One factor that affects the rate of enzyme-catalysed reactions is the pH. If too high or low, the pH interferes with the bonds holding the enzyme together. Additionally, substrate concentration also affects the rate of reaction. The higher the substrate concentration, the faster the rate of reaction. This is because it is more likely to find a substrate molecule. However, at a certain point, there will be no difference if the substrate concentration increases. Finally, temperature is also a huge factor, as it changes the rate of reaction. At the optimum temperature, normally around 40 degrees celcius, this is when the enzyme works best. If any higher, the enzyme will be too hot and will start to denature.