Kinetically perfect enzymes have a specificity constant, k/K, on the order of 10 to 10 M s. The rate of the enzyme-catalysed reaction is limited by diffusion and so the enzyme processes the substrate well before it encounters another molecule.
Some enzymes operate with kinetics which are faster than diffusion rates, which would seem to be impossible. Several mechanisms have been invoked to explain this phenomenon. Some proteins are believed to accelerate catalysis by drawing their substrate in and preorienting them by using dipolar electric fields. Some invoke a quantum-mechanical tunneling explanation whereby a proton or an electron can tunnel through activation barriers. If the proton tunneling theory remained a controversial idea, it has been proven to be the only possible mechanism in the case of the soybean lipoxygenase.