Antistasin (ATS) is a leech-derived 119-amino-acid protein which exhibits potent and highly selective inhibition of coagulation Factor Xa. It inhibits Factor Xa according to a common mechanism of serine-proteinase inhibitors in which a conformationally rigid substrate-like reactive site is presented to the enzyme. In this study a recombinant version of ATS was expressed and purified utilizing a yeast expression system in order to probe the reactive site P1 (Arg-34) and P1′ (Val-35) residues by site-directed mutagenesis. The results demonstrate the requirement for a positively charged residue in the P1 position of ATS, with an arginine residue preferred over a lysine, yielding K1 values of 61 pM and 1.28 nM respectively. Mutation of the P1 arginine residue to the non-polar amino acid leucine abolished its inhibitory potency toward Factor Xa. The role of the C-terminal domain of ATS, which shares significant amino acid sequence identity with the N-terminal domain, was investigated by creating a second reactive site in the corresponding position of the C-terminal domain. The inhibitory activity of this mutant demonstrated that the C-terminal domain of ATS is not folded into the proper conformation necessary to create a functional inhibitory domain.

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