The specificity constant  (also known as the catalytic efficiency) is a measure of how efficiently an enzyme converts a substrate into product. Although it is the ratio of  and  it is a parameter in its own right, more fundamental than . Diffusion limited enzymes, such as fumarase, work at the theoretical upper limit of , limited by diffusion of substrate into the active site.
If we symbolize the specificity constant for a particular substrate A as  the Michaelis–Menten equation can be written in terms of  and  as follows:
At small values of the substrate concentration this approximates to a first-order dependence of the rate on the substrate concentration:
Conversely it approaches a zero-order dependence on  when the substrate concentration is high:
The capacity of an enzyme to distinguish between two competing substrates that both follow Michaelis–Menten kinetics depends only on the specificity constant, and not on either  or  alone. Putting  for substrate  and  for a competing substrate , then the two rates when both are present simultaneously are as follows:
Although both denominators contain the Michaelis constants they are the same, and thus cancel when one equation is divided by the other:
and so the ratio of rates depends only on the concentrations of the two substrates and their specificity constants.