How does ph affect enzyme activity?

Each enzyme has a different optimum pH, which is the ideal pH for the enzyme to perform its job successfully.

As can be seen above, the optimum pH for the enzyme Salivary Amylase is around 7. The closer the pH is to 7, the higher the reaction rate.

As the pH distances from the optimum, however, the reaction rate decreases because the shape of the enzyme's active site begins to deform, until it becomes denatured and the substrate can no longer fit the active site.