Fig. 1.
Determination of the distribution in the number of ATP molecules bound to GroEL from the fine structure of each charge state. (A) Mass spectrum of the intact 14-mer GroEL in the presence of 5 μM ATP. The ATP binding number distribution is reflected in the fine splitting of the peaks as highlighted for the 58+ charge state (in orange). (B) Blow-up of the 58+ charge state spectrum. The fine structure of the 58+ charge state is a result of the combination of peaks that correspond in mass to different numbers of bound ATP molecules. The data demonstrate the coexistence of different populations of GroEL/ATP forms. (C) Deconvolution of the measured spectrum assuming coexistence of the following species: (GroEL)14, (GroEL)14(ATP)1, (GroEL)14(ATP)2, (GroEL)14(ATP)3, and (GroEL)14(ATP)4. A close agreement is seen between the deconvoluted spectrum and the spectrum of the charge state shown in B.