Thus the product formation rate depends on the enzyme concentration as well as on the substrate concentration, the equation resembles a bimolecular reaction with a corresponding pseudo-second order rate constant formula_10. This constant is a measure of catalytic efficiency. The most efficient enzymes reach a formula_10 in the range of . These enzymes are so efficient they effectively catalyse a reaction each time they encounter a substrate molecule and have thus reached an upper theoretical limit for efficiency (diffusion limit); and are sometimes referred to as kinetically perfect enzymes. But most enzymes are far from perfect: the average values of formula_12 and formula_13 are about formula_14 and formula_15, respectively.