Figure 4: Examples of known thermophilic archaeal protein structures containing disulfide bonds, but having an odd number of total cysteines. Two proteins from Sulfolobus islandicus M.14.25 are shown. Mapped cysteines positions are represented in pink while cysteine residues involved in a putative metal-binding site are in orange, Zn in grey. (a) Mapping of an OsmC family protein (UniProtKB : C3MY18) onto the PDB structure 2OPL. One disulfide bond is predicted; the third cysteine is located at the surface and could participate in an intermolecular disulfide bond. (b) Mapping of a probable DNA primase small subunit (UniProtKB : C3MYF5) onto the PDB structure 1ZT2. Two mapped cysteines are poised to form a disulfide bond while the third is close to a Zn-binding site, suggesting an interaction with the metal ion. Many cases of proteins with an odd number of cysteines probably reflect the presence of intermolecular disulfide bonds or participation in metal-binding sites.