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Co-factors, co-enzymes, and vitamins

Video transcript
Today, we're going to talk about co-factors and co-enzymes and how sometimes they can be essential to proper enzymatic function. But first, let's review the idea that enzymes make reactions go faster. And they do this by lowering the activation energy peak of their respective reactions. Let's also review the idea that enzymes bind their substrates at a location on the enzyme called the active site, which is where most of the reaction takes place. Now, not all enzymes are able to catalyze reactions on their own. And some need a little help. So if we have our enzyme here, trying to react with our substrate over here, sometimes something called a co-factor or a co-enzyme will be needed, which will also need to bind to the enzyme in order for it to function properly. And we're going to go over what co-enzymes and co-factors are and exactly how they work. So first, we'll talk about what a co-enzyme is. Well, co-enzymes are organic carrier molecules. And what I mean by "organic" is that they're primarily carbon-based molecules. And by "carrier," I mean that co-enzymes hold on to certain things for an enzyme to make the catalysis run a little more smoothly. And a great example of a co-enzyme is NADH, which acts as an electron carrier. And here, I've shown NADH dissociating into its oxidized form, NAD+, as well as to a hydride ion, which basically just exists as a pair of electrons that some other molecule would be grabbing. So NAD+ can accept electrons, causing the molecule to be converted to NADH, which could then carry electrons for an enzyme. Now if you remember the lactic acid fermentation reaction, where pyruvate is converted to lactic acid, you'd see that the enzyme catalyzing this reaction, lactate dehydrogenase, uses NADH as a co-enzyme in order to transfer electrons to the pyruvate molecule, in order to turn it into lactic acid. And in this sense, NADH is acting as an electron-carrying co-enzyme. Another example of a co-enzyme is co-enzyme A, which like NADH acts as a carrier molecule. But instead of carrying electrons like NADH does, co-enzyme A, which we sometimes call CoA, holds on to acyl or acetyl groups instead. And you'd see CoA appear quite often in metabolic reactions, where it will carry these two carbon acetyl groups from one molecule to another. Now, co-factors are a little different from co-enzymes. While co-enzymes are only really involved in transferring different things from one molecule to another, co-factors are directly involved in the enzyme's catalytic mechanism. They don't strictly carry something like a co-enzyme would, but might be stabilizing the enzyme or the substrates or helping the reaction convert substrates from one form to another. A great example of this is with the enzyme DNA polymerase. Remember that DNA polymerase is responsible for helping out with synthesizing new DNA during DNA replication. Now, you may remember that DNA is a very negatively charged molecule because of all the negatively charged phosphate groups that you'll find around it. Well, DNA polymerase uses a magnesium ion as a co-factor, which can use its big positive charge to stabilize all that negative charge on DNA. And you can see how this is different from a co-enzyme. Becomes instead of acting as a carrier molecule, the magnesium ion co-factor is stabilizing the DNA and is more directly involved in the actual catalysis. Now, interestingly, what people normally called vitamin and minerals, like the kinds that a doctor would tell you to make sure you get enough of in your diet, are often different co-factors and co-enzymes. And what's special about vitamins and minerals is that your body can't build them up from scratch. And you need to get them from your diet in order to stay healthy. So when we say vitamins, we typically refer to organic co-factors and co-enzymes. So two great examples are ones we just discussed. Vitamin B3, which you may see being called niacin on a food label, is actually just a precursor for NAD. And vitamin B5 is just a precursor for co-enzyme A. Minerals, on the other hand, are inorganic, meaning they aren't carbon based. And minerals are usually just co-factors in our body. So magnesium would be a great example of a mineral co-factor that an enzyme like DNA polymerase would use. Now, not all minerals act only as co-factors. Some minerals, like calcium, which can act as a co-factor, is also a critically important component of bone and teeth. And it doesn't strictly act as an enzyme co-factor here. It's actually an important part of the structure itself. So what did we learn? Well, first we learned that not all enzymes are able to function alone and some need a little help. And next, we learned that this help can come from co-enzymes, which usually act as carrier molecules, or co-factors, which directly assist with the catalysis that the enzyme is doing. And finally, we learned that the vitamins and minerals generally refer to dietary co-factors and co-enzymes.