BIOCHEMISTRY  DR. JAKUBOWSKI
4/10/16
Learning Goals/Objectives for Chapter 6B: After class and this reading, students will be able to

It is important to get a "gutlevel" 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 in 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 constants, with units of s1 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 condition when [S] << Km, the MichaelisMenten equation becomes v0 = (kcat/KM)[E0][S]. This really describes a biomolecular rate constant, with units of M1s1, for conversion of free substrate to product. Some enzyme 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. The constant kcat/Km is also referred to as the specificity constant in that it describes how well an enzyme can differentiate between two different competing substrates. (We will show this mathematically in the next chapter.)
Table: Km and kcat values for various enzymes
Km values  
enzyme  substrate  Km (mM) 
catalase  H2O2  25 
hexokinase (brain)  ATP  0.4 
DGlucose  0.05  
DFructose  1.5  
carbonic anhydrase  HCO3  9 
chymotrypsin  glycyltyrosinylglycine  108 
Nbenzoyltyrosinamide  2.5  
bgalactosidase  Dlactose  4.0 
threonine dehydratase  LThr  5.0 
kcat values  
enzyme  substrate  kcat (s1) 
catalase  H2O2  40,000,000 
carbonic anhydrase  HCO3  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 104  9 x 105  1.6 x 108 
carbonic anhydrase  CO2  1 x 106  1.2 x 102  8.3 x 107 
HCO3  4 x 105  2.6 x 102  1.5 x 107  
catalase  H2O2  4 x 107  1.1  4 x 107 
crotonase  crotonylCoA  5.7 x 103  2 x 105  2.8 x 108 
fumarase  fumarate  8 x 102  5 x 106  1.6 x 108 
malate  9 x 102  2.5 x 105  3.6 x 107  
triose phosphate isomeraser  glyceraldehyde3P  4.3 x 103  4.7 x 104  2.4 x 108 
blactamase  benzylpenicillin  2.0 x 103  2 x 104  1 x 108 
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