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Published online before print July 27, 2007, 10.1110/ps.072911607
Protein Science (2007), 16:1842-1850. Published by Cold Spring Harbor Laboratory Press. Copyright © 2007 The Protein Society
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Probing the local conformational change of {alpha}1-antitrypsin

Je-Hyun Baek1,2, Hana Im3, Un-Beom Kang2,4, Ki Moon Seong2, Cheolju Lee4, Joon Kim2, and Myeong-Hee Yu1

1 Functional Proteomics Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul 136-791, Korea
2 Laboratory of Biochemistry, School of Life Sciences and Biotechnology, Korea University, Seongbuk-gu, Seoul 136-701, Korea
3 Department of Molecular Biology, Sejong University, Gwangjin-gu, Seoul 143-747, Korea
4 Life Sciences Division, Korea Institute of Science and Technology, Seongbuk-gu, Seoul 136-791, Korea

(RECEIVED March 29, 2007; FINAL REVISION May 17, 2007; ACCEPTED May 22, 2007)

The native form of serpins (serine protease inhibitors) is a metastable conformation, which converts into a more stable form upon complex formation with a target protease. It has been suggested that movement of helix-F (hF) and the following loop connecting to strand 3 of beta-sheet A (thFs3A) is critical for such conformational change. Despite many speculations inferred from analysis of the serpin structure itself, direct experimental evidence for the mobilization of hF/thFs3A during the inhibition process is lacking. To probe the mechanistic role of hF and thFs3A during protease inhibition, a disulfide bond was engineered in {alpha}1-antitrypsin, which would lock the displacement of thFs3A from beta-sheet A. We measured the inhibitory activity of each disulfide-locked mutant and its heat stability against loop–sheet polymerization. Presence of a disulfide between thFs3A and s5A but not between thFs3A and s3A caused loss of the inhibitory activity, suggesting that displacement of hF/thFs3A from strand 5A but not from strand 3A is required during the inhibition process. While showing little influence on the inhibitory activity, the disulfide between thFs3A and s3A retarded loop–sheet polymerization significantly. This successful protein engineering of {alpha}1-antitrypsin is expected to be of value in clinical applications. Based on our current studies, we propose that the reactive-site loop of a serpin glides through between s5A and thFs3A for the full insertion into beta-sheet A while a substantial portion of the interactions between hF and s3A is kept intact.

Keywords: protein engineering; disulfide locking; {alpha}1-antitrypsin; serpin; conformational change; loop–sheet polymerization


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