Motional effects on NMR structural data. Comparison of spinach and Escherichia coli acyl carrier proteins.

Proteins in solution need not exist in a single rigid structure but can exist in a dynamic equilibrium among structural forms. The problems that this poses for structure determination using nuclear Overhauser effect data from two-dimensional NMR experiments are discussed and illustrated with data on functionally equivalent proteins from two different species. One of these proteins, acyl carrier protein from Escherichia coli, shows a single set of resonances, easily interpreted on the basis of a single rigid structure. However, the related protein, acyl carrier protein from spinach, shows two sets of resonances, suggesting that two conformers in dynamic equilibrium would be a better structural model.