Identify the N-terminal and C-terminal amino acids of the peptide.

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Accepted Answer

Hi, everybody. Let's look at our next problem. Consider the peptide and then we have a very large structure here. A skeletal structure clearly with several amino acids in it. We'll go into it in more detail in a little bit. The question says, what are its C terminal amino acid and N terminal amino acid A and terminal glycine C terminal seine B and terminal Cine C terminal glycine CN, terminal alanine, C terminal cysteine or DN terminal cysteine, C terminal alanine. So when we look at a big structure like this, we want to kind of mark off the boundaries between the different amino acids. We're looking for the two on either end, the N terminal and the C terminal. But let's recall that by convention, amino acids are written with the N terminal amino acid on the left and the C terminal on the right. So see terminals listed, listed first. And we'll want to double check that our structure is indeed written this way, make sure they haven't flipped it for us, but just something to keep in mind. And in fact, just a glance at our structure shows us we have an H two N I mean, a group on the left and a carboxylic acid group on the right. So at first glance, at least it appears that's the way it's written and terminal to c terminal left to right. So let's think about the backbone structure of an amino acid which is H two N bonded to a carbon, which is the alpha carbon that has the hydrogen and an R group, then a carbonel group. So C double water to an O and an oh and these of course can combine with another amino acid. We'll write that in the same order, but this one will be a different amino acid. So we'll give it an R prime and they can form a peptide bond between them with the elimination of a water molecule. The oh is eliminated from the carboxylic acid group of the first, the hydrogen eliminated from the amino group of the second. And as we said, they react, losing a water molecule to form that peptide bond between the two of them. So our peptide bond, which I'll highlight in blue is between the carbon carbon and the nitrogen of the second amino acid. And so you can see here in this simple dipeptide, the N terminal amino acid is the end on the left where you have this H two N by itself. And the C terminal is where you have that carboxylic acid. So sure enough, when we look at our structure on the left side we've pointed out that H two N, let's make sure that is attached to a backbone of an amino acid and not an R group. We see it's bond into a carbon which has then a carbon carbon and then the nitrogen of the next amino acid. So that will be our N terminal. And we can look over at the other side at this carboxylic acid group at the other end. And we see the carbon carbon is indeed bonded to a carbon that has a chain on it and then to a nitrogen going from right to left that goes into another amino acid. So this is the c terminal on the right. So we don't have to identify every amino acid in our structure, just the two on either end. So I'm going to label the peptide bond between the 1st and 2nd amino acid again, between that carb carbon and the nitrogen. And then on the right side between the nitrogen and the carbon carbon. So we kind of have blocked off our two terminal amino acids. So let's look again at our N terminal. Now, this one is a little bit tricky because when we get to the alpha carbon, which I will label with a red alpha, there's nothing attached to it. We know on a, it's a skeletal structure when there's nothing there on a carbon. Since it has two bonds, there must be two hydrogens. And that means that the R group in this case, is hydrogen, I'll just put H with a circle around it. Since hydrogen atoms attached to carbon atoms are not specifically indicated but implied in the skeletal structure. So the amino acid with its R group just being hydrogen is glycine or gly. So we see in choice A, it's labeled N terminal glycine. So that's correct. Choice B says the N terminal is searing but that is not correct. So we can eliminate B choice C says N terminals, alamy also incorrect. And choice D has the N terminal as cysteine. So we don't even need to look any further. We don't need to look at the other side. If we were on a test, we already have our answer choice. A as we've eliminated all the other possibilities. But as this is a practice problem, we'll be a little more thorough and look at our C terminal. So when we look at that C terminal, we have our carboxylic acid. On the end, the carbon carbon is bound to the alpha carbon, which I'll label here with that red alpha. So this R group is the chain attached to the alpha carbon which is AC H2O H and the amino acid that has that particular R group is sine or in the abbreviation of it is S er And sure enough, when we look at choice A, it says C terminal searing. So looking at the structure we're given of this polypeptide, looking for the C terminal amino acid and N terminal amino acid. Our answer is choice. A the N terminal is glycine. The C terminal is seine. See you in the next video.

Identify the N-terminal and C-terminal amino acids of the peptide.

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Key Concepts

Peptide Structure

N-terminal and C-terminal

Amino Acid Sequence