Now, if we think about what an ideal enzyme might be, it would be one that has a very high velocity and a very high affinity for its substrate. That is, it wouldn’t take much substrate to get to $$V_{max}/2$$ and the $$K_{cat}$$ would be very high. Such enzymes would have values of $$K_{cat} / K_M$$ that are maximum. Interestingly, there are several enzymes that have this property and their maximal values are all approximately the same. Such enzymes are referred to as being “perfect" because they have reached the maximum possible value. Why should there be a maximum possible value of $$K_{cat} / K_M$$. The answer is that movement of substrate to the enzyme becomes the limiting factor for perfect enzymes. Movement of substrate by diffusion in water has a fixed rate and that limitation ultimately determines how fast the enzyme can work. In a macroscopic world analogy, factories can’t make products faster than suppliers can deliver materials. It is safe to say for a perfect enzyme that the only limit it has is the rate of substrate diffusion in water.