In the plot below, circle the point(s) representing peptides w...

Question

In the plot below, circle the point(s) representing peptides with intrachain disulfides.

Graph showing electrophoresis results; circle points for peptides with intrachain disulfides.

Accepted Answer

Alright. So this practice problem says, in the plot below, circle the point or points representing peptides with intrachain disulfides. And down below, we have a diagonal electrophoresis plot where we have the first direction of electrophoresis on the x axis that's going from the left to the right. And then on the y axis, we have electrophoresis after performic acid exposure. And we know that performic acid is going to cleave all of the disulfide bonds, and the direction of this second electrophoresis is from the bottom to the top. And it's really important to take into account the direction of electrophoresis. Now, with diagonal electrophoresis, we know there's some kind of diagonal line that forms. And here we can see a diagonal line that's pretty clearly forming. And we know that these points on the plot represent small peptides or proteins, and so the peptides that actually fall onto this diagonal line that we see here, are actually peptides that have no disulfide bonds, so they do not have any disulfide bonds. And so what that means is that any of these points that lie on this diagonal line, we can pretty much ignore since they don't have any disulfides, and we're looking to circle points that represent peptides that actually have disulfide bonds, intrachain disulfides. Now recall from our previous lessons that there's 2 types of peptide, of disulfide bonds. There are interchain disulfides, which are essentially separate chain disulfides. This is when you have 2 cysteine residues on separate polypeptide chains that form a disulfide bond. And then we also have intra chain disulfides, which are pretty much same chain disulfides or when you have 2 cysteine residues on the same polypeptide chain that form a disulfide bond. And so upon cleavage of interchain disulfides, the peptides, chains are actually gonna get smaller and, therefore, they will travel faster through the gel. And upon cleavage of intra chain disulfides, the, polypeptide chain will change shape and it will therefore travel slower through the gel. And because we're looking for intra chain, disulfides or peptides with intrachain disulfides, we're looking for peptides that are going to be traveling slower after exposure to a performic acid. And so again, the direction of the second electrophoresis is going from the bottom to the top. So the proteins start at the bottom and they make their way to the top. And so proteins that are larger, or change shape and travel slower are gonna remain towards the bottom of the plot, whereas proteins that travel faster are gonna make their way really quickly to the top and they'll remain, towards the top of the gel. I'm sorry, the top of the plot here. And so, what you'll see is that, this diagonal line represents peptides that did not change their mobility, from the first electrophoresis to the second electrophoresis. But peptides that travel faster are going to move faster through this second electrophoresis, so all of these, peptides on this side of the diagonal line traveled faster and had inter chained disulfides. And all of the peptides that, essentially are on this side of the diagonal line had intra chain disulfides. And so because we're looking to circle peptides with intra chain disulfides, these are the 2 that we want to circle. And so, essentially, this concludes our answer to this problem, and I'll see you guys in our next video.

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In the plot below, circle the point(s) representing peptides with intrachain disulfides.

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Diagonal Electrophoresis practice set