The backbone atoms in adjacent strands forms hydrogen bonding within the same or different polypeptide chains. 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. 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 viruses.
Most proteins have a compact, globular shape because of the 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. Without turns, proteins would be large, extended, and loosely packed. A polypeptide backbone also may contain long bends, or loops.