a general explanation of the velocity and gross mechanism of enzyme-catalyzed reactions. First stated in 1913, the hypothesis assumes the rapid, reversible formation of a complex between an enzyme and its substrate (the substance upon which it acts to form a product). It also assumes that the rate of formation of the product, P, is proportional to the concentration of the complex. The velocity of such a reaction is greatest when all the sites at which catalytic activity can take place on the enzyme molecules (active sites) are filled with substrate; i.e., when the substrate concentration is very high. These relationships provide the basis for all kinetic studies of enzymes and also have been applied to investigations of the effects of carriers upon the transport of substances through cell membranes.