Crystal structure of an uncleaved alpha 1-antitrypsin reveals the conformation of its inhibitory reactive loop

The crystal structure of a recombinant human alpha 1-antitrypsin, in the uncleaved and uncomplexed state, has been determined by X-ray crystallographic methods and refined to an R-factor of 18.4% for 8.0-3.46 A data with good stereochemistry. This structure provides the first view at the inhibitory loop and the central beta-sheet A of the uncleaved alpha 1-antitrypsin. The reactive loop takes a distorted helical conformation and no pre-insertion of two residues in the reactive loop into the beta-sheet A is observed. The present structure is largely in agreement with the model predicted by Engh, Wright, and Huber [Prot. Eng. 3 (1990) 469-477].