Structure of the mexicain–E-64 complex and comparison with other cysteine proteases of the papain family

Mexicain is a 23.8 kDa cysteine protease from the tropical plant Jacaratia mexicana. It is isolated as the most abundant product after cation-exchange chromatography of the mix of proteases extracted from the latex of the fruit. The purified enzyme inhibited with E-64 [N-(3-carboxyoxirane-2-­carbonyl)-leucyl-amino(4-guanido)butane] was crystallized by sitting-drop vapour diffusion and the structure was solved by molecular replacement at 2.1 Å resolution and refined to an R factor of 17.7% (R_free = 23.8%). The enzyme belongs to the α+β class of proteins and the structure shows the typical papain-like fold composed of two domains, the α-helix-rich (L) domain and the β-barrel-like (R) domain, separated by a groove containing the active site formed by residues Cys25 and His159, one from each domain. The four monomers in the asymmetric unit show one E-64 molecule covalently bound to Cys25 in the active site and differences have been found in the placement of E-64 in each monomer.