Ronnie -- The list that I have in a home database contains 89 instances of a CXC in 53 files when C-C forms a disulfide bond. The records returned are ordered by the name of "X" residue from CXC motif for convenience.
There is indeed PRO in one case (2zxt), but it is not the major observation in CXC motif. hope this helps cheers -- heping pdbid | resn-X | chainids | resseq -------+---------+------------------------+-------- 1xm7 | ALA | A, B | 156 1zp0 | ALA | B | 162 2blf | ALA | A | 244 2bpb | ALA | A | 244 2c9x | ALA | A | 244 2ca3 | ALA | A | 244 2ca4 | ALA | A | 244 2b5g | ARG | B, A | 121 3bj8 | ARG | B, A | 121 2bcf | ASN | A | 101 1dbi | ASP | A | 138 1wrj | ASP | A | 30 3inj | CYS | E | 302 1ssu | GLN | A | 20 3me2 | GLU | R | 126 3me4 | GLU | A, B | 126 3nzy | GLU | B, D | 126 1jr8 | GLY | B, A | 110 1jra | GLY | B, D, C, A | 110 2bxr | GLY | B, A | 322 3i8i | GLY | 4 | 17 3i9c | GLY | 4 | 17 3lvt | GLY | A | 355 2he8 | HIS | A, B | 6 2ipg | HIS | B, A | 6 2p5n | HIS | A, B | 6 3cv6 | HIS | A, B | 6 3fac | HIS | B | 6 3fjn | HIS | A, B | 6 1z3s | ILE | B, A | 434 1z3u | ILE | A, B, C, D | 434 2gy7 | ILE | A | 434 2irm | LEU | A | 52 1cx8 | PHE | G, B, F, H, D, C, A, E | 557 1de4 | PHE | F, C, I | 557 1ewk | PHE | A, B | 290 1ewt | PHE | B, A | 290 1ewv | PHE | B, A | 290 1isr | PHE | A | 290 1iss | PHE | B, A | 290 1suv | PHE | B, A | 557 1t3n | PHE | B | 830 2nsu | PHE | A, B | 557 3lmk | PHE | A, B | 277 2zxt | PRO | A | 397 1mec | SER | 3 | 87 1nhr | SER | A | 41 1tmf | SER | 3 | 88 2mev | SER | 3 | 87 3dxu | SER | A | 311 2a4h | THR | A | 81 2ozk | VAL | C, A | 291 2w8n | VAL | A | 341 (53 rows) On 05/04/2011 13:55, Ronnie wrote: > I have a question related to protein structure, but not crystallography > per se. Has anyone see a disulfide forming between the two cys of "CXC" > in the middle of a loop, and create a sharp turn, where X is not a > proline? I seems to me that geometrically this would be possible but I > am not sure how stable it is, or how energetically unfavorable it might > be. > > thanks so much for your input! > > Ronnie
