Sathish H A, Kaul P, Prakash V
Department of Protein Chemistry and Technology, Central Food Technological Research Institute, Mysore, India.
Indian J Biochem Biophys. 2000 Feb;37(1):18-27.
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 2 x 10(-4) M of CdCl2 and 4 x 10(-4) M of ZnCl2. It is completely inactivated above 1 x 10(-3) M concentration of either ZnCl2 or CdCl2. Of the two metal ions zinc with a ki value of 5 x 10(-5) M is a more potent inhibitor than cadmium which has a ki value of 8 x 10(-5) M. Both the metal ions have higher affinity for active site than the substrate. At concentrations above 1 x 10(-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 beta-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 degrees 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 concentration has been explained.
木瓜蛋白酶是一种属于半胱氨酸蛋白酶家族的内切蛋白酶。研究了木瓜蛋白酶在两种不同金属离子（即锌和镉）存在下的催化活性。这两种金属离子均以浓度依赖的方式强烈抑制酶活性。在2×10⁻⁴ M的CdCl₂和4×10⁻⁴ M的ZnCl₂时，该酶失去50%的活性。在ZnCl₂或CdCl₂浓度高于1×10⁻³ M时，它会完全失活。在这两种金属离子中，ki值为5×10⁻⁵ M的锌比ki值为8×10⁻⁵ M的镉是更强效的抑制剂。这两种金属离子对活性位点的亲和力都高于底物。在金属离子浓度高于1×10⁻² M时，抑制作用是不可逆的。量热研究表明，这些金属离子结合后木瓜蛋白酶的热稳定性降低。远紫外圆二色光谱数据显示，这些金属离子结合后β结构含量仅有微小变化。金属离子结合后木瓜蛋白酶的表观热转变温度降低5℃，这也表明木瓜蛋白酶分子不稳定，支持了这些数据。解释了在较低和较高浓度下，这两种金属离子存在时木瓜蛋白酶部分和完全失活的机制。
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