Parallel, anti-parallel and mixed b-sheets:
In parallel b-sheets the strands all run in one direction, whereas in anti-parallel sheets they all run in opposite directions. In mixed sheets some strands are parallel and others are anti-parallel.
Below is a diagram of a three-stranded anti-parallel b-sheet. It emphasises the highly regular pattern of hydrogen bonds between the main chain NH and CO groups of the constituent strands.
In the classical Pauling-Corey models the parallel b-sheet has somewhat more distorted and consequently weaker hydrogen bonds between the strands:
b-sheets are very common in globular proteins and most contain less than six strands. The width of a six-stranded b-sheet is approximately 25 Å. No preference for parallel or anti-parallel b-sheets is observed, but parallel sheets with less than four strands are rare, perhaps reflecting their lower stability. Sheets tend to be either all parallel or all anti-parallel, but mixed sheets do occur.
The Pauling-Corey model of the b-sheet is planar. However, most b-sheets found in globular protein X-ray structures are twisted. This twist is left-handed as shown below. The overall twisting of the sheet results from a relative rotation of each residue in the strands by 30 degrees per amino acid in a right-handed sense.
Parallel sheets are less twisted than anti-parallel and are always buried. In contrast, anti-parallel sheets can withstand greater distortions (twisting and b-bulges) and greater exposure to solvent. This implies that anti-parallel sheets are more stable than parallel ones which is consistent both with the hydrogen bond geometry and the fact that small parallel sheets rarely occur (see above).
Courtesy: swissmodel expasy
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