-1, respectively.
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