Recombinant expression, characterization and phylogenetic studies of novels cystatins-like proteins of sweet orange (Citrus sinensis) and clementine (Citrus clementina).

Phytocystatins are plant cystatins that are related to several physiological processes regulating endogenous cysteine proteases involved in seed development and germination, programmed cell death and response to stress conditions. In addition, phytocystatins can act in plant defense against exogenous peptidases from herbivorous insects, pathogens and nematodes. Considering that Citrus fruits are important to human nutrition and represent a high value crop in worldwide agriculture, in the present work, we performed the identification of putative cystatins from Citrus sinensis and from Citrus clementine and submitted them to phylogenetic analysis. Six cystatins from each species were identified as orthologous and classified into three well supported phylogenetic groups. Five cystatins representative of the phylogenetic groups were recombinantly expressed and the in vitro studies revealed them to be potent inhibitors against the cysteine peptidases papain, legumain, human cathepsins (B, L, S, K) and a cathepsin B-like from Diaphorina citri (the Asian Citrus psyllid). Our findings provide the C. clementina and C. sinensis cystatins classification and an enzyme-inhibitor interactions profile, which may reflect an evolutionary process of Citrus cystatins related to gene functions as initial germination rates and seedlings development as well associated to plant defense against pathogens, as insects and nematodes.

Inhibition of cysteine proteases by a natural biflavone: behavioral evaluation of fukugetin as papain and cruzain inhibitor.

Cruzain is the major cysteine protease of Trypanosoma cruzi, the infectious agent responsible for Chagas disease, and cruzain inhibitors display considerable antitrypanosomal activity. In the present work we elucidated crystallographic data of fukugetin, a biflavone isolated from Garcinia brasiliensis, and investigated the role of this molecule as cysteine protease inhibitor. The kinetic analyses demonstrated that fukugetin inhibited cruzain and papain by a slow reversible type inhibition with K(I) of 1.1 and 13.4 µM, respectively. However, cruzain inhibition was about 12 times faster than papain inhibition. Lineweaver-Burk plots demonstrated partial competitive inhibition for cruzain and hyperbolic mixed-type inhibition for papain. Furthermore, the docking results showed that the biflavone binds to ring C' in the S2 pocket and to ring C in the S3 pocket through hydrophobic interactions and hydrogen bonds. Finally, fukugetin also presented inhibitory activity on proteases of the T. cruzi extract, with IC50 of 7 µM.