One of the factors that affects enzyme activity is the pH. If there are small changes in the pH, the enzyme will denature and catalytic activity will be lost.
Another factor is the temperature. Enzyme-catalyzed reactions will have the rate increased as the temperature increases. Because the enzymes are proteins, they will become denatured and turn inactive at the higher temperatures.
The concentration of the enzyme also affects the activity. If the concentration of the enzyme is increased while the concentration of the substrate is kept constant, the rate of the reaction will double as the enzyme concentration doubles. This is due to the fact that the molar concentration of the enzyme being used almost always will be lower than the molar concentration of the substrate.