Enzyme Kinetics -- Part I
Part V. Definition of Km and Vmax and Their Ratio - Vmax/Km.
The Km is sometimes called the Michaelis Constant. The Km is an intrinsic property of an enzyme related to the binding constant for forming the ES complex, which is an equilibrium and can be defined by the rate constants for its formation and breakdown using the simple enzyme mechanism shown above in Figure 2a:
Figure 7. The approximate relationship between the Km and the Ks for the binding of the substrate to the enzyme which leads to the formation of the E-S complex. Ks is defined by the equilibrium formed between the enzyme (E) and substrate (S) and the E-S complex, as shown above. Ks is also defined by the ratio of the rate of breakdown of the E-S complex divided by its rate of formation.
But Km also involves the breakdown of the E-S complex to E and P, which is not a component of the Ks. Thus, the rate of the breakdown of the E-S complex to make product (P) is also defined in the simple enzyme mechanism shown in Figure 2a:
Figure 8. The definition of Km by using rate constants for simple enzyme mechanism. The point of this graphic is to emphasize that the Km constant of the enzyme catalyzed reaction includes more than just the formation of the E-S complex, but also its breakdown to form product, which is of course the key to an enzyme catalyzed reaction.
So Km reflects both binding of E to S but also the catalytic constant (shown as k3 above, but also defined as kcat) of the enzyme catalyzed reaction.
The Vmax is also dependent on the catalytic constant:
Vmax = kcat [E]
So both Vmax and Km are properties of individual enzymes and not very useful for comparing enzymes.
However, the ratio Vmax/Km can be used to compare enzymes. This ratio (Vmax/Km) measures the efficiency of the enzyme. The efficiency of the enzyme is ultimately limited by the rate of diffusion of the substrate to the enzyme - thus the diffusion of substrates, which is very rapid, sets an upper limit. The most efficient enzymes like Triose-P Isomerase are limited by how fast their substrates get to them. But most enzymes are not this efficient and more limited by chemical events in the active site of the enzyme.
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©Wilbur H. Campbell, 1995; wcampbel@mtu.edu