So up until this point, we've really only talked about amino acids as free individual amino acids, and we haven't talked much about their ability to link together to form long proteins. But we're going to begin that discussion in this video by talking about peptide bonds. So recall that free amino acids can actually be linked together via peptide bonds. And so, all a peptide bond really is, is just an amide covalent linkage between neighboring amino acids in a polypeptide chain. And recall that an amide is just when you have a carbonyl group that is linked to a peptide bond. And so it turns out that the total number of peptide bonds is actually just one less than the total number of amino acids in a chain, and we'll see how that works down below in our example. Now, peptide bond formation is actually an endergonic process and recall, endergonic processes are ones that require energy and they require ATP. And so the name of this endergonic reaction that forms peptide bonds is called a dehydration synthesis reaction. And the reason it's called dehydration is that literally the molecule is dehydrated because it loses water during the peptide bond formation. And so, hydrolysis here is actually the complete opposite reaction. It's the complete opposite reaction. And so instead of being endergonic, it's actually an exergonic process. And instead of forming peptide bonds, it actually cleaves or breaks down peptide bonds. And so in our example below, what we're going to do is talk about peptide bond formation and peptide bond breakdown. And we're also going to circle all of the alpha carbons, which can be symbolized like this, and recall that the alpha carbon is just the central carbon atom of an amino acid. So let's take a look at this example and what you'll notice is over here on the far left, what we have is our dehydration synthesis reaction. And so this is again going to be for peptide bond formation. And so, over here on the far right, what we have is the complete opposite reaction of hydrolysis. And recall that hydrolysis is specifically bond breakdown. And notice that these arrows are going in opposite directions. And so, let's first talk about dehydration synthesis, then we'll talk about hydrolysis. So over here, what we have is, 2 free amino acids. We have one free amino acid here, and we have another one over here. And so notice that, their alpha carbons are right in the center. So we have an alpha carbon here, which is linked to its R group, and then we also have an alpha carbon over here, which is again linked to this R group. And you should recognize these R groups. R group of, just a hydrogen is of course going to be a glycine amino acid, and the R group of a methyl group is going to be an alanine amino acid. And so now that we've identified these, alpha carbons and these amino acids, what we can see is that during a dehydration synthesis reaction, what happens is the carboxyl group of the glycine interacts with the amino group of the alanine. And what they do is they form a water molecule, and that's why it's being dehydrated. And in the process of dehydrating the, the molecule and forming a water, a peptide bond is made, and the peptide bond is shown here in red. And so this is our peptide bond. And what you'll notice is that the peptide bond is indeed an amide linkage because we have our carbonyl group and the carbonyl group is linked to a nitrogen atom. So this is an amide linkage, and an easy way to be able to find, peptide bonds is to just look for the carbonyl group. Once you find the carbonyl group in the backbone, then you know that the, the bond to the nitrogen linking the carbonyl group is going to be the peptide bond. And so, again, we can label these alpha carbons here, so we have the alpha carbon here and we could circle them, so we have the circled alpha carbons. And here, we have an alpha carbon as well, so we can circle it. So we have all the alpha carbons circled. And you can see that these 2 amino acids, glycine and alanine, are linked together via a covalent peptide bond, amide linkage. So now let's talk about hydrolysis. And again, hydrolysis is going to be the complete opposite reaction. So we're going to take this peptide bond here and we're going to, instead of dehydrate it, we're going to add water to it. So we're going to add water and what that's going to do is initiate the, peptide bond cleavage reaction. And so notice that the arrow again is going in the opposite direction here, and so, basically, what happens is we break down this dipeptide which has 2 amino acids in it, and we form these 2 separate amino acids up above. And so, this process here is going to be an ex process. Whereas over here with the peptide bond formation, it's actually an endergonic process. So it requires energy or ATP in order for the peptide bond to form. And so, notice that, when we have 2 amino acids, so, we have 2 amino acids in this chain, we have one over here and one over here. But even though we have 2 amino acids, we only have one peptide bond. So the number of peptide bonds is always going to be one less than the number of amino acids in the protein. And so, what we have here in the middle is a free energy diagram. So what we have is the free energy of the system on the y axis and the reaction coordinate on the x axis or the time that passes as the reaction progresses. And so down here, what we have are reactants, and so what we have are 2 separate amino acids. They are not linked by a peptide bond. They are free amino acids. And up here, what we have is a peptide, a small dipeptide with 2 amino acids, and these 2 amino acids are linked by a peptide bond which is in red right here. And so, what I want you to know is that the formation of this peptide bond, so going from here up over to this process over here, is an endergonic reaction like we said earlier. And so because it's endergonic, it requires energy input. And the opposite process, essentially, of going from this, peptide bond and breaking it down into 2 separate amino acids is actually an exergonic process, so it's spontaneous. So you might be wondering, if it's a spontaneous reaction, why is it that it's possible for peptide bonds to be stable? How is it that proteins can be stable, and why is it that all the proteins just break down into their their amino acids quickly? And the reason is that it is an exergonic process, but the reason that it happens very very slowly is because of this energy barrier. So there's a big energy barrier between the peptide bond here and, the, the barrier here. So there's a big energy barrier that makes hydrolysis happen very very slowly. So even though it is ex ergonic, as you can see by the energy difference between the reactants and the product here, it is exergonic, this concludes our lesson, stable. And so, this concludes our lesson on peptide bonds and peptide bond formation and breakdown, and I'll see you guys in our practice videos. We'll be able to practice these concepts. So I'll see you guys there.
Peptide Bond: Study with Video Lessons, Practice Problems & Examples
Created using AIA peptide bond is an amide covalent linkage formed between amino acids during dehydration synthesis, which is an endergonic process requiring ATP. The number of peptide bonds is always one less than the number of amino acids in a chain. Conversely, hydrolysis is an exergonic reaction that breaks peptide bonds by adding water. Despite being spontaneous, hydrolysis occurs slowly due to a significant energy barrier. Understanding these processes is crucial for grasping protein structure and function, as they highlight the dynamic nature of polypeptide formation and breakdown.
Peptide Bond
Video transcript
Considering that peptide bond hydrolysis is exergonic, how is the stability of a peptide bond accounted for?
Highlight the peptide bonds in the figure below & circle all the α-carbons. How many peptide bonds are there?
Which of the following best represents the backbone atom arrangement of two peptide bonds?
