KM - The Michaelis constant, KM, is often associated with the affinity of the enzyme for substrate, but this is not always correct. A more accurate statement is that, for reactions obeying Michaelis-Menten kinetics, KM is a measure of the substrate concentration required for effective catalysis to occur. That is, an enzyme with a high KM requires a higher substrate concentration to achieve a given reaction velocity than an enzyme with a low KM. Table 11.1 lists values of KM, kcat, and kcat/KM for selected enzymes.

kcat - The Michaelis-Menten equation, V = Vmax[S]/(KM + [S]), can be rewritten as

V = kcat[E]t[S] / ( KM + [S])

where Vmax = kcat[E]t. kcat incorporates the rate constants for all the reactions between ES and E + P in a multistep enzymatic process. For a two-step reaction, kcat = k2. For more complex reactions, kcat depends on which steps in the process are rate-limiting.

kcat gives a direct measure of the catalytic production of product under optimum conditions (saturated enzyme). The units of kcat are seconds^-1. The reciprocal of kcat can be thought of as the time required by an enzyme molecule to "turn over" one substrate molecule. Alternatively, kcat measures the number of substrate molecules turned over per enzyme molecule per second. Thus, kcat is sometimes called the turnover number. Some typical turnover numbers are shown in Table 11.1, as well.

kcat/KM - This ratio is often thought of as a measure of enzyme efficiency. Either a large value of kcat (rapid turnover) or a small value of KM (high affinity for substrate) makes kcat/KM large.

When [S] << KM (dilute solution), equation 11.27 becomes

V (kcat/KM)[E][S]

Here, kcat/KM behaves as a second-order rate constant for the reaction between substrate and free enzyme. This ratio is important, for it shows what the enzyme and substrate can accomplish when abundant enzyme sites are available, and it allows direct comparison of the effectiveness of an enzyme toward different substrates. When an enzyme has a choice of two substrates, A or B, present at equal, dilute concentrations,

Table 11.2 shows that when an enzyme has different substrates on which it can work, the range of efficiencies may vary considerably. Note that for chymotrypsin the kcat/KM ratio varies 1-million fold.

As a second-order rate constant, kcat/KM has a maximum possible value, which is determined by the frequency with which enzyme and substrate molecules can collide. A reaction which attains such a velocity is said to be "diffusion-limited" because every encounter leads to reaction. If every collision results in formation of an enzyme-substrate complex, diffusion theory predicts that kcat/KM will attain a value of about 10⁸ to 10⁹ (mol/L)^-1s^-1. The enzymes carbonic anhydrase, fumarase, and triose phosphate isomerase actually approach this limit.