Active vs. Inactive Enzymes

Instructor: Erika Steele

Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.

Enzymes are proteins that do work in cells. All enzymes have a specific job to do, but sometimes they are not needed. For this reason, the cell has to have a way to control them. This lesson will explain how enzymes are activated or inactivated.

What is an Enzyme?

Enzymes are proteins found in cells that help speed up chemical reactions such as breaking down nutrients, building DNA, and making energy. Enzymes are catalysts, which means they help in the reaction, but are not changed. You can think of them as molecular assembly workers; they work by building or breaking down other molecules at super fasts rates.

These hardworking proteins make chemical reactions occur at rates fast enough to maintain life. Without enzymes, these reactions would never occur and the cell could not survive.

For example, allegedly, Twinkies last forever, but if you eat one, your body will use enzymes to digest and get energy from it. Enzymes are the reason Twinkies and other foods are converted into energy instead of just sitting in your stomach forever.

How Enzymes Work

Enzymes work by transforming specific substrates (molecules) into specific products. Each enzyme has an action site where the chemical transformation occurs. In order to convert a substrate to a product, an enzyme must have a very specific molecular structure so that the action site is properly arranged. If the active site is misshapen in any way, the substrate will not fit and will remain unaffected.

To illustrate, let's say that there is an enzyme called 'Twinkiase' that breaks down Twinkies to get energy (ATP). In this example, the Twinkie is the substrate and energy is the product. If the Twinkie couldn't fit into the active site, 'Twinkiase' could not convert it into energy (ATP).

To put it another way, think of the active site of an enzyme as a dressing room that can transform you into a superhero, but only if you are the exact shape to fit inside. You may want to be transformed, but since you aren't really the superhero for whom the dressing room was designed, you can't fit into it.

Figure 1: Enzymes will only work if the substrate can fit in the active site.

How Does an Enzyme Become Active?

Apoenzymes are enzymes that do not need help to be turned on and work. If the substrate is present, the enzyme will do its job. Other enzymes have to be made active. These enzymes aren't lazy, they are just tightly regulated by molecules called effectors or in other ways that will be described. If an effector is required to regulate an enzyme, the enzyme is an allosteric enzyme.

Try to imagine 'Twinkiase 1' from Figure 1 as an apoenzyme. If a Twinkie is present, it is converted to energy. Maybe 'Twinkiase 2' from Figure 1 could convert Twinkies into energy if something happened to make it active. This lesson will summarize the most common ways enzymes become active.


In addition to the active site, some enzymes have allosteric sites where molecules called effectors can bind. An activator is an effector that activates the enzyme. It can do this by changing the shape of the active site. Also, an activator may be required to turn the enzyme on even if the substrate is bound.

Figure 2: Some enzymes cannot bind the substrate until an activator is bound.


Another type of enzyme that needs activation is called a holoenzyme. To become active, this enzyme needs a chemical compound called a cofactor to fill in and make the active site whole. 'Holo' kind of sounds like the word 'whole', so that might help you to remember this type of enzyme.

Holoenzymes won't become active until their active sites are made complete (whole) by adding the cofactor. The cofactor helps the substrate fit into the active site.

Figure 3: Some enzymes will only become active if a cofactor makes the active site whole.


Some enzymes require modification in order to become active. For example, many enzymes require ATP to transfer a phosphate group over before they can be active. Once the phosphate group is removed, the enzyme becomes inactive again. This is called covalent modification because the phosphate group forms a temporary covalent bond with the enzyme.

Other enzymes requiring modification need something removed instead of added. Enzymes like zymogen or proenzyme start out inactive; to become active, a small piece of them has to be removed.

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