Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Enzymes
Each enzyme has a characteristic KM value for a given substrate

The K_M value is a key parameter of the Michaelis-Menten Equation. It characterizes The behavior of a given enzyme toward a particular substrate under specific Temperature and pH conditions. An approximate value of K_M can be obtained by a simple graphical method, as shown in Fig. 9-4. However, it is difficult to accurately determine the V_max value from such a curve, because this value corresponds to a limit that the curve approaches but never reaches. By applying algebraic transformations of the Michaelis-Menten equation, as described in Box 9-2, a more accurate K_M value can be obtained from a plot of the same data in a different coordinate system, known as a double-reciprocal plot.

Table 9-4 lists the K_M values for several Enzymes. Note that some enzymes, such as Carbonic anhydrase and catalase, require a relatively high Substrate Concentration to reach half-maximal velocity. Other Enzymes, such as Brain hexokinase, which catalyzes The transfer of a phosphate group from ATP to glucose, reach half-maximal velocity at a very low substrate concentration. Enzymes with two or more substrates, such as hexokinase or aspartate aminotransferase (which catalyzes a reversible reaction), can have different K_M values for different substrates. If an enzyme, such as Chymotrypsin, acts on several different substrates sharing a common structural feature, the K_M values for these substrates can vary widely (Table 9-4).

Class="center">Aspartate + α-Ketoglutarate ⇄

⇄ Oxaloacetate + Glutamate,

Table 9-4. Michaelis-Menten Constants (K_M) for Some Enzymes

Under intracellular conditions, enzymes are usually not saturated with substrate and therefore do not operate at their maximum possible rates. By altering intracellular substrate concentrations, the rates of enzymatic reactions in The Cell can be regulated to some extent.