Fill-in the missing R-groups for the following peptide from me...

Question

Fill-in the missing R-groups for the following peptide from memory: H-E-K. Circle the acidic amino acids.

Diagram of a peptide with labeled N-terminus and C-terminus, highlighting charged amino acids.

Accepted Answer

So this practice problem wants us to fill in the missing r groups for the following peptide from memory, HEK, so histidine, glutamic acid, and lysine. And then it says it says to circle all of the acidic amino acids. Alright. So first what we're going to do is recall our mnemonic for remembering the charged amino acids. And of course, that's going to be dragons eat knights riding horses. And so the first amino acid that we have listed here is H, and remember that H stands for the horse. And of course, histidine is just going to be alanine, our leader, with a 5 membered ring. And so let's go ahead and draw that in. So we can draw an alanine here, so c h, and then we know the hydrogens are gonna change, so c h 2, with a 5 membered ring. So 1, 2, 3, 4, 5. And then what we need to remember is that just like the letter H has 2 parallel lines, there's gonna be 2 hydrogen, 2 double bonds, and 2 nitrogen atoms. And so it's also going to look like a sideways horse. And so we can draw in our back legs, our front legs, we can put in our nitrogen atom here, and then we can draw in a little tail if we'd like for the back. We know that the nitrogen here, the neck of the horse here is going to be positively charged, so we can draw in a hydrogen here. And then we can make this a positive charge. And then the last thing to remember, now that we have 2 double bonds and 1 nitrogen, we know there's gonna need to be a second nitrogen. And the, thing to remember is that the horse is not stepping on the second nitrogen, and so that means that the only position that it can go in is is here, because if it were in either this position or this position, then the horse would be stepping on it, so it can't go into those positions. And so, all we need to do is fill in our last hydrogen and that's the structure of histidine. Now, for the next one, what we have is e, and e here is for glutamic acid or just for glutamate. And, of course, glutamate or glutamic acid is just gonna be building off of aspartic acid, and aspartic acid is just alanine with a carboxyl group. And so because glutamate, has the GLUT in it, it's just gonna have an extra CH 2. And here, we're kind of drawing it, the opposite direction. So here's our carbon. What we're gonna have to do is do a c h two here. We're gonna need an extra c h 2, so that's our second c h 2 for the glute part. And then we can draw in our carboxylate group, so c double bond O, O minus. And so, again, what we have is our carboxylate group here, extending off of our, 2 c h twos. And so that's it. That's the structure of glutamic acid. And then last, what we have is k, and k is for lysine. And just like lysine stands for the knight, its its r group actually looks like a knight's sword. And we wanna count how many pointy ends do you have that you can try to poke somebody with in the letter k. And so so there are 4 pointy ends, 1, 2, 3, and 4, that you could really try to poke somebody with. So that helps us remember that there's a 4 carbon, start to the chain. So we know that there's gonna be a CH 2, there's gonna be another CH 2, there be another CH 2, and one more, CH 2. Perfect. So there's our 4 carbons start to our chain. And then, of of course, at the very end of our sword, there's going to be an amino group. And so what we can do is draw in our amino group, NH 3, and we know that it's going to be positively charged. And so there it is, that's the structure of lysine. And so, finally, it asks us to circle the acidic amino acids. And recall here that with, our mnemonic, that it actually groups our amino acids into acidic amino acids and basic amino acids. And remember that the dragons eat or the d and the e, they are negatively charged, and dragons, they spit fire and acid out of their mouth. So the d and the e here are the acidic ones. So these are the acidic ones, whereas these over here are the basic ones. And so, all we need to do here is circle e, so we circle e here as being acidic. We can also circle it here. Then we've circled the acidic amino acids. And that concludes this practice problem. I'll see you guys in the next one.

Struggling with Biochemistry?

Fill-in the missing R-groups for the following peptide from memory: H-E-K. Circle the acidic amino acids.

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