ENGINEERING AN ANION-BINDING CAVITY IN ANTICHYMOTRYPSIN MODULATES THESPRING-LOADED SERPIN-PROTEASE INTERACTION

Expressed in a kinetically trapped folding state, a serpin couples the thermodynamic driving force of a massive beta-sheet rearrangement to the inhibition of a target protease. Hence, the serpin-protease intera ction is the premier example of a ''spring-loaded'' protein-protein in teraction. Amino acid substitutions in the hinge region of a serpin re active loop can weaken the molecular spring, which converts the serpin from an inhibitor into a substrate. To probe the molecular basis of t his conversion, we report the crystal structure of A349R antichymotryp sin in the reactive loop cleaved state at 2.1 Angstrom resolution. Thi s amino acid substitution does not block the beta-sheet rearrangement despite the burial of R349 in the hydrophobic core of the cleaved serp in along with a salt-linked acetate ion. The inhibitory activity of th is serpin variant is not obliterated; remarkably, its inhibitory prope rties are anion-dependent due to the creation of an anion-binding cavi ty in the cleaved serpin.