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Enzymes and activation energy

Uncover how enzymes accelerate biochemical reactions by reducing activation energy. Learn about catalytic strategies such as acid/base and covalent catalysis, and grasp the concept of transition states. Understand that enzymes modify the reaction pathway, but don't alter the reactants, products, or get used up in the process. By Ross Firestone. Created by Ross Firestone.

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  • hopper cool style avatar for user ☣Ƹ̵̡Ӝ̵̨̄Ʒ☢ Ŧeaçheя  Simρsoɳ ☢Ƹ̵̡Ӝ̵̨̄Ʒ☣
    How is it that enzymes speed reactions? By promoting the exchange of electrons, it lowers the energy needed for the reaction? thanks, T.s.
    (13 votes)
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  • hopper jumping style avatar for user Smart-guy
    How do enzymes lower the activation energy?
    (5 votes)
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    • leaf red style avatar for user FTB
      The enzyme does not lower the activation energy, what it does is provide an alternate route that is at a lower energy level, thus more molecules are able to react. Now those at a higher energy can still react via the route without the enzyme, but can also go the route of lower energy through use of the enzyme.
      (17 votes)
  • mr pants teal style avatar for user Yossi Hapanecha
    I am a little confused. I know a family member of mine, takes certain enzymes to allow their body to break down certain foods down better. However, in the video it says that enzymes can not be consumed (surely Newtons second law applies to this); yet, why does a person need to continue to take these enzymes if they're not consumed?
    (5 votes)
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    • piceratops seedling style avatar for user ajmorquillas
      Enzymes are really not consumed in a reaction but they also 'age' overtime as proteins normally do and undergo ubiquitination. Thus, new enzymes are needed when these enzymes 'age'. Also take note that enzymes are well regulated thus when there is a surmounting amount of it, the body reacts via negative feedback and inhibits its formation. Perhaps your family member is ingesting a metal or organic cofactors that might increase enzymatic activity.
      (3 votes)
  • aqualine ultimate style avatar for user Cheryn
    Can enzymes be reused indefinitely if there's some source able to repair them if they degrade with time?
    (2 votes)
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    • aqualine ultimate style avatar for user Alessandro.M.Rosa
      Enzymes generally have very short half-lives, some on the order of minutes. While they are not degraded in the reactions themselves, their structural stability is such that they break down rapidly. It is not a good thing for the body to have a molecule like pepsin in its active form for an indefinite period of time, because once it has finished catalyzing the reactions with its substrate, in this case the food that is eaten, it would start to break down self proteins.
      (2 votes)
  • blobby green style avatar for user David Choi
    Enzymes apparently help speed up chemical reactions but do they speed up all chemical reactions?
    (2 votes)
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    • blobby green style avatar for user Robert Lyday
      The general definition of enzymes is that they speed up reactions and the great majority of the time they do, on occasion, they can slow the reaction back down by not working as effectively, or in effect shutting the enzyme off. This is done by certain molecules binding to their allosteric sites which makes enzyme activity less able or effective, overall that will drop reaction speed.
      (1 vote)
  • starky ultimate style avatar for user scottmorford99
    I'd rather climb the hill than use the shovel to be honest lol
    (2 votes)
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  • male robot donald style avatar for user Dan
    if enzymes only affect the kinetics (how fast) a reaction takes place and not the thermodynamics, then how would reactant (A) ever get enough energy to traverse the free energy of activation? If enzymes don't change the thermodynamics then that means that the reaction would take place without their input but just at a slower pace, right?
    (1 vote)
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    • aqualine ultimate style avatar for user Alessandro.M.Rosa
      Enzymes provide an alternate pathway through the transition state that is lower in energy than the transition state of the reaction going from A to B.

      What you have to remember is that we are talking about biological conditions. so often, without the aid of an enzyme, the environment inside the cell, or the body of a multicellular organism does not provide enough energy, or harsh enough conditions to overcome the energy of activation. Enzymes also control reactions. If you think about the amount of energy that is release in most chemical reactions, we would likely spontaneously combust anytime anything took place within our body in a spontaneous and uncontrolled manner.
      (2 votes)
  • piceratops ultimate style avatar for user Basem Fouda
    When we write A --> B on the diagram, do we mean energy of all reacting molecules at point A and then energy of all products at point B OR do we mean molecule A becomes molecule B ?
    (1 vote)
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  • blobby green style avatar for user kevin li
    How are spontaneous reactions able over come the free activation energy barrier?
    (1 vote)
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  • blobby green style avatar for user nonsogent
    Does a spontaneous reaction occur randomly or need an enzyme before it take place?
    (1 vote)
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    • piceratops ultimate style avatar for user Iman Baharmand
      Hi there!

