Papain-like cysteine proteases cystatins

In addition to being an inhibitor of papain-like cysteine proteases, cystatin C has recently been shown be an efficient inhibitor of some of the cysteine proteases of another family of cysteine proteases, called the peptidase family C13, with human legumain as a typical enzyme (C6). Human legumain has, like cathepsin S, been proposed to be involved in the class n MHC presentation of antigens (M3). It has also been shown that the cystatin C inhibitory site for mammalian legumain does not overlap with the cystatin C inhibitory site for papain-like cysteine proteases (Fig. 1) and that the same cystatin C molecule therefore is able to simultaneously inhibit one cysteine protease of each type (A 10). 

The Cystatin Superfamily of Inhibitors of Papain-like Cysteine Proteases. . 64... 

Fig. 1. Amino acid sequence and schematic structure of human cystatin C. The shaded area marks the inhibitory site for papain-like cysteine proteases, which does not overlap with the inhibitory site for mammalian legumains comprising, inter alia, the Asn39 residue. The arrow indicates the Leu68 residue, which is replaced with a Gin residue in the cerebral hemorrhage producing cystatin C variant. The asterisk marks the Pro3 residue, which is partly hydroxylated.

The cystatins, which are a superfamily of proteins that inhibit papain-like cysteine proteases, are a classic example of these inhibitors. The cystatins (Fig. 3) insert a wedge-hke face of the inhibitor that consists of the protein N-terminus and two hairpin loops into the V-shaped active site of a cysteine protease. The N-terminal residues bind in the S3-S1 pockets in a substrate-like manner, but the peptide then turns away from the catalytic residues and out of the active site. The two hairpin loops bind to the prime side of the active site, which provides most of the binding energy for the interaction. Thus, both the prime and the nonprime sides of the active site are occupied, but no interactions are actually made with the catalytic machinery of the enzyme (23). 

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