Kinetic Constants
The Meaning of Kinetic Constants
It is important to get a "gutlevel" understanding of the significance of the rate constants. Here they are:

K_{m}: The Michaelis constant, with units of molarity (M), is operationally defined as the substrate concentration at which the initial velocity is half of V_{max}. 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.

k_{cat}: 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 v_{0} = k_{cat}[ES]

k_{cat}/K_{m}: Under conditions when [S] << K_{m}, the MichaelisMenten equation becomes v_{0} = (k_{cat}/K_{M})[E_{0}][S].This really describes a bimolecular rate constant, with units of M^{1}s^{1}, for conversion of free substrate to product. Some enzymes have k_{cat}/K_{m} values around 10^{8}, 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 k_{cat} values for various enzymes
Km values  
enzyme  substrate  Km (mM) 
catalase  H_{2}O_{2}  25 
hexokinase (brain)  ATP  0.4 
DGlucose  0.05  
DFructose  1.5  
carbonic anhydrase  HCO_{3}^{}  9 
chymotrypsin  glycyltyrosinylglycine  108 
Nbenzoyltyrosinamide  2.5  
bgalactosidase  Dlactose  4.0 
threonine dehydratase  LThr  5.0 
k_{cat} values  
enzyme  substrate  k_{cat} (s^{1}) 
catalase  H_{2}O_{2}  40,000,000 
carbonic anhydrase  HCO_{3}^{}  400,000 
acetylcholinesterase  acetylcholine  140,000 
blactamase  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 M1s1)  
enzyme  substrate  kcat (s1)  Km (M)  kcat/Km (M1s1) 
acetylcholinesterase  acetylcholine  1.4 x 10^{4}  9 x 10^{5}  1.6 x 10^{8} 
carbonic anhydrase  CO2  1 x 10^{6}  1.2 x 10^{2}  8.3 x 10^{7} 
HCO3  4 x 10^{5}  2.6 x 10^{2}  1.5 x 10^{7}  
catalase  H2O2  4 x 10^{7}  1.1  4 x 10^{7} 
crotonase  crotonylCoA  5.7 x 10^{3}  2 x 10^{5}  2.8 x 10^{8} 
fumarase  fumarate  8 x 10^{2}  5 x 10^{6}  1.6 x 10^{8} 
malate  9 x 10^{2}  2.5 x 10^{5}  3.6 x 10^{7}  
triose phosphate isomeraser  glyceraldehyde3P  4.3 x 10^{3}  4.7 x 10^{4}  2.4 x 10^{8} 
blactamase  benzylpenicillin  2.0 x 10^{3}  2 x 10^{4}  1 x 10^{8} 