Well from my IGCSE knowledge I know that the factors that affect enzyme activity are:
1. Temperature. An enzyme is most active at its optimum temperature, which is about 40 degrees Celsius in human beings. A temperature exceeding this value denatures or destroys the enzyme whereas a temperature below this makes our enzyme inactive.
2. pH levels. Similarly an enzyme works best in a particular condition. For instance, the enzyme amylase works best in alkaline solutions. This pH level is referred to as the optimum pH of this enzyme. If the enzyme is exposed too pH level exceeding its optimum pH then the enzyme gets denatured. If the enzyme is below its pH levels then it gets de-activated.
Basically these are two main factors that affect enzyme activity.