The Theme of Regulation in Energy Flow and Enzymes
If you were to look at a diagram of all of the enzyme-catalyzed reactions in a cell, the complexity of the map and the spider web-like qualities of the pathways would be enough to make your head spin more than the Mad Tea Party teacups at Disneyland. Yet, each of your cells keeps track of every single one of these reactions.
Even more amazingly, each of these reactions is carefully regulated. Since many enzyme reactions are limited by the interaction of the substrate with enzymes through diffusion, the enzymes can increase the efficiency of their biochemical pathways if enzymes from the same pathway spatially group together. Large, multi-enzyme complexes can be found during many cellular processes. Keep an eye out for them as you study DNA replication and protein synthesis in the weeks to come.
The cell can control enzymatic reactions in many other ways as well. Cells can regulate how much enzyme is produced, or even target an enzyme for destruction. In another scenario, a biological regulator, in the form of a molecule, may bind to an enzyme near its active site. This molecule can affect the ability of the enzyme to catalyze a desired reaction by either inhibiting or stimulating catalysis. This regulatory molecule may also bind positionally farther away from the active site on the enzyme and still regulate its activity. Take that.
During feedback inhibition, an enzyme that does its job early on in a biological pathway is bound and inhibited by a product generated much later in the pathway. Feedback inhibition is a nifty mechanism for the cell; when a lot of product has been generated, the cell can then slow down or turn off the generation of the product. This regulation is called noncompetitive regulation because the inhibitor binds to a different site than the substrate yet still regulates its activity. On the other hand, in competitive inhibition, an inhibitor competes with the substrate for binding to an enzyme’s active site. Competitive inhibition can be compared to you duking it out with your little brother over the last Double-Stuf Oreo in the package. You grabbing it prevents him from grabbing it, and vice versa.
Many protein-based enzymes also consist of small molecules or metals called coenzymes. Our bodies cannot synthesize many of these coenzymes, and therefore, we must consume them as a part of our diet, or as supplemental vitamins. Flintstones Chewables, here we come. Coenzymes bind at or near an enzyme’s active site and assist in catalysis. Coenzymes provide another way for the cell to regulate its enzymatic activity.