      Great question; it's important to realize that spontaneity is a factor of a reactions 'Thermodynamics'. Whereas as an enzyme effects a reaction's 'Kinetics'.

      That is to say, an enzyme will lower a reaction's activation energy (EA) but it will not necessarily make a reaction happen spontaneously. The presence of an enzyme will, however, make a spontaneous reaction occur faster.

      The occurrence of a reaction randomly or it's requirement for an enzyme is a factor of how much energy is needed to overcome the activation energy barrier.
      (1 vote)

Video transcript

Today, we're going to talk about how enzymes can influence a reaction's activation energy. But first, let's review the idea that enzymes make biochemical reactions go faster. And in order to do that, they use a bunch of different catalytic strategies. Now, there are lots of different catalytic strategies that enzymes use. But a couple of the key ones are acid/base catalysis, where enzymes use their acidic or basic properties to make reactions go faster by helping out with proton transfer. There's also covalent catalysis where enzymes covalently bind to a reacting molecule to help with the electron transfer. There's electrostatic catalysis where enzymes use charged molecules or metal ions to stabilize big positive or negative charges. And we also have proximity and orientation effects, where enzymes make collisions between reacting molecules happen a little more often. So what effect do these catalytic strategies actually have on a reaction? Well, let's look at a sample reaction where we're having molecule A being converted to molecule B. Now, we can look at the process of this reaction using something called a reaction coordinate diagram. And here, we'll plot the energy state of our molecules against the progress of the reaction. So essentially, using this graph, we'll follow the energy level of molecule A as it's converted to molecule B. Remember that a molecule's energy level is related to its stability. And something that has a lower energy state is more stable. And for something to transform to a more unstable form, it needs an input of energy to get there. So looking at this graph, you'll notice that the energy of molecule A will rise up pretty high and then drop all the way down to the energy of molecule B. And we can actually define a couple of values from this graph. The transition state of a reaction, which is represented by this double dagger symbol, is the highest energy point on the path from A to B. And it's where you'll find the most instability throughout the entire reaction. Now the difference between the energy level where we start and the top of our graph at our transition state is what we call the delta G double dagger or the free energy of activation. And this is the amount of energy that A needs to have in order to break the reaction barrier to ultimately get to point B. You'll also notice that there is a difference in energy between point A and point B. And we call this the standard free energy change for the entire reaction. And it represents the net change in energy levels between our reactant and our product. And it's also the energy that is released into the environment once the reaction is over. Reactions you typically look at will have their products at a lower energy state than their reactants since that makes the reaction spontaneous. Now, it's important to recognize that it is the free energy of activation energy value, which is the difference between point A and the transition state, that usually determines how quickly a reaction will go. And usually this energy value is much higher than the free energy change for the reaction, which is why enzymes speed up a reaction by lowering the reaction's activation energy. Now, I want to quickly point out that you may see delta G double dagger written out as EA in some textbooks. And you may see the standard free energy change for the reaction written out as E reaction. And I'm just letting you know that might see both sets of terms used from time to time. Now, let's look at an analogy to get a closer look at how this all works. And let's say there's a giant hill that you're trying to climb. And it's a pretty steep hill, that goes up really high. But you need to get to the other side of the hill. Now, this would be a pretty scary thing on its own since you would need to go all the way up and then all the way down the mountain to get to the finish line. But if I were to give you a shovel, then now you could dig your way through the mountain and not have to climb up so high. In this example, the shovel represents an enzyme and the hill represents the activation energy barrier that prevents you from getting to start to finish. By using the shovel, you're able to lower the height of the hill you have to climb. But in both cases, it's important to recognize that you still started and finished at the same points. So let's go back to our example from before with our reaction coordinate diagram. But now, let's say that the reaction has a catalyst. So with the catalyst, the activation energy barrier that molecule A has to overcome in order to get to point B is much smaller. And this will mean that your reaction will have a transition state with a much lower energy, meaning that it's more stable with the enzyme and also that the reaction as a whole have a much lower activation energy. Now it's really important to recognize that like our example where you're trying to climb the hill, the enzyme will not be changing the starting and ending points of the reaction. It doesn't change molecule A or molecule B. Your starting and ending points are always the same. And the only thing that changes is the path that you take to get from A to B. Now since our starting and ending points aren't changing, it follows that the enzymes are not used up when they catalyze a reaction. And there is no permanent change to the enzyme following a reaction. So what did we learn? Well, first we learned that enzymes work by lowering the free energy of activation of a reaction, making it much easier for the reactants to transition and form products. And we also learned that the free energy of the reaction doesn't really change when you use an enzyme and when you don't. Second, we learned that, despite the change in pathway to get from A to B, the reactants and products do not change when using an enzyme versus when not using an enzyme. And finally, we learned that enzymes are not consumed when they catalyze a reaction and the same enzyme can catalyze reactions over and over again.