pH is also a factor which affects the rate of an enzyme catalyzed reaction. The following graph shows how three different enzymes are affected by pH.
From the graph above, it can be seen that Enzyme #1 works well at pH 4. It means that the optimum pH of this particular enzyme is 4. Above or below this pH, the rate of the reaction slows down. The optimum pH for enzymes #2 and #3 are 6 and 9 respectively. This means that below or above these pH, the reactions catalyzed by these enzymes will slow down.