Enzyme reaction velocity and pH | Cellular energetics | AP Biology | Khan Academy

In this video, we're going to talk about enzymes. In particular, we're going to talk about the effect of pH on enzymes—how acidic or basic the environment is and how that affects enzyme activity.

So just as a bit of review, enzymes are molecules that help catalyze various reactions, and they are all throughout biological systems. Most enzymes are proteins—large proteins often times made up of chains of amino acids. I could draw a chain of amino acids where each of these circles represents an amino acid, and the primary structure is just a sequence of the amino acids. But there's 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 form some type of hydrogen bond. Just as a review, a hydrogen bond is an interaction between hydrogen and a more electronegative atom. 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. 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.

Then the hydrogen ends of a water molecule get a partially positive charge. This is the Greek lowercase delta for partially positive charge; it's what it's typically used for. 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.

These hydrogen bonds not only help to define the secondary structure of the proteins, which helps to define the shape of the protein, but they can also interact with the substrate of the protein—the things that the proteins are trying to catalyze reactions on. 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.

If you want to see a more complex picture of that, this is a detailed schematic of a substrate interacting with an enzyme. What you see circled in yellow is the substrate here, and you see these dotted lines—those are the hydrogen bonds. You can see a hydrogen bond between a hydrogen and a nitrogen, a hydrogen bond between an oxygen and a hydrogen. The yellow part is the substrate, and all the stuff that's wrapping around it is the enzyme itself.

So, 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 classes, the negative log of the hydrogen ion concentration. 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? If you have a bunch of, depending on how many hydrogen ions you have floating around, 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.

You can 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 will often see a diagram that looks like this. In the vertical axis, you will often see reaction velocity, where reaction velocity goes higher as we go higher in the vertical direction. In this axis right over here, you might see our level of pH. 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. That is associated with acidic environments.

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. If your pH is around seven, then that would be a neutral situation. But different enzymes' activities peak at different pHs. 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. You would typically see this type of an enzyme in, say, a place like the stomach, which is a very acidic environment.

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. For example, this peak might be at, say, a pH of seven. Then you might have other enzymes that do better in a basic environment. 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 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—a more neutral environment, or even slightly basic—its optimal activity is at a pH of 8. So, I will leave you there. The big picture is that a lot of an enzyme's shape or its ability to interact with the substrate is based on hydrogen bonds. You can imagine hydrogen bonds could be influenced by hydrogen ion concentration. 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.