Peptides - Online Tutor, Practice Problems & Exam Prep

Peptides are formed when two or more amino acids bond through a peptide bond, which is a covalent bond between the carboxyl group of one amino acid and the amino group of another. This process occurs via a condensation reaction, resulting in the loss of water. Peptides have directionality, with the N terminus (free amino group) on one end and the C terminus (free carboxyl group) on the other. The number of amino acids in a peptide is indicated by prefixes: di- for two, tri- for three, and tetra- for four, while longer chains are referred to as polypeptides.

Peptides Concept 1

Now here, we're going to say that peptides are formed when 2 or more amino acids bond with each other through a peptide bond. The peptide bond, also known as an amide bond, is formed when the carboxyl group of one amino acid covalently bonds with the amino group of another amino acid. Pay attention here. We have the carboxyl group in its anionic form, so it's in its negatively charged form, and we have the amino group in its positively charged form. We're talking about the formation of a peptide bond.

We've kind of seen this before when we talked about the formation of an amide bond. Remember, that happens through a condensation reaction. We have the loss of water. So here we have alanine, which is our amino acid, and we have threonine here. We're going to have the loss of water.

With the loss of water, we would lose this oxygen here, and we'd lose two hydrogens here from the NH₃ group. So, we have the loss of water here, indicated by this arrow pointing up, showing that the water is leaving the two structures. What's left behind combined together to form our peptide bond. So we lost this oxygen, we lost two hydrogens, meaning that nitrogen still has one hydrogen on it. That's why it's still here. We didn't change anything in terms of this carbonyl group, so it's still intact.

The bond that they form as a result of them losing water is now a dipeptide, and we show this by giving the three-letter code for each amino acid connected to each other by a hyphen. This hyphen represents the peptide bond between the two amino acids. Here, we say that the number of amino acids in peptides is indicated by prefixes such as di, tri, and tetra. A polypeptide is a peptide consisting of a larger number of amino acid residues. A residue is just an individual amino acid contained within a peptide. Here, this is called a dipeptide, "di" because it's two amino acids connected by one peptide bond. If we have three amino acids, it'd be a tripeptide.

If we have four amino acids, it'd be a tetrapeptide. Beyond that, it can be referred to as a more complicated chain of amino acids connected by peptide bonds. But just remember, we've seen something like this before when we talked about the formation of amides in earlier chapters. Now we're relating it to the connection of two different amino acids to one another.

So, just remember, a peptide is two amino acids connected by a peptide bond.

Peptides Example 1

Circle all peptide bonds and amino residues in the following tetrapeptide. Alright, so remember, a peptide bond is basically an amide bond. So we're going to look for where we have a carbonyl single bonded to a nitrogen. If we take a look, here goes one of them.

This is an amide bond. Here goes a carbonyl carbon connected to a nitrogen. There goes another one. And then there goes our last one here. So we have 3 peptide bonds.

They say that it's a tetrapeptide, so that means we have 4 peptides connected to one another. So remember, this would represent 1 peptide. Remember, the amide bond is what connects 2 different peptides or 2 different amino acids to one another. So this is 1. Here goes the second one.

We're going to cut the amide bond. That's a second. Then here, this is another one. There goes another one. We should be circling it, but just for us to see me tracing where the peptide is.

And then finally, we have this last one here. So this represents our 4 peptides involved.

Drawing & Naming Peptides Concept 2

Now, when it comes to drawing and naming peptides, remember that every peptide has directionality. What we mean by that is that we look at it from the N-terminus to the C-terminus end. Now, when we say N-terminus, this is just the residue with the free amino group. And when we say C-terminus, this is the residue with the free carboxyl group. Now, when drawing or naming, always arrange from left to right and have it in N-terminus to C-terminus when looking at a peptide.

