Low-populated folding intermediates of Fyn SH3 characterized by relaxation dispersion NMR.

Many biochemical processes proceed through the formation of functionally significant intermediates. Although the identification and characterization of such species can provide vital clues about the mechanisms of the reactions involved, it is challenging to obtain information of this type in cases where the intermediates are transient or present only at low population. One important example of such a situation involves the folding behaviour of small proteins that represents a model for the acquisition of functional structure in biology. Here we use relaxation dispersion nuclear magnetic resonance (NMR) spectroscopy to identify, for two mutational variants of one such protein, the SH3 domain from Fyn tyrosine kinase, a low-population folding intermediate in equilibrium with its unfolded and fully folded states. By performing the NMR experiments at different temperatures, this approach has enabled characterization of the kinetics and energetics of the folding process as well as providing structures of the intermediates. A general strategy emerges for an experimental determination of the energy landscape of a protein by applying this methodology to a series of mutants whose intermediates have differing degrees of native-like structure.