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Kinetic Constants

The Meaning of Kinetic Constants

It is important to get a "gut-level" understanding of the significance of the rate constants. Here they are:

  • Km: The Michaelis constant, with units of molarity (M), is operationally defined as the substrate concentration at which the initial velocity is half of Vmax. It is equal to the dissociation constant of E and S only if E, S and ES are in rapid equilibrium. It can be thought of as an "effective" Kd in other cases.

  • kcat: The catalytic rate constant, with units of s-1, is often called the turnover number. It is a measure of how many bound substrate molecules turnover or form product in 1 second. This is evident from equation v0 = kcat[ES]

  • kcat/Km: Under conditions when [S] << Km, the Michaelis-Menten equation becomes v0 = (kcat/KM)[E0][S].This really describes a bimolecular rate constant, with units of M-1s-1, for conversion of free substrate to product. Some enzymes have kcat/Km values around 108, indicating that they are diffusion controlled. That implies that the reaction is essentially done as soon as the enzyme and substrate collide.

Table: Km and kcat values for various enzymes

Km values
enzyme substrate Km (mM)
catalase H2O2 25
hexokinase (brain) ATP 0.4
D-Glucose 0.05
D-Fructose 1.5
carbonic anhydrase HCO3- 9
chymotrypsin glycyltyrosinylglycine 108
N-benzoyltyrosinamide 2.5
b-galactosidase D-lactose 4.0
threonine dehydratase L-Thr 5.0
kcat values
enzyme substrate kcat (s-1)
catalase H2O2 40,000,000
carbonic anhydrase HCO3- 400,000
acetylcholinesterase acetylcholine 140,000
b-lactamase benzylpenicillin 2,000
fumarase fumarate 800
RecA protein (ATPase) ATP 0.4

Table: diffusion controlled enzymes

Enzymes with kcat/Km values close to diffusion controlled (108 - 109 M-1s-1)
enzyme substrate kcat (s-1) Km (M) kcat/Km (M-1s-1)
acetylcholinesterase acetylcholine 1.4 x 104 9 x 10-5 1.6 x 108
carbonic anhydrase CO2 1 x 106 1.2 x 10-2 8.3 x 107
HCO3- 4 x 105 2.6 x 10-2 1.5 x 107
catalase H2O2 4 x 107 1.1 4 x 107
crotonase crotonyl-CoA 5.7 x 103 2 x 10-5 2.8 x 108
fumarase fumarate 8 x 102 5 x 10-6 1.6 x 108
  malate 9 x 102 2.5 x 10-5 3.6 x 107
triose phosphate isomeraser glyceraldehyde-3-P 4.3 x 103 4.7 x 10-4 2.4 x 108
b-lactamase benzylpenicillin 2.0 x 103 2 x 10-4 1 x 108