Movable lobes and flexible loops in proteins. Structural deformations that control biochemical activity.

Two classes of protein whose structure is modified by small ligands are reviewed. Proteins of one group contain two massive domains joined by a flexible link; in response to small molecules, the two lobes approach and enclose the ligand. In the other, a short segment of amino acids moves as a flexible loop over the ligand which often is trapped in a non-aqueous environment. Biochemical reaction rates are altered dramatically by these movements.