Despite the above limitation, enzymes steer all of the reactions in cells through specific reaction paths. This is because enzymes are both highly selective and very precise, usually catalyzing only one particular reaction. In other words, each enzyme selectively lowers the activation energy of only one of the several possible chemical reactions that its bound substrate molecules could undergo. In this way, sets of enzymes can direct each of the many different molecules in a cell along a particular reaction pathway (Figure 2–24).
The success of living organisms is attributable to a cell’s ability to make enzymes of many types, each with precisely specified properties. Each enzyme
Figure 2–23 enzymes cannot change the equilibrium point for reactions. enzymes, like all catalysts, speed up the forward and backward rates of a reaction by the same factor. Therefore, for both the catalyzed and the uncatalyzed reactions shown here, the number of molecules undergoing the transition X → Y is equal to the number of molecules undergoing the transition Y → X when the ratio of Y molecules to X molecules is 3 to 1. in other words, the two reactions reach equilibrium at exactly the same point.
Figure 2–22 lowering the activation energy greatly increases the probability of a reaction. at any given instant, a population of identical substrate molecules will have a range of energies, distributed as shown on the graph. The varying energies come from collisions with surrounding molecules, which make the substrate molecules jiggle, vibrate, and spin. for a molecule to undergo a chemical reaction, the energy of the molecule must exceed the activation-energy barrier for that reaction (dashed lines). for most biological reactions, this almost never happens without enzyme catalysis. even with enzyme catalysis, the substrate molecules must experience a particularly energetic collision to react (red shaded area). Raising the temperature will also increase the number of molecules with sufficient energy to overcome the activation energy needed for a reaction; but in marked contrast to enzyme catalysis, this effect is nonselective, speeding up all reactions (Movie 2.2).
Figure 2–24 Directing substrate molecules through a specific reaction pathway by enzyme catalysis. a substrate molecule in a cell (green ball) is converted into a different molecule (blue ball) by means of a series of enzyme-catalyzed reactions. as indicated (yellow box), several reactions are energetically favorable at each step, but only one is catalyzed by each enzyme. sets of enzymes thereby determine the exact reaction pathway that is followed by each molecule inside the cell.
has a unique shape containing an active site, a pocket or groove in the enzyme into which only particular substrates will fit (Figure 2–25). Like all other catalysts, enzyme molecules themselves remain unchanged after participating in a reaction and therefore can function over and over again. In Chapter 3, we discuss further how enzymes work.
how enzymes find Their substrates: The enormous Rapidity of molecular motions
An enzyme will often catalyze the reaction of thousands of substrate molecules every second. This means that it must be able to bind a new substrate molecule in a fraction of a millisecond. But both enzymes and their substrates are present in relatively small numbers in a cell. How do they find each other so fast? Rapid binding is possible because the motions caused by heat energy are enormously fast at the molecular level. These molecular motions can be classified broadly into three kinds: (1) the movement of a molecule from one place to another (translational motion), (2) the rapid back-and-forth movement of covalently linked atoms with respect to one another (vibrations), and (3) rotations. All of these motions help to bring the surfaces of interacting molecules together.