Infuence of metal ions on structure and catalytic activity of papain

Abstract

Papain is an endoprotease belonging to cysteine protease family. The catalytic activity of papain in presence of two different metal ions namely zinc and cadmium has been investigated. Both the metal ions are potent inhibitors of the enzyme activity in a concentration dependent manner. The enzyme loses 50% of its activity at 2x10^-4 <i>M </i>of CdCI₂ and 4x10^-4 <i>M </i>of ZnCI₂. It is completely inactivated above 1x10^-3 <i>M </i>concentration of either

ZnCI₂ or CdCI₂. Of the two metal ions zinc with a k_i value of 5x10^-5 <i>M </i>is a more potent inhibitor than cadmium which has a k_i value of 8x10^-5 <i>M. </i>Both the metal ions have higher affinity for active site than the substrate. At

concentrations above 1x10^-2 M of metal ions the inhibition is not reversible. Calorimetric studies showed decreased thermal stability of papain upon binding of these metal ions. Far UV circular dichroic spectral data showed only small changes in the β<span style="font-size:14.0pt;font-family:HiddenHorzOCR;mso-bidi-font-family:HiddenHorzOCR">-structure content upon binding of these metal ions. These data are also supported by decrease in the apparent thermal transition temperature of papain by 5°C upon binding of metal ions indicating destabilization of the papain molecule. The mechanism of both partial and complete inactivation of papain in presence of these two metal ions both at lower and higher <span style="font-size:14.0pt;font-family:HiddenHorzOCR; mso-bidi-font-family:HiddenHorzOCR">concentration has been explained.

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