Beta sheets
Also known as the beta pleated sheet due to the pleated appearance of the protein structure from a side view.
No strict rules to how they are formed because the hydrogen bonds can be formed between distant amide hydrogen and carbonyl oxygen.
They are two or more strands distant from each other in the primary structure that form hydrogen bonds with each other side by side.
There are two types of beta sheet structures; parallel and anti-parallel. Parallel beta sheets have strands that run in the same direction as each other and anti-parallel beta sheets have strands that run in opposite direction to each other. The hydrogen bonding in anti-parallel beta sheets are usually more linear.
The N-H and C=O groups on the outer edge of the beta sheet structure are not hydrogen bonded to other strands of the primary sequence.
If the R-groups along the outer edge of the beta sheets are polar, it can interact with solvents such as water. If they are non-polar, they can interact with hydrophobic structures such as lipids.
They can also pack closely against side chains of nearby alpha helix structures.
Almost all of the polar amide groups are hydrogen bonded to each other in the beta sheet structure.
Parallel sheets are almost always buried in the inside of the structure of a protein and anti-parallel sheets are mostly exposed to solvent due to the amino acids that make up that part of the structure. Therefore, anti-parallel sheets are seen as being more stable structures than parallel sheets.
Parallel sheets usually have other structures, such as helices, separating them from other parallel sheets.
They have a right handed twist to the beta strands due to the steric factors of the L-amino acid configuration.
Isoleucine and valine are often found in these beta sheets because they are hydrophobic.
Beta strands can be amphipathic because of the alternating side chains of amino acids next to each other. These amphipathic strands are found on the surface of proteins.
A large anti-parallel beta sheet can also form a barrel structures (such as retinol binding protein). The last strand of the beta sheet is hydrogen bonded to the first strand so it forms a closed barrel shape.
The exterior of the structure is usually surrounded by solvent as it is hydrophilic and the interior is where the hydrophobic residues are found so non-polar species can be found here (e.g retinol).
