Much evidence indicates that motions are continually occurring within folded protein molecules. Protein motions can be roughly grouped into several classes, as shown in Table 6.5.

Class 1 motions occur even within protein molecules in crystals and account, at least in part, for the limits of resolution obtainable in x-ray diffraction studies.

The larger, slower motions in classes 2 and 3 are more likely to occur in solution. Some of these, like the opening and closing of clefts in molecules, are thought to be involved in the catalytic functions of enzymes.

How long it takes for a protein to bind or release a small molecule may depend on the time required for the protein to open or close a cleft. Similarly, the protein "gates" that pass molecules and ions through membranes rapidly change from open to closed states. The dynamic behavior of proteins is at least as important in their function as the static details of their structure.