Mimicry by asx‐ and ST‐turns of the four main types of β‐turn in proteins

Hydrogen‐bonded β‐turns in proteins occur in four categories: type I (the most common), type II, type II', and type I'. Asx‐turns resemble β‐turns, in that both have an NH…OC hydrogen bond forming a ring of 10 atoms. Serine and threonine side chains also commonly form hydrogen‐bonded turns, here called ST‐turns. Asx‐turns and ST‐turns can be categorized into four classes, based on side chain rotamers and the conformation of the central turn residue, which are geometrically equivalent to the four types of β‐turns. We propose asx‐ and ST‐turns be named using the type I, II, I', and II' β‐turn nomenclature. Using this, the frequency of occurrence of both asx‐ and ST‐turns is: type II' > type I > type II > type I', whereas for β‐turns it is type I > type II > type I' > type II'. Almost all type II asx‐turns occur as a recently described three residue feature named an asx‐nest.