Kinetic perfection, also known as catalytic perfection, refers to enzymes that are diffusion-limited; that is, the reaction they catalyze occurs as quickly as the reactants diffuse to the enzyme. The time needed for the reaction to occur is negligible compared to the time it takes for the substrate to enter, and product to leave, the active site.
Kinetic perfection: "Their [the enzymes] catalytic velocity is restricted only by the rate at which they [the enzymes] encounter substrates in a solution."
This means that the rate of the enzyme catalysed reaction is actually limited by diffusion. The enzyme 'processes' the substrate well before it encounters another molecule. An example of a kinetically perfect enzyme is Triose phosphate isomerase, which is involved in the glycolytic pathway.
Some kinetically perfect enzymes may employ the 'Circe Effect', the use of electostatic forces to attract substrate molecules into active sites.
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 susbstrates) there is no more advantage to increase the speed even further. Increasing the catalytic speed past the diffusion speed will not aid the organism in any way. 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.
See also 
- Biochemistry, Stryer et al. 2006.