Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Protein molecules generally adopt a tertiary structure in which all backbone and side chain conformations are arranged in local energy minima; however, in several well‐refined protein structures examples of locally strained geometries, such as <jats:italic>cis</jats:italic> peptide bonds, have been observed. Staphylococcal nuclease A contains a single <jats:italic>cis</jats:italic> peptide bond between residues Lys 116 and Pro 117 within a type VIa<jats:italic>β</jats:italic>‐turn. Alternative native folded forms of nuclease A have been detected by NMR spectroscopy and attributed to a mixture of <jats:italic>cis</jats:italic> and <jats:italic>trans</jats:italic> isomers at the Lys 116‐Pro 117 peptide bond. Analyses of nuclease variants K116G and K116A by NMR spectroscopy and X‐ray crystallography are reported herein. The structure of K116A is indistinguishable from that of nuclease A, including a <jats:italic>cis</jats:italic> 116–117 peptide bond (92% populated in solution). The overall fold of K116G is also indistinguishable from nuclease A except in the region of the substitution (residues 112–117), which contains a predominantly <jats:italic>trans</jats:italic> Gly 116–Pro 117 peptide bond (80% populated in solution). Both Lys and Ala would be prohibited from adopting the backbone conformation of Gly 116 due to steric clashes between the <jats:italic>β</jats:italic>‐carbon and the surrounding residues. One explanation for these results is that the position of the ends of the residue 112–117 loop only allow <jats:italic>trans</jats:italic> conformations where the local backbone interactions associated with the ϕ and Ψ torsion angles are strained. When the 116–117 peptide bond is <jats:italic>cis</jats:italic>, less strained backbone conformations are available. Thus the relaxation of the backbone strain intrinsic to the <jats:italic>trans</jats:italic> conformation compensates for the energetically unfavorable <jats:italic>cis</jats:italic> X‐Pro peptide bond. With the removal of the side chain from residue 116 (K116G), the backbone strain of the <jats:italic>trans</jats:italic> conformation is reduced to the point that the conformation associated with the <jats:italic>cis</jats:italic> peptide bond is no longer favorable.</jats:p>