Antiparallel and Parallel Beta Sheets - Online Tutor, Practice Problems & Exam Prep

Antiparallel and parallel β sheets are key secondary structures in proteins. Antiparallel β sheets have strands aligned in opposite directions, with a rise per residue of 3.5 angstroms, resulting in stronger, perpendicular hydrogen bonds. In contrast, parallel β sheets have strands aligned in the same direction, with a rise of 3.2 angstroms, leading to weaker, distorted hydrogen bonds. This structural difference contributes to the stability and prevalence of antiparallel β sheets in nature. Understanding these distinctions is crucial for grasping protein folding and function.

Antiparallel And Parallel Beta Sheets

In this video, we're going to begin distinguishing between antiparallel and parallel beta sheets. Now, you might be wondering why talk about antiparallel beta sheets before we talk about parallel beta sheets? Well, it turns out that the antiparallel beta sheets are actually easier than the parallel beta sheets. So, we'll talk about the easy stuff first, and then we'll talk about the harder stuff later. And so, with the antiparallel beta sheets, it turns out that the beta strands are going to be aligned in opposite directions in terms of their N- and C-terminal ends. And the rise per residue for an antiparallel beta sheet is, to no surprise, 3.5 angstroms since we said in our previous lesson videos that beta sheets have a rise per residue of 3.5 angstroms. And so, down below in our example, it's pretty clear to see that this image on the left here is an antiparallel beta sheet. And so, we can see that we have a single polypeptide chain that's all connected to one another, and these arrows here represent beta strands. And the beta strands that are adjacent to one another, right next to another, are pointing in opposite directions. And so, when they're pointing in opposite directions, in terms of their N- and C-terminal ends, that identifies them as being antiparallel beta sheets. And so, we know that the arrows are pointing towards their C-terminal end. And because this is the beginning of the chain, this is the N-terminal end, and the arrows pointing towards the C-terminal end are going to make this the C-terminal end over here. And again, because this is one continuous polypeptide chain, that makes this an intrachain antiparallel beta sheet.

Moving on to the parallel beta sheets, in contrast, the beta strands are actually aligned in the same direction in terms of their N- and C-terminal ends instead of the opposite direction. And the rise per residue for a parallel beta sheet is actually a little bit more condensed. It's 3.2 Å. And so, even though 3.2 Å is a little bit more condensed, it's still, in comparison to an alpha helix, a lot more extended. Because an alpha helix has a rise per residue of 1.5 angstroms. So that's really really condensed. 3.2 Å is all very extended in comparison to the alpha helix. It's just that this antiparallel beta sheet is a little bit more extended than the parallel beta sheet. So, something interesting to keep in mind. And so down below on the right, we have an image of the parallel beta sheet and we can tell because the beta strands are all pointing in the same direction in terms of their N- and C-terminal ends. And so, this is one continuous polypeptide chain here, so we know that this is an intrachain parallel beta sheet. And this is the back of the arrow over here, so this will be the N-terminal end. And this is what the arrows are pointing to, so this is the C-terminal end. And so this concludes our initial lesson on the differences between antiparallel and parallel beta sheets. In our next lesson video, we're going to be able to talk even more differences between these two, specifically the hydrogen bonding differences between the two. But before we get there, let's get some practice. So I'll see you guys in that practice video.

Silk fibroin contains predominantly β sheet conformation. Which of the following is true regarding its β sheets?

• A. Its antiparallel β sheets are oriented in the same direction.
• B. Its parallel β sheets are oriented in opposite directions.
• C. Its antiparallel β sheets are slightly more extended.
• D. Its parallel β sheets are slightly more extended.

