Screengrab of animation of Acid-Base catalysis from the Physical Chemistry book
Enzymes are the catalysts of biological systems. They are remarkable molecular designs that determine the pattern of chemical transformations.
The most striking characteristics of enzymes are their catalytic power and specificity. By using the full gamut of intermolecular forces, enzymes bring substrates together in an optimum orientation.In short, they catalyze reactions by stabilizing transition states. By doing this selectively, an enzyme determines which one of several potential chemical reactions actually occurs. This interesting subject is covered in Chapter 10.9 in the Physical Chemistry textbook.
Enzymes and their catalytic power
Enzymes accelerate reactions; and in the absence of enzymes, most reactions in biological systems do not occur at noticeable rates. Even a reaction as simple as the hydration of carbon dioxide is catalyzed by an enzyme called carbonic anhydrase.
H2O + CO2 ⇌ HCO3‾ + H⁺
The transfer of CO2 from the tissues into the blood and then to the alveolar air would be less complete in the absence of this enzyme.
Enzymes are highly specific
Enzymes are highly specific both in the reaction catalyzed and in their choice of reactants, which are called substrates. An enzyme usually catalyzes a single chemical reaction or a set of closely related reactions. The degree of specificity for substrate is usually high and sometimes virtually absolute.
Enzymes cannot alter reaction equilibria
Since an enzyme is a catalyst, consequently it cannot alter the equilibrium of a chemical reaction. This means that an enzyme accelerates the forward and reverse reaction by precisely the same factor.
Kinetic properties of enzymes
Much of the catalytic power of enzymes comes from their bringing substrates together in a favourable orientation in enzyme-substrate (ES) complexes. The existence of the ES complexes has been shown by Leonor Michaelis who interpreted the maximum rate of an enzyme-catalyzed reaction in terms of the formation of an ES complex.
At a constant concentration of enzyme, the reaction rate increases with increasing substrate concentration until maximum rate is reached. In contrast, uncatalyzed reactions do not show this saturation effect. At a sufficiently high substrate concentration, the catalytic sites are filled and so the reaction rate reaches a maximum.
The Michaelis-Menten model accounts for the kinetic properties of enzymes. Using this model, an enzyme (E) combines with a substrate (S) to form an enzyme-substrate (ES) complex, with a rate constant k1. The ES complex has two possible fates: it can either dissociate into E and S with a rate constant k2 or it can proceed to form product P with a rate constant k3.
For many enzymes, the rate of catalysis (V) varies with the substrate concentration [S], in the manner shown in Figure 10.7 .
Click to enlarge
Figure 10.7 is a plot of the variation rate as a function of the substrate concentration for an enzyme that obeys Michaelis-Menten kinetics. The Michaelis-Menten equation is therefore used to explain the kinetic data given (in Figure 10.7 above). The equation can be written as such:
Where, V is the rate when the enzyme is fully saturated with substrate and KM is the Michaelis constant.
Again, by referring to Figure 10.7, we can see that at very low substrate concentration when [S] is much less than KM, ν = [S]V/KM; that is, the rate is directly proportional to the substrate concentration. At high substrate concentration, when [S] is much greater than KM, ν = V; that is, the rate is maximum and it is independent of substrate concentration.
In summary, we can say that the Michaelis constant is the substrate concentration at which the reaction rate is half maximum (as shown in Figure 10.7). When [S] = KM, then ν = V/2. Thus KM is equal to the substrate concentration at which the reaction rate is half of its maximum value.