Question

The catalytic efficiency of two different enzyme can be compared by the

• A. The K${}_m$ value
• B. The pH optimum value
• C. Formation of the product
• D. Molecular size of the enzyme

Answer 1

The correct option is A The K${}_m$ value

For an enzyme catalyzed reaction, which follows Michaelis-Menten kinetics, the substrate concentration at which V${}_0$ (initial rate or initial velocity), is half maximal is defined as K${}_m$, the Michaelis constant.

K${}_m$ can be used to compare enzyme efficiency but it is practically not a very useful measurement specially for multistep reactions where more than one reactions are responsible for product formation. It is useful to define a more general rate constant, k${}_{cat}$, to describe the limiting rate of any enzyme-catalyzed reaction at saturation. If the reaction has several steps and one is clearly rate limiting, k${}_{cat}$ is equivalent to the rate constant for that limiting step.The kinetic parameters k${}_{cat}$ and K${}_m$ are generally useful for the study and comparison of different enzymes, whether their reaction mechanisms are simple or complex.

Each enzyme has values of k${}_{cat}$ and K${}_m$ that reflect the cellular environment, the concentration of substrate normally encountered in vivo by the enzyme, and the chemistry of the reaction being catalyzed. The parameters k${}_{cat}$ and K${}_m$ also allow us to evaluate the kinetic efficiency of enzymes, but either parameter alone is insufficient for this task.

Two enzymes catalyzing different reactions may have the same k${}_{cat}$ (turnover number), yet the rates of the uncatalyzed reactions may be different and thus the rate enhancements brought about by the enzymes may differ greatly. Experimentally, the K${}_m$ for an enzyme tends to be similar to the cellular concentration of its substrate. An enzyme that acts on a substrate present at a very low concentration in the cell usually has a lower K${}_m$ than an enzyme that acts on a substrate that is more abundant. The best way to compare the catalytic efficiencies of different enzymes or the turnover of different substrates by the same enzyme is to compare the ratio k${}_{cat}$/K${}_m$ for the two reactions. This parameter, is sometimes called the specificity constant.