Beta Sheet

The beta sheet is a major secondary structural element in globular proteins. These structures are formed from beta strands and are in an almost fully extended conformation with psi,phi bond angle pairs in the wide allowed region in the upper left-hand corner of the Ramachandran Plot. These strands are aligned adjacent to each other allowing hydrogen bonds to form between C'=O groups of one beta strand and NH groups on the adjacent one and so forth. Successive Calpha atoms are located above and below the plane of the sheet. The strands of the beta sheet are not fully extended due to the constraints of hydrogen bonding, giving the sheet a 'pleated' appearance. In addition there is a left-handed twist between adjacent strands when looking at right angles to the strand direction. The beta strands can be arranged to form parallel or antiparallel beta sheets.