Methionines forming part of an S-aromatic motif are less likely to be oxidized.
(A) For each of the 1118 methionines present in the set of highly H2O2-sensitive proteins with known structures, the distance from the sulphur atom to the geometric centre of the aryl moiety of any aromatic residue was computed. The distribution of such distances was plotted and shows a characteristic bimodal distribution. Any methionine sulphur atom within 7 Å (arrow) of the aromatic ring centre from any aromatic amino acid was considered to be part of an S-aromatic motif. (B) The interrelationship between the propensity to form S-aromatic motifs and the propensity to be oxidized is shown as a mosaic plot, which allows us to easily see that the proportion of S-aromatic motifs that are oxidized is very different from the proportion of S-aromatic motifs that are not oxidized. The significance of the difference (p-value = 4 × 10−10) was assessed using Fisher’s exact probability test.