The Michaelis constant KM reflects the affinity of an enzyme for its substrate; kcat reflects the catalytic ability of an enzyme. The ratio of these, kcat/KM, is the specificity constant, which is a measure of how good the enzyme is at its job. A high specificity constant means that a reaction goes fast (kcat is big) and the enzyme does not need a high concentration of substrate (KM is small). When an enzyme has relatively low specificity, that is, it can work on a number of different substrates, the substrate that has the largest specificity constant is the preferred substrate for the enzyme.
A reaction cannot go faster than the rate at which enzyme and substrate actually collide. In some enzymes this rate of collision is the factor limiting the overall rate, and such enzymes are said to be "diffusion limited" and are considered to have reached a perfection of biological design. Diffusion-limited values of kcat/KM can be calculated to be in the range 108-1010 liter per mole per second. Table 11.1 shows kcat, KM, and kcat/KM for a number of enzymes.
Speed Isn't Everything
In this chapter we emphasize enzymes as tools for performing a reaction that is required in the cell. For this type of enzyme, the faster the reaction occurs, the better, although in many cases an "optimized" enzyme must be controlled so that it only operates at maximum rate when lots of product is needed.
For other enzymes, slowness is a virtue. We will meet trimeric G proteins in Chapter 16. Like the GTPases that we have already met (EF-tu, Ran, and Rab), these are active when they have GTP bound, and switch to an inactive state when they hydrolyze the GTP to GDP. While they are active, they turn on their target processes. In these enzymes, therefore, a slow rate of reaction allows them to act as timer switches. They are activated when GDP is ejected and GTP bound. They then remain active until they hydrolyze the GTP. For the typical trimeric G protein listed in Table 11.1, kcat is 0.02 per second, so the "timer" is set to 50 s (the reciprocal of 0.02 s-1).
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