Opening image: Concanavalin, a carbohydrate-binding protein from plants.
The other secondary structure found in proteins is the β-sheet. The β-sheet is a sheetlike structure formed by interactions between two or more extended polypeptide chains, each one known as a β-strand. The α-helix is organized into a rodlike structure by hydrogen bonds between peptide groups of a single segment of the polypeptide chain. But extended β-strands are organized into complex β-sheets by hydrogen bonding interactions between distant parts of the polypeptide chain or even between different polypeptide chains. Although β-sheets are important as building blocks in normal proteins, they are also toxic elements in pathological amyloid disorders, such as Alzheimer's disease and mad cow diease.
to the basic unit: the β-strand, with the polypeptide chain almost fully extended. How many residues are there?
alternate views of the zig-zag conformation of the backbone. Green dots mark the location of the side chains. Observe how the side chains alternate above and below the backbone.
Now the β carbons.
Click to display the R-groups. This last view shows the characteristic pattern of side chains along a β-strand.
In a β-sheet the polypeptide chains are linked in regular manner by hydrogen bonds between the main chain C=O and N-H groups. Let's start with two β-strands.
Click to display one such H-bond as a dashed line. Can you identify locations for additional H-bonds between the two β-strands? Look for five more...
All six H-bonds are now displayed. But do the dashed lines really represent peptide H-bonds between the two β-strands? in order to visualize the van der Waals radii of the atoms.
If two β-strands run in opposite directions, they form an antiparallel β-sheet. Note that the H-bonds are parallel to each other in an antiparallel β-sheet; for a parallel β-sheet (strands oriented in the same direction) the H-bonds are not parallel. However, in both types of sheets, the H-bonds define the β-sheet. This becomes more apparent in the cartoon mode.
Cartoon Representation of β-sheets.
The concept of a β-sheet is elegant, but recognizing them in protein structures would be nearly impossible without Jsmol's ability to display β-strands as arrows. The arrowhead always designates the C-terminus of the strand.
the β carbons of the two strands. Observe the highly regular pattern of the β carbons, alternating above and below the backbone ribbon.
Next, the side chains in this β sheet. The β carbons are shown in green. In the next section, we will learn how to connect these two antiparallel strands with a very tight turn.
both strands. The α carbons are colored dark gray.
The stability of the β-sheet depends on other favorable interactions in addition to hydrogen bonds. In fact, in proteins we never find a β-sheet by itself, i.e. one in which both sides of the sheet are exposed to water.