Antiparallel And Parallel Beta Sheets

So now that we understand the fundamental differences between anti-parallel and parallel beta sheets, let's talk about the differences between hydrogen bonding of the anti-parallel and parallel beta sheets. It's important to note that both anti-parallel and parallel beta sheets are stabilized by hydrogen bonds that form between the carbonyl group and the amino groups on the backbones of adjacent beta strands. What this means is that the R groups are not involved in stabilizing the beta sheet, and that's something very important, and this is also true for alpha helices. This is true for secondary structures. Our groups are not involved; it's all about the backbone. This is how beta sheet hydrogen bonding works. Each amino acid residue in both anti-parallel and parallel beta sheets only forms 2 hydrogen bonds, but the hydrogen bonding is still slightly different.

With anti-parallel beta sheets, they actually have stronger bonds that are pretty much perfectly perpendicular. They have stronger hydrogen bonds that are perfectly perpendicular, and because these bonds are stronger, they are more stable. Because they are more stable hydrogen bonds, anti-parallel beta sheets are more prevalent and more common in proteins found in nature than parallel beta sheets, but that's not to say that parallel beta sheets are never found because they are. With parallel beta sheets, they actually have weaker hydrogen bonds and their hydrogen bonds are slightly distorted. They are not perfectly perpendicular. Because parallel beta sheets have weaker, distorted hydrogen bonds that are not perfectly perpendicular, they are less stable and because they are less stable, they are less commonly found in proteins found in nature. This is something important to keep in mind.

In our example below, we're going to distinguish between anti-parallel and parallel beta sheet hydrogen bonding or H bonding. Notice on the left here, we have our anti-parallel beta sheets, and on the right, we have our parallel beta sheet. What you'll see is that the anti-parallel beta sheet is slightly more extended because it has a rise per residue of 3.5 angstroms, so it's a bit more extended. Whereas, the parallel beta sheet only has a rise per residue of just 3.2 angstroms, so it's slightly less extended. Both residues in both anti-parallel and parallel beta sheets form 2 hydrogen bonds. However, there are some slight differences. For antiparallel beta sheets, both of the hydrogen bonds on a residue actually link to just one residue on the other strand. If we were to highlight a single residue, notice that the 2 hydrogen bonds of this single residue, highlighted in red with these dotted lines, only connect to a single residue on the opposite strand. The hydrogen bonds link from one residue to just one residue on the other strand, which is why the anti-parallel beta sheets are more straightforward.

With parallel beta sheets, however, notice that we have one residue over here and it still has 2 hydrogen bonds, but both hydrogen bonds of a residue link to 2 different residues on the other strand. We've got 2 hydrogen bonds, slightly distorted and not perfectly perpendicular, slightly off at different angles. They connect to two different residues on the opposite strand. So it's connecting to one residue over here with this hydrogen bond, and the other hydrogen bond over here is connecting to another residue. Both hydrogen bonds of a residue connect to one residue on the other strand in parallel beta sheets, allowing for hydrogen bonds to be stronger and perpendicular. This is why anti-parallel beta sheets are more commonly found and more stable.

That concludes our lesson here on the differences between anti-parallel and parallel beta sheet hydrogen bonding, and in our next couple of videos, we'll be able to get some practice. I'll see you guys in those practice videos.

The diagram illustrates: 

• A. The polypeptide chain of an alpha helix.
• B. Two polypeptides of a β-sheet running in a parallel fashion.
• C. Two polypeptides of a β-sheet running in an antiparallel fashion.
• D. Two polypeptides of a coiled coil.

The major reason that antiparallel β-sheets are more stable than parallel β-sheets is that the latter:

• A. Are in a slightly less extended configuration than antiparallel strands.
• B. Do not have as many disulfide crosslinks between adjacent strands.
• C. Do not stack in sheets as well as antiparallel strands.
• D. Have fewer lateral hydrogen bonds than antiparallel strands.
• E. Have weaker hydrogen bonds laterally between adjacent strands.

Which (phi, psi) pair of bond angles is closest to those of the residues shown in the figure below?

• A. (-90, -90).
• B. (-90, 90).
• C. (90, -90).

What type of β-sheet is presented in the figure below? Draw all hydrogen bonds between appropriate groups.

• A. Antiparallel β-sheet.
• B. Parallel β-sheet.

