Beta-Pleated Sheet
Definition and meaning of Beta-Pleated Sheet in chemistry.
A beta-pleated sheet is a common form of protein secondary structure in which extended strands of the polypeptide chain lie side by side and are held together by hydrogen bonds. The strands give the sheet a rippled, pleated appearance.
In more detail
Secondary structure refers to local folding patterns in a protein that are held in place by hydrogen bonds between the backbone atoms, not the side chains. The beta-pleated sheet is one of the two most common patterns, alongside the alpha helix. Instead of coiling, the chain stretches out into nearly straight segments called beta strands, and several of these strands line up next to one another to form the sheet.
The sheet is stabilized by hydrogen bonds between the backbone N-H group of one strand and the backbone C=O group of a neighboring strand. Because these bonds run between strands rather than within a single strand, a stable sheet needs at least two strands.
The strands do not lie perfectly flat; the tetrahedral geometry of the backbone carbons gives the sheet its characteristic pleats, like folds in paper. Beta sheets come in two arrangements. In a parallel sheet, neighboring strands run in the same direction, from the amino end to the carboxyl end.
In an antiparallel sheet, they run in opposite directions. Antiparallel sheets tend to be a bit more stable because their hydrogen bonds line up more directly. In both types, the side chains point alternately above and below the plane of the sheet.
Beta sheets appear in many important proteins and give them useful mechanical properties. Silk fibers owe much of their strength to stacked antiparallel beta sheets. Beta sheets also form the core of many enzymes and antibodies.
On the harmful side, misfolded proteins that stack into abnormal beta-sheet structures form the amyloid deposits linked to diseases such as Alzheimer's. The pattern was first proposed by Linus Pauling and Robert Corey.
Key facts
| Field | Biochemistry |
|---|---|
| Structure level | secondary structure |
| Building block | extended beta strands |
| Held together by | backbone hydrogen bonds |
| Minimum strands | two |
| Two types | parallel and antiparallel |
| Side chains | alternate above and below |
| Proposed by | Pauling and Corey |
Silk from silkworms and spiders is built largely from antiparallel beta-pleated sheets. The tightly packed, hydrogen-bonded strands make silk strong and resistant to stretching along the fiber.
Frequently asked questions
What holds a beta-pleated sheet together?
Hydrogen bonds between the backbone N-H group of one strand and the backbone C=O group of a neighboring strand. Because these bonds run between strands, at least two strands are needed.
How is a beta sheet different from an alpha helix?
A beta sheet is made of extended strands lying side by side, while an alpha helix is a single coiled strand. Both are secondary structures stabilized by backbone hydrogen bonds, but their shapes differ.
What is the difference between parallel and antiparallel sheets?
In parallel sheets the neighboring strands run in the same direction; in antiparallel sheets they run in opposite directions. Antiparallel sheets are usually slightly more stable due to better-aligned hydrogen bonds.
Why are beta sheets important in disease?
Some proteins misfold and stack into abnormal beta-sheet structures called amyloid. These deposits are associated with disorders such as Alzheimer's disease.