The stabilisation of the cysteine protease of Carica papaya (papain) and the catalytic domain of the Cathepsin L-like cysteine protease of Trypanosoma congolense (TcoCATL)
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Proteases play an intricate role in the numerous functions of a living organism. Proteases are responsible for the cleavage of proteins into smaller fragments by catalysing the hydrolysis of peptide bonds. The class of cysteine proteases have a cysteine thiol group in their active site have been found in lower and higher organisms. They have been investigated as promising drug targets for various diseases due to their fundamental functions in catabolism and protein processing. The thermal stability of a protease is a key characteristic feature that is largely dependent on its amino acid sequence and composition and quantified through the determination of its melting temperature (TM). Papain is the most well characterised cysteine protease and is commonly used as a model for other cysteine proteases. Congopain is the major cysteine protease of Trypanosoma congolense which has been identified as the main causative agent of trypanosomiasis in livestock. The thermal stability for papain and congopain were investigated in this study via the thermal shift assay. Papain was purchased and the catalytic domain of congopain was expressed using the Pichia pastoris yeast expression system. The thermal stability of the proteases were determined under neutral pH conditions and the effect of pH and ligand binding were evaluated to determine if the proteases could be further stabilised. The most stable forms of papain and the catalytic domain of congopain in its monomeric form was observed at pH 5.0 with 50 μM chymostatin. The thermal stability of both cysteine proteases was successfully evaluated via the thermal shift assay and conditions to further stabilise papain and the catalytic domain of congopain were determined. The thermal shift assay has been proven to be a reliable technique in identifying factors which increase the stability of a protein. More specifically, the technique serves as a simple and primary diagnostic tool to screen potential inhibitors of a protein and detect changes in the TM of a protein.