CRYSTAL STRUCTURE OF A STABLE ALPHA-1-ANTITRYPSIN VARIANT REVEALS THE STABILIZATION MECHANISM. Seong-Eon Ryu, Hee-Jung Choi, Kee Nyung Lee, Ki-Sun Kwon and Myeong-Hee Yu, Protein Engineering Division Korea Research Institute of Bioscience and Biotechnology, KIST P.O. Box 115, Yusong, Taejon 305-600, South Korea

[[alpha]]-1-antitrypsin, which is a member of serpin family, controls the level of neutrophil elastase in plasma by inhibiting its activity. The conformation of the mobile reactive loop and the relative instability of the native [[alpha]]-1-antitrypsin are implicated in the inhibitory mechanism. The crystal structure of a mutant form of the uncleaved [[alpha]]-1-antitrypsin with stabilization mutations at seven different positions (hepta [[alpha]]-1-antitrypsin) has been determined at 2.7 Å resolution. The structure was compared with the structures of other serpins to understand the stabilizing effect of the mutations. We found that hepta [[alpha]]-1-antitrypsin is stabilized by various mechanisms. i) relaxation of a conformational strain by removing unfavorable overlaps of Van der Waals radii. ii) stabilization of hydrophobic cores by addition of better hydrophobic interactions. iii) destabilization of the loop inserted structures. Among these, the stabilization by the relaxation of the conformational strain by the F51L mutation underscores the importance of the central hydrophobic core region in the sheet A opening of the serpins.