Inactive conformation of the serpin alpha(1)-antichymotrypsin indicates two-stage insertion of the reactive loop: Implications for inhibitory function and conformational disease

The serpins are a family of proteinase inhibitors that play a central role in the control of proteolytic cascades. Their inhibitory mechanism depends on the intramolecular insertion of the reactive loop into beta-sheet A afte r cleavage by the target proteinase, Point mutations within the protein can allow aberrant conformational transitions characterized by beta-strand exc hange between the reactive loop of one molecule and beta-sheet A of another . These loop-sheet polymers result in diseases as varied as cirrhosis, emph ysema, angio-oedema, and thrombosis, and we recently have shown that they u nderlie an early-onset dementia, We report here the biochemical characteris tics and crystal structure of a naturally occurring variant (Leu-55-Pro) of the plasma serpin alpha(1)-antichymotrypsin trapped as an inactive interme diate. The structure demonstrates a serpin configuration with partial inser tion of the reactive loop into beta-sheet A. The lower part of the sheet is filled by the last turn of F-helix and the loop that links it to s3A, This conformation matches that of proposed intermediates on the pathway to comp lex and polymer formation in the serpins, In particular, this intermediate, along with the latent and polymerized conformations, explains the loss of activity of plasma alpha(1)-antichymotrypsin associated with chronic obstru ctive pulmonary disease in patients with the Leu-55-Pro mutation.