There are five main factors which affect the rate of enzyme-mediated reactions.
1) Enzyme concentration. Adding enzyme molecules will speed up the reaction, until a saturation point is reached.
2) Substrate concentration. Assuming sufficient enzyme concentration, adding substrate molecules will increase the rate, again until saturation is reached.
3) Temperature. Reactions occur faster at warmer temperatures because the rate of collisions between molecules increases. However, if the temperature gets too hot the enzyme may denature and fail to function. All enzymes have an optimal temperature range determined by these two factors.
4) pH level. Enzymes also have an optimal pH level; if the environmental pH is too high or too low, the fold structure of the enzyme may be affected, causing the molecule to change shape and lose functionality.
5) Concentration of salts in the medium. Again, there is an optimal range, outside which the enzyme molecules may denature.