betastrand

Betastrand, or beta strand, is a type of secondary structure element in proteins. It refers to a relatively extended segment of polypeptide chain that participates in the formation of beta sheets through interstrand hydrogen bonds. Beta strands are typically 3 to 15 amino acids long and adopt a pleated conformation caused by the backbone dihedral angles (phi and psi) that place successive carbonyl and amide groups in positions to form stabilizing hydrogen bonds with neighboring strands.

Strands can align in parallel sheets, where neighboring strands run in the same direction, or antiparallel sheets, where they run in opposite directions. The hydrogen bonds between strands are usually more linear in antiparallel arrangements, and the sheets can be twisted or curved in three dimensions. Beta strands are relatively rigid compared with helices and contribute to the overall topology of the protein fold.

Beta strands are common in many protein structures, contributing to the stable cores of globular proteins and forming the interior of beta-barrel proteins in membranes. They may occur as isolated stretches, within beta-hairpins, or as part of longer beta sheets that span large regions of the polypeptide. Protein structure can be determined by X-ray crystallography or NMR, and beta strands are identified by characteristic backbone hydrogen bonds and dihedral angles. Sequence-based predictions and structural databases annotate beta strands to describe protein topology.

Misfolding and aggregation of beta-strand rich regions are associated with amyloid diseases, where extended beta sheets form insoluble fibrils. The term beta strand is used in contrast to alpha helix, which is stabilized by intrastrand hydrogen bonds.