Structural and Functional Studies of Chagasin, a Parasite Protease Inhibitor

Sammanfattning: The cysteine proteases are enzymes found in every kingdom of life. Clan CA is the largest of the cysteine protease clans, with members all displaying a Cys/His/Asn(Asp) catalytic triad responsible for hydrolysing the peptide bond within the substrate. To regulate the activity of the proteases, another group of proteins act as natural inhibitors (cysteine protease inhibitors) and bind to the protease in order to block the active site, thus leaving no space for substrate to enter. Chagas´ disease is a tropical parasitic disease caused by the flagellate protozoan Trypanosoma cruzi. T. cruzi is transmitted to humans and other mammals by blood-sucking assassin bugs such as Triatoma infestans. Chagasin is a 110 amino acid long, potent and reversible cysteine protease inhibitor produced by T. cruzi. It inhibits members of clan CA of cysteine proteases but shows no sequence similarity to corresponding human inhibitors (cystatins), although the function is very much alike. In this work high-quality chagasin in quantities necessary for crystallization and detailed kinetic studies was produced. The human cysteine proteases cathepsin L and cathepsin B, both with the active site cysteine replaced to alanine (in the case of cathepsin B also with the substitution H110A), were crystallized in complex with chagasin. In addition, active papain was carboxymethylated and crystallized in complex with chagasin with good result. Five different structures were solved; the free molecule of chagasin (PDB code 2NNR), chagasin in complex with cathepsin L (2NQD), two structures of chagasin in complex with cathepsin B (3CBJ, 3CBK) and chagasin in complex with papain (3E1Z). In all the described structures, the inhibitory loops creating the enzyme-binding epitope have almost the same architecture, but specific interactions within the complexes occurs. In parallel, the function of chagasin to inhibit these proteases was studied by enzyme kinetic experiments, revealing chagasin as a very potent inhibitor of papain-like proteases in clan CA.