Beta-barrel

Beta-barrel
Identifiers
Symbol	Beta-barrel
Pfam	PF00560
InterPro	IPR001107
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

A beta-barrel is a large beta-sheet that twists and coils to form a closed structure in which the first strand is hydrogen bonded to the last. Beta-barrels are a common structural motif in proteins and are found in many different types of proteins, including porins, lipocalins, and transmembrane proteins.

Structure

Beta-barrels are composed of beta-strands that are arranged in a circular fashion, forming a cylindrical shape. The strands are connected by hydrogen bonds, creating a stable structure. The number of strands in a beta-barrel can vary, but they typically range from 8 to 22 strands. The strands are usually antiparallel, meaning that adjacent strands run in opposite directions.

The interior of a beta-barrel is often hydrophobic, allowing it to accommodate hydrophobic ligands or to span a lipid bilayer in the case of transmembrane beta-barrels. The exterior of the barrel is typically hydrophilic, interacting with the aqueous environment.

Function

Beta-barrels serve a variety of functions in biological systems. In membrane proteins, beta-barrels can form channels or pores that allow the passage of molecules across the membrane. For example, porins are beta-barrel proteins that form pores in the outer membrane of Gram-negative bacteria, allowing the diffusion of small molecules.

In the case of lipocalins, beta-barrels serve as binding sites for small hydrophobic molecules, such as vitamins and hormones. The beta-barrel structure provides a stable environment for these molecules, protecting them from the aqueous surroundings.

Examples

• Porins: Found in the outer membrane of bacteria, mitochondria, and chloroplasts, porins form channels that allow the passage of ions and small molecules.
• Lipocalins: These proteins bind small hydrophobic molecules and are involved in various biological processes, including transport and storage of lipophilic substances.
• Green fluorescent protein (GFP): A well-known example of a beta-barrel protein, GFP is used as a fluorescent marker in molecular biology.

Also see

• Alpha helix
• Protein structure
• Transmembrane protein
• Porin
• Lipocalin

This diagram (which is interactive) of protein structure uses PCNA as an example. (PDB: 1AXC​)