Cooperative motion and hydrogen exchange stability in protein beta-sheets.

Abstract

Protein molecules are dynamical structures due to the continual exchange of thermal energy between them and the solvent environment1,2. This dynamic behaviour is manifest in hydrogen exchange experiments, which reflect transient solvent accessibility of groups usually buried in the protein interior3,4. However, studies of hydrogen exchange kinetics in pancreatic trypsin inhibitor (PTI) reveal a small subset of amide protons which exchange very slowly5,6. Four of these groups form successive interchain hydrogen bonds in the central region of an antiparallel β-sheet7 (Fig. 1). Here I suggest that the unusual exchange stability of these β-sheet protons reflects the structure's intrinsic flexibility. This property allows transient energy fluctuations to be accommodated as cooperative motions which do not locally strain the interchain hydrogen bonds.