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Enzyme reaction velocity and pH

Enzymes are proteins that catalyze reactions in biological systems. The pH of an enzyme’s surroundings affects the bonds that maintain the enzyme’s structure, which can also affect the enzyme’s activity. Different enzymes have optimal activity at different pH levels, with some enzymes working best in acidic environments and others in neutral or basic environments.

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  • blobby green style avatar for user tlee25
    Sal mentions how a too-high concentration of H+ ions could affect the secondary and tertiary protein structures. How would a concentration that is too low, resulting in a high pH, decrease the effectiveness of the enzyme? Thanks!
    (2 votes)
    Default Khan Academy avatar avatar for user
    • stelly blue style avatar for user ³oɔiiᴎ
      Here are some examples:

      Damage to Structure:
      Enzymes, which help chemical reactions happen, are like keys that fit into specific locks (substrates). A high pH can mess up the shape of these keys, making them unable to fit into their locks properly.

      Less Stickiness:
      Enzymes need to stick to their substrates to work. A high pH makes enzymes less sticky, so they can't hold onto their substrates as well, slowing down reactions.

      Not in the Right Zone:
      Enzymes work best in a certain pH range. If the pH is too high, they're not in their comfort zone and don't work as well.

      Changes in Chemistry:
      Some reactions enzymes help with involve transferring protons (H+ ions). A high pH changes the chemistry, making these reactions harder to do.
      (1 vote)

Video transcript

- [Instructor] In this video, we're gonna talk about enzymes. And in particular, we're going to talk about the effect of pH on enzymes, how acidic or basic the environment is, how that affects enzyme activity. So just as a bit of a review, enzymes are molecules that help catalyze various reactions, and they are all throughout biological systems. Most enzymes are proteins, large proteins oftentimes, made up of chains of amino acids. And so I could draw a chain of amino acid, where each of these circles represents an amino acid. And the primary structure is just a sequence of the amino acids, but there is also a secondary structure on how this amino acid backbone interacts with itself. And a lot of that is based on hydrogen bonding. So that amino acid and that amino acid, maybe it forms some type of hydrogen bond. And just as a review, a hydrogen bond is an interaction between hydrogen and a more electronegative atom. So for example, we've seen this in water. Let me draw a couple of water molecules right over here. And this is all review. In water, you have these covalent bonds between the oxygen and the hydrogen. So arguably, they're sharing the electrons. But because oxygen is more electronegative, it likes to hog the electrons more, the electrons spend more time around the oxygens. So the oxygen end of a water molecule gets a partially negative charge, and then the hydrogen ends of a water molecule get a partially positive, partially positive charge. This is the Greek lowercase delta for partially, partial charge is what it's typically used for. And so the partially negative ends would be attracted to the partially positive ends of another molecule, and that's what hydrogen bonds are. It's not always between hydrogen and oxygen. In fact, oftentimes it's between hydrogen and nitrogen, which is another electronegative atom. And these hydrogen bonds, not only do they help define the secondary structure of the proteins, which helps to find the shape of the protein, they can also interact with the substrate of the protein, the things that the protein's trying to catalyze reactions on. So for example, if that's the substrate, and I'm just doing it as a big red circle, parts of it might form hydrogen bonds with the enzyme itself. And if you want to see a more complex picture of that, this is a detailed schematic of a substrate interacting with an enzyme, where what you see in, circled in yellow, that is the substrate here. And you see these dotted lines, those are the hydrogen bonds. So you could see a hydrogen bond between a hydrogen and a nitrogen, a hydrogen bond between an oxygen and a hydrogen. And so the yellow part is a substrate. And all the stuff that's wrapping around it, that is the enzyme itself. So with that out of the way, how does pH play into it? Well, we just have to remind ourselves what pH is. pH, which is often viewed as the power of hydrogen, that's where the p comes from, is the negative log, or at least the way it's introduced in many introductory chemistry class, the negative log of the hydrogen ion concentration. And a hydrogen ion is essentially a proton. Well, how would this affect an enzyme's shape and its ability to interact with the substrate, the thing that it's trying to act on? Well, if you have a bunch of, depending on how many hydrogen ions you have floating around, and oftentimes it'll be in the form of hydronium, which is a water molecule where the oxygen is bonded to one extra hydrogen proton, well, it might mess with these hydrogen bonds, where some of these hydrogen protons usurp the bond with the negative end of one of these molecules or repel some of the positive ends of some of these molecules in a certain way. And so you could imagine that different enzymes might have a different level of activity at different levels of pH, and that actually is the case. In fact, you'll often see a diagram that looks like this. So in the vertical axis, you will often see reaction, reaction velocity, where reaction velocity goes higher as we go higher in the vertical direction. And in this axis right over here, you might see our level of pH. And remember, pH, because you have this negative out front, a high hydrogen ion concentration, because of this negative, that will give you a low pH, and that is associated with acidic, acidic environments. And a low hydrogen ion concentration, that's associated with a high pH, once again because of this negative out front, and that's associated with a more basic situation. And if your pH is around seven, then that would be a neutral situation. But different enzymes' activities peak at different pHs. So for example, you might have an enzyme like this whose activity peaks at a pH of, let's say this is right over here, a pH of four, which is relatively acidic. And you would typically see this type of an enzyme in, say, a place like the stomach, which is a very acidic environment. And then you might see other enzymes that actually don't do too well in an acidic environment, but do quite well in a more neutral environment. So for example, this peak might be at, say, a pH of seven. And then you might have other enzymes that do better in a basic environment. And we actually do see this in the human body. For example, lipase, which is an enzyme that breaks down fat, when it's found in the stomach, that particular version of lipase, it actually has optimum activity closer to this, at a pH of roughly four or five. While lipase that is secreted from the pancreas, which acts in the small intestines, which is a more neutral environment or even slightly basic, its optimal activity is at a pH of eight. So I will leave you there. Big picture is, is that a lot of an enzyme's shape or its ability to interact with a substrate is based on hydrogen bonds. And so you can imagine hydrogen bonds could be influenced by hydrogen ion concentration. And so because of that, depending on your enzyme and sometimes the substrate that you're dealing with, you might have different reaction velocities at different pHs, with some enzymes doing better in acidic environments and other enzymes doing better in neutral or basic environments.