Cooperativity in the unfolding transitions of cysteine proteinases. Calorimetric study of the heat denaturation of chymopapain and papain.
Differential scanning calorimetry (DSC) was employed to study the thermal unfolding of chymopapain (EC 3.4.22.6) and papain (EC 3.4.22.2), two highly homologous cysteine proteinases from Carica papaya. Under all pH conditions used, both enzymes showed irreversible thermal denaturation. However, results from experiments performed at two different scanning rates suggest that interpretation of data in terms of equilibrium thermodynamics is not unreasonable. For papain, the ratio of calorimetric (delta Hcal) to van't Hoff (delta HvH) enthalpies approximated to 2.0. This value indicates that papain domains unfold almost independently, as it has been reported previously. In contrast, chymopapain displayed a more cooperative behavior with a delta Hcal to delta HvH ratio of 1.3-1.4. DSC curves were analyzed in terms of a mechanism that includes domain-domain interactions. The results showed a negligible interdomain free energy in the case of papain, but a significant value of approx. 1.0 kcal/mol (1 cal = 4.184 J) for chymopapain. These two proteins also differed in the unfolding heat-capacity change, delta Cp, which suggests that their native structures bury different amounts of nonpolar surface area.
Solís-Mendiola
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Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, México City, México.
Rojo-Domínguez
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Hernández-Arana
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D002021:Buffers; D002152:Calorimetry, Differential Scanning; D002917:Chymopapain / Q000737:chemistry*; D003546:Cysteine Endopeptidases / Q000737:chemistry; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D010206:Papain / Q000737:chemistry*; D010944:Plants / Q000201:enzymology; D011487:Protein Conformation; D011489:Protein Denaturation
D002021:Buffers; D003546:Cysteine Endopeptidases; D010206:Papain; D002917:Chymopapain