Answer
Enzyme activity can be affected by pH, temperature, and regulatory molecules.
Work Step by Step
Enzymes usually work best within a narrow range of temperatures and pH values. For example, human enzymes work best at about 37$^{\circ}$C. Some enzymes such as pepsin work best in acidic, low pH conditions such as those found in the stomach. Certain regulatory molecules can either activate or deactivate enzymes, depending on the needs of the cell.
