Enzymes are generally not allowed to run uncontrolled in a cell. While they are useful in catalyzing reactions, there is a time and place for everything. In this lesson, we'll discuss enzyme activators and inhibitors that regulate these reactions positively and negatively, respectively.
So, let's say you stopped the baker from baking by taking away the oven. Then over time, the baked goods are eaten or sold. A holiday comes by, and you realize another few hundred cookies would be a pretty good idea. Thank goodness it's quite easy to ask the baker to start up again. He's happy to oblige. After all, he is a baker, and this is what he loves to do.
Enzymes don't usually have free reign of the cell and their substrates either. They are not allowed to run amok in a cell, baking up a storm. Instead, they are carefully regulated. This regulation can be positive or negative and serves to make sure an enzyme is working when it needs to and not working when it's no longer needed.
Enzyme regulation can be positive or negative
An enzyme activator is a molecule that positively regulates an enzyme's activity. Many enzymes require activators to begin or continue a process, recognize a substrate, or reach their maximum reaction rate. On the flip-side of enzyme activation is inactivation. This is achieved by an inhibitor, or a molecule that binds to an enzyme and disrupts its activity. Activators are the green light of enzyme regulation, while inhibitors are the red light of enzyme regulation. Neither activators nor inhibitors are substrates. These regulators can be proteins or other molecules.
Inhibition can be either competitive or noncompetitive and is often reversible. Remember that in a normal enzymatic interaction, an enzyme will recognize and bind to a substrate in order to catalyze a reaction. It will then release the products.
Competitive inhibition is the interruption of an enzyme's ability to bind to a substrate due to a different molecule binding to the active site. Competitive inhibitors bind with the active site. They may or may not catalyze a reaction. All competitive inhibitors prevent the substrate from binding to the enzyme, literally competing or fighting for the use of the active site. In our enzyme as a baker example, this would be similar to a dinner chef competing with the baker for the kitchen.
A competitive inhibitor prevents the substrate from binding to the enzyme
An example of competitive inhibition is used in medical treatments. Your cells contain an enzyme called alcohol dehydrogenase that converts alcohols into other chemicals. Methanol is a type of alcohol found in antifreeze and not one that we normally ingest. Methanol competes with other alcohols for the active site of alcohol dehydrogenase. Therefore, if antifreeze or other methanol-containing substances are accidentally ingested, alcohol dehydrogenase will recognize and bind to this substrate.
In this case, a reaction is catalyzed, but methanol is converted to a toxic chemical called formaldehyde, which can cause blindness. Competitive inhibition can be used to stop this from happening. Intravenous ethanol can be given to a patient, which will then compete with the methanol for the active site of alcohol dehydrogenase, thereby decreasing the amount of formaldehyde that is made.
In a different type of inhibition, there is no competing for the kitchen. Instead, in noncompetitive inhibition, a non-substrate molecule binds to an enzyme to disrupt enzyme activity but does not bind the active site. In this example, a noncompetitive inhibitor binds to a different area of the enzyme. This often causes the enzyme to change shape and may make the active site unrecognizable to a substrate. If the substrate can no longer bind, the reaction cannot take place. To our baker, this might be like shutting off the oven so that bread can no longer be baked.
A real-life example of noncompetitive inhibition could happen right in your backyard. Hopefully, you know to avoid picking and eating wild mushrooms. It's difficult to distinguish wild mushrooms that are safe to eat from the ones that are poisonous. What makes some of these fungi toxic? A molecule called alpha-amanitin can be found in a genus of mushrooms.
Alpha-amanitin binds to RNA polymerase II and prevents transcription
Alpha-amanitin binds to the enzyme RNA polymerase II. RNA polymerase II performs an important process in your cells called transcription, which makes mRNA from DNA. Alpha-amanitin does not bind to the RNA polymerase II active site, but the location it binds to interrupts RNA polymerase II activity, making it slower. The amount of transcription decreases until mRNA is no longer made. Transcription is an essential process. Alpha-amanitin is a noncompetitive inhibitor that prevents transcription and causes cell death.
In summary, in today's lesson we learned about how enzymes are regulated. Enzymes can be regulated by an activator, which is a protein or molecule that positively regulates enzyme function, or an inhibitor, which is a protein or molecule that negatively regulates enzyme function.
Two types of reversible inhibition include competitive inhibition, or the inhibition due to a non-substrate binding to an enzyme's active site and blocking substrate binding, and noncompetitive inhibition, or the inhibition due to a non-substrate binding to an enzyme at a location other than the active site, which blocks enzyme function.
At the end of this lesson, you will be able to:
- Explain how activators and inhibitors work
- Describe the two types of inhibition: competitive and noncompetitive