Now that we know about the beta strand, we can talk about beta sheets. Beta sheets consist of two or more beta strands arranged side by side and are also known as beta pleated sheets. If you Google pleated anything, like pleated cups, pleated pants, or pleated shorts, pleated means zigzag. Taking a quick look at the example of our beta sheets over here, you'll see the zigzag type of structure in the beta sheets. Notice that the R groups, which are shown in yellow, go down, then up, repeatedly, creating this zigzag structure. That's really what the pleated sheets are referring to. These R groups, which move up and down, are actually perpendicular to the plane of the beta sheets. They go straight up and straight down, above and below the beta sheet plane. Whereas, the hydrogen bonds, which are in red here, are perpendicular to the direction of the beta strands. If we consider the directions, the beta strands can be portrayed as arrows always pointing towards the C-terminal end. This type of structure is something important to keep in mind. Unlike alpha helices, the hydrogen bonds in the beta sheets can form between either separate protein chains, which means they are interchain hydrogen bonds, or within the same chain, indicating intrachain hydrogen bonds. We can distinguish this in our example down below. Beta sheets typically have only between two to ten or even more beta strands arranged side by side. Let's take a look at our example to distinguish between inter and intrachain beta pleated sheets.
Again, beta sheets, known for their zigzag structure, are seen here with the R groups moving down and up repeatedly. In this image, notice the two beta strands, shown with arrows, are not connected. They represent separate polypeptide chains. This setup is an example of an interchain beta sheet, where the hydrogen bonds form between opposite polypeptide chains. We see one polypeptide chain at the top and another below, with hydrogen bonds depicted in dotted black lines between them.
On another note, we have a continuous polypeptide chain that contains three beta strands, indicating an intrachain beta sheet. This single chain forms multiple beta strands, all part of one continuous polypeptide. This is distinctly different from alpha helices, which cannot form interchain hydrogen bonds. Beta sheets' ability to form both interchain and intrachain structures makes them uniquely different from alpha helices, only capable of forming intrachain hydrogen bonds. That concludes this lesson on beta sheets. We will get some practice and then we'll talk about beta sheet bond angles in a Ramachandran plot. I'll see you guys in that practice video.