Draw a two-stranded antiparallel β-sheet with appropriate hydrogen bonding between the following peptides:                                   

1) L-A-D-Y.                   

2) G-A-G-A.

So this practice problem wants us to draw a 2 stranded antiparallel beta sheet with appropriate hydrogen bonding between the following peptides, Lady Gaga. So we're drawing a Lady Gaga antiparallel beta sheet here. And so first, what we're gonna do is draw the lady peptide. And recall that the steps for drawing a peptide, we first need to draw the backbone and identify alpha carbons. So for this lady here, remember the way that we draw the peptide backbone, it is by drawing an NCC set for each amino acid residue. So, we have 4 amino acid residues in this lady peptide over here, which means that we're gonna have 4 NCC sets. So let's go ahead and draw that over here. So let's put in our nitrogen, here's our carbon, carbon, and that's one set, so then we can do that again. Nitrogen, carbon, carbon, 2 sets, nitrogen, carbon, carbon, that's 3, and then nitrogen, carbon, carbon. Perfect. That's 4 sets. So now we need to identify alpha carbons and remember that the NCC, the middle one's going to be the alpha carbon. So NCC, the middle one's alpha carbon. NCC, middle one's an alpha carbon. NCC, middle one's an alpha carbon. Great. So, now that we have this strand, lady going from left to right in this direction, what we need to do is consider drawing the Gaga peptide, but going in the opposite direction. So remember, if this one's going from left to right, N to C terminal, the other one, Gaga, needs to go from right to left, N to C terminal. So then we can go ahead and draw in our Gaga peptide. We can go ahead and draw the same backbone. So it also has 4 residues, so we're gonna need 4 NCC sets, but this time, we're gonna go in the opposite direction. And so, what we wanna do is since we started up, over here with this nitrogen, we'll start up over here with this nitrogen. So we'll have a nitrogen, it'll come down, and then NCC. Then we'll have NCC, NCC, and NCC. Perfect. So now what I'm gonna do is shift these guys down a little bit and give us some a little bit more space and move this up. Move this guy back down. Here, let's just get rid of him and draw him back in. Alright. So now what we've got is our 2 peptides and now we need to identify the alpha carbons here and we know it's gonna be the middle carbon. So NCC, the middle one's alpha carbon. NCC, middle one's an alpha carbon. NCC, middle one's an alpha carbon. NCC, middle one's an alpha carbon. Alright. So now that we've got these, we can move on to our 2nd step for drawing a peptide, which is just to draw in the carbonyl groups and to identify amino acid chirality. So, when we're drawing in the carbonyl groups, it's just gonna be on the ones that do not have an alpha carbon on them. So, this C over here is gonna be a carbonyl group. And we know that it's going to also be the last residue is gonna have a free carboxyl group, so we'll put an O and we'll make it charged, so an O minus. Then we can draw in the other carbonyl groups, so for all of them that do not have an alpha carbon on them. So top peptide's done. Now we can do the bottom one. So here we have 1. Here we have 1 going down. Here's one going up. And then here's one going down. And again, this is gonna be the end, so it's going to have, carboxyl anion. It's gonna be free since it's the last residue. And so we know that the first residue is gonna have a free amino group, so let's go ahead and put in NH3+ from both of these. So okay. So now we're done with step number 2. We need to move on to identify amino acid chirality. And so all the chirality is saying is that if the R group if the R group is going down, then it should be on a dash, But if the R group is going up, then it should be on a wedge. And so, we can take a look at each of our, alpha carbons here and we can say, okay, well, this one's going up. This alpha carbon here, it's kind of going up, so its R group is going to go up. When the R group is going up, it has to be on a wedge. So, this is going to be on a wedge here and then down on a dash. Up on a wedge, down on a dash. Perfect. Alright. So we can do the same for the bottom one. So down on a dash, up on a wedge. Down on a dash, up on a wedge. Alright. So now we've got our chirality. The last step is to just draw in the R groups. So, for, the first peptide, which is on the bottom, so this is peptide number 1, this is peptide number 2 up here. So, for peptide number 1, that's lady. So L here is for leucine. Remember that leucine's R group is literally just a, it's just a loose extension of valine, so it's essentially just going to be CH2. It's gonna be valine with an extra CH2, so it'll be CH2 and then CH, and then 2 methyl groups coming off. So there is Leucine's R group. Next, what we have is A, and A is for alanine, and alanine's first letter is, its one letter code is just an A. And just like A is the first letter of the alphabet, alanine's R group is pretty easy to remember just like A is the first letter of the alphabet and that's easy to remember. So it's literally just gonna be a methyl group, so it's just gonna be a CH3. So there's alanine right there. D is for aspartic acid and aspartic acid is literally just alanine with a carboxyl group. So all we need to do is redraw alanine and we know the hydrogens are gonna change, it's gonna end up just having 2, and then we just draw a carboxyl group, and we can make it a carboxyl anion, make it charged. And then, so that is aspartic acid, and then Y is gonna be tyrosine, and tyrosine is literally just phenylalanine with a hydroxyl group baby. So phenylalanine is literally just alanine with a phenyl group. So group baby. So there is our lady peptide. So l

