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The four cysteines ring motif in proteins

Disulfide bonds play fundamental roles in proteins. This work is devoted to highly rare motifs containing disulfide bonds. A search for four cysteines, forming a 16‐atom membered ring (4CR) embodying two disulfide bonds, was carried out against all entries in the Protein Data Bank. Searching the crystallographic subset, only few protein molecules, all dimeric, were found to embody this peculiar structural feature, which establishes a covalent link between two different polypeptide chains. In contrast, in a peptide studied in solution by NMR, the four cysteines moiety includes only residues from one chain. A comparative analysis provided evidence for similarity and difference. It emerged that 4CR motif is highly rare and may serve to gain a specialized function. © 2009 Wiley Periodicals, Inc. Biopolymers 91: 1048–1055, 2009.