Exploring the entire conformational space of proteins by high-pressure NMR

A protein in solution is a thermodynamic entity, spanning, in principle, the entire allowed conformational space from the fully folded N to the fully unfolded U. Although some alternately or partially folded higher-energy conformers may coexist with N and U, they are seldom detected spectroscopically because their populations are usually quite low under physiological conditions. I describe here a new type of experiment, a combination of multidimensional NMR spectroscopy with pressure, that is capable of detecting and analyzing structures and thermodynamic stability of these higher-energy conformers. The idea is based on the finding that under physiological conditions the conformational order of a globular protein normally decreases in parallel with its partial molar volume (negative AV), so that under equilibrium conditions, the population is shifted to a less-and-less-ordered conformer with increasing pressure. In principle, with the high space resolution of the multidimensional NMR, the method enables one to explore protein structure and stability in atomic detail in a wide conformational space from N to U with pressure and temperature as variables. The method will provide us with a strong basis for understanding the fundamental phenomena of proteins: function, folding, and aggregation.