On the occurrence of linear groups in proteins

Linear groups—polypeptide conformations based on a single repeating ϕ,ψ‐pair—are a foundational concept in protein structure, yet how they are presented in textbooks is based largely on theoretical studies from the early days of protein structure analysis. Now, ultra‐high resolution protein structures provide a resource for an accurate empirical and systematic assessment of the linear groups that truly exist in proteins. Here, a purely conformation‐based survey of linear groups shows that only three distinct ϕ,ψ‐regions occur: a diverse set of extended conformations mostly present as β‐strands, a broad population of polyproline‐II‐like spirals, and a tight cluster that includes the highly populated α‐helix and the conformationally‐similar but much less populated 3₁₀‐helix. Rare, short left‐handed α‐/3₁₀‐helical turns with repeating ϕ,ψ‐angles occur, but none are longer than three residues. Misperceptions dispelled by this study are the existence of 2.2₇‐ and π‐helices as linear groups, the existence of specific ideal ϕ,ψ‐angles for each linear group, and the existence of a substantive difference in the ϕ,ψ‐preferences for parallel versus antiparallel β‐strands. This study provides a concrete basis for updating and enhancing how we think about and teach the basics of protein structure.