When K2>> K3 M (31) The dissociation constant of the es complex is given by [EJ[S Ro KES (32) k In other words, KM is equal to the dissociation conslant of the ES complex ifk is much smaller than k. When this condition is met, KM is a measure of the strength of the ES complex: a high KM indicates, weak binding; a low KM indicates strong binding. It must be stressed that KM indicates the affinity of the es complex only when ko is much greater than ks
When K2 >> K3
The turnover munber of an enzvme is the number of substrale molecules converted inlo product by an enzyme molecule in a unit line when the enzyme is fully saturated with substrate. It is equal to the kinetic constant k]. The maximal rate Vmax reveals the turnover number of an enzyme if the con- centration of active sites [Er] is known, because Vmax =kler] (33 For example, a 10-6m solution of carbonic anhydrase catalyzes the for- mation of 0.6 M HoCO3 per second when it is fully saturated with sub- strate. Hence, ks is 6 X 105 s-l. This turnover number is one of the larg- est known. Each round of catalysis occurs in a time equal to l/k3, which is 1.7 us for carbonic anhydrase. The turnover numbers of most enzymes ith their physiological substrates fall in the range from 1 to 10 per second(Table 8-3)
Table 8-3 Maximum turnover numbers of some enzymes Turnover Enzyme number (per second) Carbonic anhydrase 600.000 3-Ketosteroid isomerase 280,000 Acetylcholinesterase 25,000 Penicillinase 2,000 Lactate dehydrogenase 1.000 Chymotrypsin 100 DNA polymerase I 15 Tryptophan synthetase isozyme 0.5
KINETIC PERFECTION IN ENZYMATIC CATALYSIS: THE Kcat/KM CRITERION Under physiological conditions, the [s/ Km ratio is typically be tween 0.01 and 1.0. When [S]< KM, the enzymatic rate is much less than ka because most of the active sites are unoccupied