Like the Alpha sheet, the Beta sheet is a repeating secondary structure, compromising 20-28% of all residues in globular proteins. The beta sheet is often called pleated because sequentially neighboring carbon atoms are alternately above and below the plane of the sheet, resulting in a pleated appearance.
The basic unit of a beta sheet is a beta strand with n = 2 residues per turn. The approximate backbone dihedral angles are phi = -120 degrees and psi = +120 degrees. This translates to 3.2 - 3.4 Angstroms per residue in antiparallel strands and parallel strands, respectively. Hydrogen bonds in beta sheets are on average 0.1 Angstroms shorter than those found in alpha helices.
Beta sheets are designated as parallel or antiparallel based on the relative direction of the two interacting beta strands. The average length of a beta sheet is about 6 residues and most beta sheets contain fewer than 6 strands. The side chains of the amino acids of a strand in a beta sheet are found on opposite sides of the sheet and don't interact with one another. Therefore, like alpha helices, beta sheets potentially have amphipathic regions.
Beta sheets are prevalent due to their extremely stable structure. The geometry of the backbone of this conformation is such that hydrogen bonding molecules are nearly perfectly aligned for H-bonding. Further, close internal packing of the backbone atoms optimizes van der Waals interactions and minimizes hydrophobic interactions. The net result is a reduction of the free energy of the beta sheet, increasing overall stability.
One of the best sites out there on beta sheets has a practice quiz and excellent images: Beta sheets.
Another good site describes how evolutionary biology is related to secondary structure based on beta globin genes: Beta Globin genes.
Related Links
C & I Web Site
Secondary Structure Main Page
Secondary Structure -- Alpha Helix
