Abstract

Serine proteinase cleavage of proteins is essential to a wide variety of biological processes and is primarily regulated by protein inhibitors. Many inhibitors are conformationally rigid simulations of optimal serine proteinase substrates, which makes them highly efficient competitive inhibitors of target proteinases. In contrast, members of the serpin family of serine proteinase inhibitors display extensive flexibility and polymorphism, particularly in their reactive site segments and in β‐sheet secondary structure, which can take up and expel strands. Reactive site and β‐sheet polymorphism appear to be coupled in the serpins and may account for the extreme stability of serpinproteinase complexes through the insertion of the reactive site strand into a β‐sheet. These unusual properties may have opened an adaptive pathway of proteinase regulation that was unavailable to the conformationally rigid proteinase inhibitors.