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[ccp4bb]: differences between SO4 and PO4 anions binding



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Dear colleagues,
I have a question related to differences between SO4 and PO4 anions binding.

We have solved to 1.7 Angstroem a GTP-binding protein (GTPase)
in a nucleotide-free form but with a sulfate or phosphate anion
(strong density with 4 branches)
bound to usual phosphate-binding sites of the nucleotide
(to see the following PDB coordinates below).
Both sulfate and phosphate were present in my crystallization set-up
at pH=7 or slightly above with a concentration of 12 mM for each.
Of course NO NEW CRYSTALS CAN BE OBTAINED.

Since SO4 or PO4 have a different effect in kinetics upon nucleotide binding,
I would like to understand which anions is likely to be bound and do not
know how to answer this question.
Could I use the coordinates to help me in this "quest of the Truth" and how ?

Thank you for any suggestions or comments.
louis





------- PDB coordinates of the anion with its protein-binding sites --------

REMARK chain ID A= anionic binding sites of the protein
REMARK chain ID B= anion arbitrary choosen as a PO4 anion (could be a SO4)
ATOM     76  N   GLY A  17      12.646 -18.957   2.568  1.00 22.38      A
ATOM     77  CA  GLY A  17      11.388 -18.856   3.274  1.00 21.77      A
ATOM     78  C   GLY A  17      11.414 -17.658   4.194  1.00 24.92      A
ATOM     79  O   GLY A  17      12.487 -17.140   4.518  1.00 22.99      A
ATOM     80  N   ASP A  18      10.231 -17.218   4.604  1.00 26.18      A
ATOM     81  CA  ASP A  18      10.090 -16.081   5.502  1.00 28.93      A
ATOM     82  CB  ASP A  18       8.610 -15.766   5.718  1.00 30.24      A
ATOM     83  CG  ASP A  18       8.071 -14.760   4.722  1.00 34.40      A
ATOM     84  OD1 ASP A  18       8.667 -14.604   3.635  1.00 35.97      A
ATOM     85  OD2 ASP A  18       7.037 -14.128   5.028  1.00 38.34      A
ATOM     86  C   ASP A  18      10.729 -16.390   6.847  1.00 29.39      A
ATOM     87  O   ASP A  18      10.908 -17.552   7.209  1.00 28.01      A
ATOM     88  N   GLY A  19      11.070 -15.345   7.592  1.00 31.49      A
ATOM     89  CA  GLY A  19      11.662 -15.568   8.896  1.00 30.95      A
ATOM     90  C   GLY A  19      10.617 -16.190   9.798  1.00 30.55      A
ATOM     91  O   GLY A  19       9.428 -15.903   9.657  1.00 31.60      A
ATOM     92  N   GLY A  20      11.048 -17.059  10.707  1.00 30.13      A
ATOM     93  CA  GLY A  20      10.117 -17.692  11.625  1.00 30.13      A
ATOM     94  C   GLY A  20       9.309 -18.847  11.056  1.00 29.78      A
ATOM     95  O   GLY A  20       8.317 -19.265  11.654  1.00 30.98      A
ATOM     96  N   THR A  21       9.723 -19.373   9.907  1.00 27.19      A
ATOM     97  CA  THR A  21       9.006 -20.484   9.294  1.00 26.77      A
ATOM     98  CB  THR A  21       8.918 -20.308   7.765  1.00 24.35      A
ATOM     99  OG1 THR A  21      10.228 -20.137   7.220  1.00 24.97      A
ATOM    100  CG2 THR A  21       8.076 -19.085   7.425  1.00 22.50      A
ATOM    101  C   THR A  21       9.653 -21.833   9.631  1.00 27.31      A
ATOM    102  O   THR A  21       9.263 -22.873   9.094  1.00 27.57      A
ATOM    103  N   GLY A  22      10.650 -21.796  10.514  1.00 27.94      A
ATOM    104  CA  GLY A  22      11.323 -23.005  10.953  1.00 27.47      A
ATOM    105  C   GLY A  22      12.496 -23.509  10.134  1.00 29.05      A
ATOM    106  O   GLY A  22      12.893 -24.665  10.286  1.00 28.16      A
ATOM    107  N   LYS A  23      13.065 -22.657   9.284  1.00 29.71      A
ATOM    108  CA  LYS A  23      14.193 -23.060   8.451  1.00 31.21      A
ATOM    109  CB  LYS A  23      14.614 -21.910   7.527  1.00 31.71      A
ATOM    110  CG  LYS A  23      13.535 -21.480   6.530  1.00 32.92      A
ATOM    111  CD  LYS A  23      14.116 -20.648   5.385  1.00 33.10      A
ATOM    112  CE  LYS A  23      14.740 -19.343   5.870  1.00 35.29      A
ATOM    113  NZ  LYS A  23      13.760 -18.459   6.560  1.00 34.04      A
ATOM    114  C   LYS A  23      15.399 -23.527   9.270  1.00 33.80      A
ATOM    115  O   LYS A  23      15.992 -24.569   8.981  1.00 32.73      A
ATOM    116  N   THR A  24      15.759 -22.757  10.290  1.00 36.63      A
ATOM    117  CA  THR A  24      16.903 -23.104  11.126  1.00 39.65      A
ATOM    118  CB  THR A  24      17.168 -22.006  12.170  1.00 41.70      A
ATOM    119  OG1 THR A  24      17.442 -20.772  11.495  1.00 44.77      A
ATOM    120  CG2 THR A  24      18.358 -22.373  13.046  1.00 46.02      A
ATOM    121  C   THR A  24      16.689 -24.438  11.835  1.00 40.17      A
ATOM    122  O   THR A  24      17.531 -25.335  11.753  1.00 39.91      A
ATOM   8942  P   PO4 B 250      13.429 -19.350  10.716  1.00 34.84      A
ATOM   8943  O1  PO4 B 250      12.562 -19.867  11.803  1.00 35.13      A
ATOM   8944  O2  PO4 B 250      12.810 -19.659   9.397  1.00 37.25      A
ATOM   8945  O3  PO4 B 250      13.576 -17.879  10.860  1.00 35.16      A
ATOM   8946  O4  PO4 B 250      14.767 -19.991  10.799  1.00 35.54      A
END