Active Sites and Substrate Binding Models

Some enzymes are so fast and so selective that their k2/Km ratio approaches the molecular diffusion rates (108-109m s-1). Such enzymes are called kinetically perfect [21]. With these enzymes, the reaction rate is diffusion controlled, and every collision is an effective one. However, since the active site is very small compared to the entire enzyme, there must be some extra forces which draw the substrate to the active sites (otherwise, there would be many fruitless collisions). The work of these forces was dubbed by William Jencks in 1975 as the Circe effect [22], after the mythological sorceress of the island of Aeaea, who lured Odysseus men to a feast and then turned them into pigs [23,24]. 

There is an interesting parallel between substrate binding and adsorption. Since each enzyme molecule has one active site, and since these active sites all have the same structure, we can think of enzyme molecules in solution as a surface with many equivalent adive sites. In this case, k2 in the Michaelis-Menten kinetics (Eq. 5.1 see Chapter 2 for a detailed discussion) represents the rate of adsorption, k x the rate of desorption, and k2 the rate of the surface readion followed by fast product desorption. Moreover, this system fits the assumptions of the Langmuir isotherm (all sites identical, one molecule per site, no lateral interadions) even better than the adive sites on some real solid catalysts  

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