Here’s what students ask on this topic:

What are the main differences between antiparallel and parallel beta sheets?

Antiparallel and parallel β sheets are secondary structures in proteins. In antiparallel β sheets, the β strands are aligned in opposite directions, with a rise per residue of 3.5 Å. This alignment results in stronger, nearly perpendicular hydrogen bonds, making them more stable and prevalent in nature. In contrast, parallel β sheets have β strands aligned in the same direction, with a rise per residue of 3.2 Å. The hydrogen bonds in parallel β sheets are weaker and slightly distorted, leading to less stability. Understanding these differences is crucial for grasping protein folding and function.

Why are antiparallel beta sheets more stable than parallel beta sheets?

Antiparallel β sheets are more stable than parallel β sheets due to the nature of their hydrogen bonds. In antiparallel β sheets, the hydrogen bonds are nearly perpendicular to the β strands, making them stronger and more stable. Each residue forms two hydrogen bonds with a single residue on the adjacent strand. In contrast, parallel β sheets have hydrogen bonds that are slightly distorted and not perfectly perpendicular, resulting in weaker bonds. Each residue in parallel β sheets forms hydrogen bonds with two different residues on the adjacent strand, contributing to their reduced stability.

How do hydrogen bonds differ between antiparallel and parallel beta sheets?

In antiparallel β sheets, hydrogen bonds are nearly perpendicular to the β strands, making them stronger and more stable. Each residue forms two hydrogen bonds with a single residue on the adjacent strand. In parallel β sheets, the hydrogen bonds are slightly distorted and not perfectly perpendicular, resulting in weaker bonds. Each residue in parallel β sheets forms hydrogen bonds with two different residues on the adjacent strand. This difference in hydrogen bonding contributes to the greater stability of antiparallel β sheets compared to parallel β sheets.

What is the rise per residue in antiparallel and parallel beta sheets?

The rise per residue in antiparallel β sheets is 3.5 Å, while in parallel β sheets, it is 3.2 Å. This means that antiparallel β sheets are slightly more extended compared to parallel β sheets. The rise per residue is a measure of the distance between adjacent residues along the β strand. Despite the small difference, this contributes to the overall structural differences and stability between antiparallel and parallel β sheets.

Why are antiparallel beta sheets more common in proteins found in nature?

Antiparallel β sheets are more common in proteins found in nature due to their greater stability. The hydrogen bonds in antiparallel β sheets are nearly perpendicular to the β strands, making them stronger and more stable. This stability is crucial for the proper folding and function of proteins. In contrast, parallel β sheets have weaker, slightly distorted hydrogen bonds, making them less stable and less prevalent in natural proteins. The structural integrity provided by antiparallel β sheets is essential for maintaining protein structure and function.