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. Evolution It is worth noting that there are not many kinetically perfect enzymes. This can be explained in terms of natural selection. An increase in catalytic speed may be favoured as it could confer some advantage to the organism. However, when the catalytic speed outstrips diffusion speed (i.e. substrates entering and leaving the active site, and also encountering substrates) there is no more advantage to increase the speed even further. The diffusion limit represents an absolute physical constraint on evolution. Increasing the catalytic speed past the diffusion speed will not aid the organism in any way and so represents a global maximum in a fitness landscape. Therefore, these perfect enzymes must have come about by 'lucky' random mutation which happened to spread, or because the faster speed was once useful as part of a different reaction in the enzyme's ancestry. Examples Acetylcholinesterase β-lactamase Catalase Carbonic anhydrase Carbon monoxide dehydrogenase Cytochrome c peroxidase Fumarase Superoxide dismutase Triosephosphate isomerase See also Diffusion-controlled reaction Enzyme Enzyme catalysis Enzyme kinetics Enzyme engineering References Catalysis Enzyme kinetics Chemical reaction engineering