Enzymes are protein. They are biological catalyst that increases the rate of chemical reaction but remain unchanged by reaction. There are specific enzymes for specific substrates. For example, when starch meets amylase which is inside our mouth in our saliva, it is broken down to sugar as a product. Protein is also broken down to amino acids when dealt by pepsin.
There are also factors that affect enzyme activity. Like temperature, pH level, substrate concentration. When temperature is increased, the activity of enzymes increase. But then, when maximum temperature has reached, the enzymes die and no longer work.It is called enzyme denaturing. pH level works in a different way. Different enzymes have different optimum pH level. For instance, amylase works in saliva which is just about 6. So when amylase goes to stomach which has pH level of 3, then it won`t work as it did in the mouth. Also, in terms of substrate concentration, the more substrate there are, the more the activity of the enzymes. But when it has reached the optimum level of working, then it will just be the same speed all through out.