Eglin C
 
Eglin c is a widely studied, strong inhibitor of serine proteinases [see Serine Proteinase Inhibitors, Protein]. Its reactive site P1 Leu45 residue endows it with specificity toward many, but not all, subtilisins, chymotrypsins, and elastases. It is an exceptionally strong inhibitor (K_I < 10^–13 M) of Streptomyces griseus proteinases A and B. As an efficient inhibitor of human leukocyte elastase, cathepsin G, and proteinase 3, the three human leukocyte enzymes responsible for the lung damage in emphysema and in related diseases, eglin c is of great interest to pharmaceutical companies. Eglinc consists of a single polypeptide chain of 70 amino acid residues. In spite of its lack of disulfide bridges, it is a standard-mechanism canonical inhibitor. It is a member of the potato I family, a family of inhibitors whose members either lack disulfides or contain only one. Most members of the potato I family are isolated from plants. In contrast, eglin c was isolated from the leech Hirudo medicinalis, where its physiological function is not known.