Chemical modification of papain for use in alkaline medium

Chemical modification is a useful method to recognize and modify functional determinants of enzymes. Papain, an endolytic cysteine protease (EC3.4.22.2) from Carica papaya latex has been chemically modified using different dicarboxylic anhydrides of citraconic, phthalic, maleic and succinic acids. These anhydrides reacted with five to six amino groups of the lysine residues in the enzyme, thereby changing the net charge of the enzyme from positive to negative. The resultant enzyme had its optimum pH shifted from 7 to 9 and change in temperature optima from 60 to 80 °C. The modified papain also had a higher thermostability. Stability of the modified papain was further increased by immobilization of the enzyme either by adsorption onto inert matrix or by entrapment in polysaccharide polymeric gels. Entrapment in starch gel showed better retention of enzyme activity. Incorporation of modified and immobilized enzymes to branded domestic detergent powders was found to have very good activity retention. The papain entrapped in starch gel showed better stability and activity retention than in other carbohydrate polymers when added to domestic detergent powders.