a₁-Antitrypsin 
 
a₁-Antitrypsin is a proteinase inhibitor of the serpin family that consists of a single polypeptide chain of 294 amino acid residues and four carbohydrate side chains. It also known as a₁-proteinase inhibitor. Present at a concentration of 1 g/L, it is the predominant proteinase inhibitor in human plasma in terms of concentration. Its reactive-center peptide bond between Met358 and Ser359 specifically targets the elastases that are involved in connective tissue remodeling. Genetic deficiency of a₁-antitrypsin is relatively common and predisposes to the development of the lung disease emphysema, with an associated risk of liver cirrhosis .
References
R. W. Carrell, J.-O. Jeppsson, C.-B. Laurell, S. O. Brennan, M. C. Owen, L. Vaughan, and D. R. Boswell (1982) Structure and variation of human 1-antitrypsin, Nature 298, 329–334.