Cells have DNA to serve as a blueprint for proteins and a high fidelity system for translating this code into the finished protein product. In fact, nature has put considerable effort into making a system whereby individual molecules of a particular protein are identical. Using numbers experimentally determined for transcription and translation error rates in E. coli, one can calculate that the probability of producing alkaline phosphatase with no errors in it is >90%. Even if one did accidently put in an incorrect amino acid, this does not guarantee that the protein will have a different activity. Many positions within a protein can tolerate different amino acids without affecting the protein's activity.
Having established that Nature tries to make identical proteins identical, cells have mechanisms to ensure that this is not the case. Proteins can be post-translationally modified. Such modifications are not controlled nearly as stringently as the primary sequence of the protein. This results in the transformation of a previously homogeneous enzyme population into a heterogeneous one. Why would Nature try to make molecules of a particular protein identical and then turn around and make them different? What is going on? I would really like to know the answer. That is what my research group is interested in.
Were still early on in the game so we don't have the answers yet. However, we suspect that Nature first produces a protein and then modifies it post-translationally to adjust their properties to match the current conditions that exist in the cell. We are currently growing cells under different conditions, harvesting enzymes and doing single molecule assays to determine the distribution of individual enzyme molecule characteristics and how they are changed as the cell adapts to different growth conditions.