Abstract: The kinetics of the structural changes affecting cardosin A, a plant aspartic proteinase (AP) from Cynara cardunculus L., in the presence of a mixture of acetonitrile (AN) in water (W) was studied. Incubation of cardosin A with 10% (v/v) AN resulted in a gradual increase in protein helicity, accompanied by changes in the tertiary structure, seen by changes in the intrinsic fluorescence of tryptophan. Differential scanning calorimetry (DSC) revealed that the temperature of denaturation of cardosin A decreased upon the addition of AN. With longer incubation times, the small chain of cardosin A denatured completely, consequent exposure of the single tryptophan residue accounting well for the observed spectral shift intrinsic fluorescence of the protein. Enzymatic activity assays demonstrated that the kinetically determined unfolding of the small chain of cardosin A does not result in loss of the activity of this enzyme.
Abstract: Cardosin A, a plant aspartic proteinase, capable of synthesising peptides, was investigated through synthesis of five methyl esters amino acid substrates as amino donors and nine benzyloxycarbonyl amino acid and peptide carboxyl donors. It was found that cardosin A is able to catalyse the synthesis of several peptide bonds, being the preference order for the carboxyl components the following: CBz.Phe >CBz.Trp. Unpredictably, Tyr could not be accepted in P1. Results were compared and discussed according to the known specificity of pepsin, the most studied aspartic proteinase.
