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Any change in the pH of a protein’s environment will cause observable changes in the solubility of the protein. Solubility changes, in turn, reflect changes in the three-dimensional structure of the protein. The effect of pH on protein solubility explains why most enzymes function well at an optimum pH, and why their activity decreases substantially at pH values other than the optimum. This demonstration examines the effect of pH on the solubility and structure of casein, a milk protein.