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Enzymes review

Key Terms

CatalystA substance that speeds up a chemical reaction without being changed
EnzymeA biological catalyst (usually a protein)
SubstrateThe reactant molecule that an enzyme works on
Active siteThe part of the enzyme where the substrate binds

Enzyme structure and function

Enzymes are catalysts. They are usually proteins, though some RNA molecules act as enzymes too.
Enzymes lower the activation energy of a reaction - that is the required amount of energy needed for a reaction to occur. They do this by binding to a substrate and holding it in a way that allows the reaction to happen more efficiently.
Reaction coordinate diagram showing the course of a reaction with and without a catalyst. With the catalyst, the activation energy is lower than without.
Image modified from OpenStax, CC BY 3.0.
The part of the enzyme where the substrate binds is called the active site. Here, the enzyme changes shape slightly, fitting tightly with the substrate and forming the enzyme/substrate complex.

Factors affecting enzyme activity

Enzyme activity can be affected by a variety of factors, such as temperature, pH, and concentration.
Enzymes work best within specific temperature and pH ranges, and sub-optimal conditions can cause an enzyme to lose its ability to bind to a substrate.
  • Temperature: Raising temperature generally speeds up a reaction, and lowering temperature slows down a reaction. However, extreme high temperatures can cause an enzyme to lose its shape (denature) and stop working.
  • pH: Each enzyme has an optimum pH range. Changing the pH outside of this range will slow enzyme activity. Extreme pH values can cause enzymes to denature.
  • Enzyme concentration: Increasing enzyme concentration will speed up the reaction, as long as there is substrate available to bind to. Once all of the substrate is bound, the reaction will no longer speed up, since there will be nothing for additional enzymes to bind to.
  • Substrate concentration: Increasing substrate concentration also increases the rate of reaction to a certain point. Once all of the enzymes have bound, any substrate increase will have no effect on the rate of reaction, as the available enzymes will be saturated and working at their maximum rate.

Common mistakes and misconceptions

  • Enzymes are "specific." Each type of enzyme typically only reacts with one, or a couple, of substrates. Some enzymes are more specific than others and will only accept one particular substrate. Other enzymes can act on a range of molecules, as long as they contain the type of bond or chemical group that the enzyme targets.
A substrate entering the active site of the enzyme.
Image modified from "Enzymes: Figure 2," by OpenStax College, Biology, CC BY 3.0.
  • Enzymes are reusable. Enzymes are not reactants and are not used up during the reaction. Once an enzyme binds to a substrate and catalyzes the reaction, the enzyme is released, unchanged, and can be used for another reaction. This means that for each reaction, there does not need to be a 1:1 ratio between enzyme and substrate molecules.

Want to join the conversation?

  • blobby green style avatar for user Frank Bignami
    Do enzymes in the human body have the same optimal peak rate?
    (19 votes)
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  • orange juice squid orange style avatar for user Prairie
    So, an actual field question! But this section got me wondering.

    I take lactase to assist in the breakdown of the few milk products I consume because it appears I'm lactose intolerant. I'm instructed to take one (or two, if one doesn't seem to be enough over time) every time I consume dairy.

    Why do I need to continue to take these over time? Enzymes are reusable, right? They're not reactants. They should stick around after managing my lactose intake. So why do I need to keep taking these things every time I consume dairy? Isn't there a point after prolonged use where there should be a sufficient buildup of lactase enzymes present in the gut to address this issue long term?

    Is my immune system doing away with the lactase for some reason? Do I have really garbage gut pH? What's going on here?
    (14 votes)
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    • male robot donald style avatar for user Tybalt
      Take this with a pinch of salt, but I am pretty sure that enzymes do not last forever. They will eventually wear out, and will be broken down into amino acids for other proteins. The supplements may also have to fight through the acidity of the stomach, and if you are warmer than usual, it is more likely that the enzymes will denature. Because they will wear out or potentially get damaged, you would need to replenish these enzyme stores to ensure that your body can break down the lactose found in dairy.

      Like diabetes, you would have to keep taking the supplements in order to manage the condition. Again, though, please take my answer with a pinch of salt.
      (15 votes)
  • marcimus pink style avatar for user Joelle
    Can an enzyme have more than 2 active site?
    (15 votes)
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    • female robot ada style avatar for user SparrowHarvest
      Yes they actually can-- Here's a bit I found on Reddit ( any search will bring up a number of more reliable sources saying the same thing however ;D)

      Yes. Some enzymes will bind a substrate and then there will be a change in its quaternary structure that could open up another active site for a different type of substrate. Other enzymes have more then one of the same active site and they can bind multiples of the same substrate.

      Hope that helps!
      (10 votes)
  • leaf blue style avatar for user Miri
    where are enzymes created? in the cell?
    (6 votes)
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  • starky tree style avatar for user Sarah Fox
    What is the pH?
    (4 votes)
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  • aqualine ultimate style avatar for user antonio.grant226
    What are the different types of enzymes and what are the differences?
    (5 votes)
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    • starky sapling style avatar for user sarahhigashiyama
      If you are asking the types of enzymes by how they break reactant, there are 7 types (some websites say there are 6 because after 2018, one type was added (-_-)).
      Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerase, Ligases, and Translocase. (Maybe you don't need to learn them at this point.)

      If you are asking the types of enzymes by what reactant they break, there are...well there are many (it is said there are about 2000 kinds).
      I have never counted but maybe there will be 20 ~30 enzymes you will see learning biology. One third of them will be about plants and the rest will be about human body. Don't worry, you will learn them step by step.

      As you can see the above, basically the difference is determined by how they break or what they break.
      (11 votes)
  • aqualine seed style avatar for user Noah Baxley
    Why does each enzyme have an optimal pH range. Sure, it will get denatured if pH gets too high or too low, but how does pH affect the enzyme activity within that range? Additionally how does changing it outside of that range slow down the reaction?
    Does the concentration of H+ affect certain amino acids and change the shape of the enzyme?
    (8 votes)
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  • blobby green style avatar for user Charlotte
    In the last paragraph it mentions that an enzyme is not a reactant.
    What is a reactant?
    (4 votes)
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  • mr pants purple style avatar for user Jasmine
    Why can enzymes work again after being inactivates but may not work after being denatured?
    (0 votes)
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    • eggleston blue style avatar for user IsotonicFlaccidCell21
      When they are denatured, the shape of the protein is changed permanently, thus the active site is changed permanently as well. Therefore the enzyme cannot work like it used to. But an inactive enzyme is caused when the temperature is too low for example, the enzyme is fine because it is really cold, there are virtually no collisions/very less collision, meaning that no reaction is taking place. As there is nothing wrong with the enzyme, if you just merely increase the heat, the enzyme will work as it should because there will be collisions.
      (11 votes)
  • duskpin ultimate style avatar for user jsugino
    Can enzyme denaturing also be caused by extremely low temperatures?
    (4 votes)
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