If we take a look here, we have our N-terminus on the left, and that's because the free amino group is on the left side here on that end. And then they're connected to another amino acid through this peptide bond here. And this is the C-terminus because the carboxyl group is on this end. Right? So we're looking at this structure, this peptide, 2 amino acids connected by a peptide bond, and we're looking on the left side, we have our N-terminus end.

And on the right side, we have our C-terminus end. Keep this directionality in mind whenever you're naming or writing a peptide.

Peptides Example 2

Draw a dipeptide structure consisting of given amino acids in the following order. Remember that we have to write this from the N-terminus to the C-terminus, meaning that the amino acid on the left side will have its carboxyl group interacting with the amino group of the amino acid on the right.

This is a condensation reaction where we will have the loss of water. We are going to lose the oxygen from the carboxyl group and 2 hydrogens from the amino group. After that, everything left behind will come together to form our dipeptide structure.

We have our benzene ring with our OH group connected to this CH₂ group, which is connected to this carbon with the NH₃⁺. Next, our carboxyl group is drawn here, and I am going to draw the peptide bond in red. It's connected now to this nitrogen, which still has one of its hydrogens, and it's still connected to all this stuff here, CH₂, and then we have SH here, and then we have the carboxyl group here. This will be our dipeptide structure.

Now, we need to provide the three-letter code. The amino acid on the right is easy to spot because it has an SH group, which is unique to cysteine. Cysteine's three-letter code is CYS, based on its first three letters.

The other amino acid has an OH group, a polar group, and this one is tyrosine. Its three-letter code is TYR, again based on its first three letters. Therefore, the dipeptide we have here would be TYR-CYS. This is the dipeptide name using our three-letter code, and this is the dipeptide structure.

Draw a tripeptide structure consisting of following amino acids in following order. Name tripeptide using 3 letter codes.

Draw a tripeptide structure consisting of the following amino acids in the following order. Here, we also need to name the tripeptide using a three-letter code. Alright. So, we're going to start out by looking at each of these. Remember, we look at the structure, the N-terminus is on the left side, and then our C-terminus will be on the right side. So we know that those will be the left and right ends of the tripeptide. We're going to say, in between, are a bunch of condensation reactions. So we're going to say here, here are our carboxyl groups in yellow, and then we have this amino group and this amino group that are going to be involved in our peptide bond formation. Remember, peptide bond formation is a condensation reaction with the loss of water. We're going to lose oxygen and two hydrogens here in the form of water. Oxygen and two hydrogens here in the form of water. Now we're going to draw our tripeptide. So we're going to have here what? We're going to have our NH₃⁺ connected to our alpha carbon, connected to this structure. And so we have here we're going to draw it as best as we can. Now here is our first peptide bond connected to our nitrogen, which still has one hydrogen on it. And then here, we know that this is a CH₂, this is a CH₂ connected to this carbonyl. We're going to write it as a skeletal structure just to make it easier and faster to write. Then we have our second peptide bond. Now this nitrogen lost its two hydrogens, so it has none left. And then we're connected to this carboxyl group here. So this will represent our tripeptide. Now, here if we take a look at each one of these structures, our R groups here is our R group. Here is our R group. And this here is our R group. For the first one, that R group represents a charged amino group. So it's a charged amino acid. It is histidine. We know that its three-letter code is just HIS as it follows the first three letters. Next, we have this R group. This second amino acid is glutamine. Remember, glutamine is a polar amino acid. It's one of the four amino acids that doesn't follow the first three-letter rule. Here, it's going to be GLN instead of GLU. And then finally, we have this last amino acid here, which is our proline. We don't have to worry about it because it follows the first three-letter rule. So this would be the 3 amino acids that were used, their names, and their three-letter codes. For the tripeptide that we have down here below, we would connect these three-letter names or codes with one another through hyphens. So here we'd say that this structure would be HIS-GLN-PRO. So this would be us naming this tripeptide bond or the tripeptide structure. And the tripeptide structure is given below here in black. Remember, it's connecting the 3 amino acids to one another through 2 peptide bonds.