In the alpha-helix tutorial, you saw a secondary structure formed by a single stretch of a polypeptide chain. The other central type of secondary structure, the beta-sheet, comprises two or (usually) more stretches of the polypeptide coming together.

Section 1: Exploring a Small Beta-Sheet.

Beta-sheets are relatively but not perfectly flat. Click and drag on the small beta sheet at right to see its overall shape.

A beta-sheet is composed of several extended lengths ("beta-strands") of a polypeptide that run alongside each other, forming an extended, somewhat bumpy, flattened surface.

Trace the alpha carbons to show the underlying strands of the polypeptide that comprise the sheet.
Grow Shrink

Many beta-sheets, like this one, are formed by separate sections of a polypeptide coming together.

How are the two strands connected in the whole protein?

The four beta strands of the sheet do not line up perfectly - the blue section of the chain is twisted down out of the plane of the red section. Many beta-sheets are not perfectly flat; larger ones often twist gradually into more complex shapes.

The sidechains of the amino acids in a beta-sheet project out on either side of the sheet.

Sidechains off/on.

Another way to see this is to start at one end of the sheet and take cross-sections (buttons below) through it. Do any red sidechains appear between the grey backbone strands?

Cross-section:
In Out

Why don't the sidechains project inbetween the beta-strands of a beta-sheet? Continue the tutorial to find out. Section 2: What Holds a Beta-Sheet Together?

When a section of polypeptide is in an extended conformation, the backbone is kinked, not perfectly straight. In a beta-strand the kinks result in a pleated appearance. (This is due to the lack of free rotation around the peptide bonds of the chain.)

If two pleated strands line up next to each other in just the right way, hydrogen bonds can form between them. A single hydrogen bond cannot hold a secondary structure together, but the repeating pattern of the backbone atoms enables many hydrogen bonds to form.

Each of the four beta-strands is hydrogen bonded to its neighbor.

Rebuild the beta-sheet. (Do this before going to Section 3).

Consider again the effect of a sidechain situated between these beta-strands. How would hydrogen bonding be affected?
Discussion. Section 3: Beta-Sheets Can Be Amphipathic

Like alpha-helices, beta-sheets are often amphipathic, and fold into the rest of the protein so as to protect the hydrophobic sidechains from watery environments.

Use the buttons below to explore the nature of the sidechains of this sheet.

Before proceeding, make sure you have the full beta-sheet showing. If not, click here:

Color the
backbone
hydrophilic sidechains
hydrophobic sidechains (Only two atoms change color - why?)

Original color scheme.

Based on the hydrophobicity and hydrophilicity of the amino acids sidechains, can you reach some conclusions as to the position of the sheet within the context of the whole protein?

Discussion.

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