Conformational changes in serpins: I. The native and cleaved conformationsof alpha(1)-antitrypsin

The serpins (SERine Proteinase INhibitors) are a family of proteins with im portant physiological roles, including but not limited to the inhibition of chymotrypsin-like serine proteinases. The inhibitory mechanism involves a large conformational change known as the S --> R (stressed --> relaxed) tra nsition. The largest structural differences occur in a region around the sc issile bond called the reactive centre loop: Ln the native (S) state, the r eactive centre is exposed, and is free to interact with proteinases. In inh ibitory serpins, in the cleaved (R) state the reactive centre loop forms an additional strand within the beta-sheet. The latent state is an uncleaved state in which the intact reactive centre loop is integrated into the A she et as in the cleaved form, to give an alternative R state. The serpin structures illustrate detailed control of conformation within a single protein. Serpins are also an unusual family of proteins in which hom ologues have native states with different folding topologies. Determination of the structures of inhibitory serpins in multiple conformational states permits a detailed analysis of the mechanism of the S --> R transition, and of the way in which a single sequence can form two stabilised states of di fferent topology. Here we compare the conformations of alpha(1)-antitrypsin in native and cle aved states. Many protein conformational changes involve relative motions o f large rigid subunits. We determine the rigid subunits of a,antitrypsin an d analyse the changes in their relative position and orientation. Knowing t hat the conformational change is initiated by cleavage at the reactive cent re, we describe a mechanism of the S --> R transition as a logical sequence of mechanical effects, even though the transition likely proceeds in a con certed manner. (C) 2000 Academic Press.