In many proteins the entire beta sheet twists to some extent rather than lying in a perfectly flat plane. In other cases the entire beta sheet winds into a cylinder, forming a beta barrel. The beta sheet in any of these forms allows even greater opportunities for hydrogen bonding than the alpha helix, making the beta sheets highly rigid, stable structures. Individual beta strands are frequently depicted in protein diagrams by an arrow with the arrow pointing toward the C-terminal end of the polypetide chain. Beta strands and sheets occur in many natural proteins, usually in combination with alpha-helical segments.
Beta barrel occur in some enzymes and are common in the coat proteins of icosahedral viruses.
The silk spun by silkworms is composed largely of beta sheets, which contribute to the unusually high tensile strength of these protein fibers. The Beta-Turn Most proteins have compact, globular shapes due to numerous reversals of the direction of their polypeptide chains. Analyses of the three dimensional structures of numerous proteins have revealed that many of these chain reversals are accomplished by a common structural element called the Beta-turn. Thus, a polypeptide chain can abruptly reverse its direction. The beta-turns often connect antiparallel beta strands.