Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online


B:  Kinetics of Simple and Enzyme-Catalyzed Reactions



Learning Goals/Objectives for Chapter 6B:  After class and this reading, students will be able to

  • write appropriate chemical and differential equations for the rate of disappearance of reactants or appearance of products for 1st order, pseudo first order, second order, reversible first order reactions
  • draw and interpret graphs for integrated rate equations (showing reactant or product concentrations as a function of time) and initial rate equations (showing the initial velocity vo as a function of reactant ;
  • derive kinetic rate constants from data and graphs of integrated rate and initial rate equations;
  • write appropriate chemical, differential equations, and initial rate equations for the rate of disappearance of reactants or appearance of products for simple enzyme catalyzed reaction;
  • differentiate between rapid and steady state assumptions;
  • simplify the initial rate equation containing rate constants for an enzyme catalyzed reactions to one replacing the rate constants with kcat and KM, and give operational and mathematical definitions of those constants;

B7.  Meaning of Kinetic Constants

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

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



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