On the role of covalent strain in protein function

Strain has frequently been suggested to play an important role in the function of several proteins. In this paper, we review various definition of strain and discuss how it may be quantified. We show that covalent strain (i.e. strain caused by covalent interactions) plays a minor role for the function of all proteins we have studied, e.g. blue copper proteins, desulforedoxin, CuA in cytochrome c oxidase, and antibodies (the immuno­chromic effect). However, in small molecules, such as the haem group and macrocyclic model complexes covalent strain may have a pronounced effect. In general, significant strain is seen when a molecule is constrained by covalent bonds in rings, whereas forces present in proteins, i.e. torsional constraints and non-bonded interactions are too weak to distort a bound molecule or metal significantly.