Refinement and model quality statistics.

The quality indicating factor Rcryst was calculated for observed structure factor amplitudes Fobs (square root of I) and their model counterparts Fcalc. As independent criterion for the agreement of model atomic coordinates with X-ray data, 5% of reflections were not used for refinement, but for calculation of Rfree. B-factor values that reflect thermal atom motions are in the normal range in both structures. In particular, the nucleotide GMP-PNP with a relatively lower average B-factor appears to be more tightly bound than the unspecific ligand citrate. Deviations from standard bond lengths and angles are low, as indicated by the respective RMSD values. Also the dihedral angles of the polypeptide backbone corroborate the good geometry of both models, with only one outlier in the Ramachandran plots.*alues in parentheses refer to the highest resolution shell. Rcryst is calculated with 95% of reflections (working set).bRfree is calculated with the same formula, using 5% of reflections (test set). with x either being bond lengths or angles, for the calculation with ideal and observed parameters. In case of two atomic coordinate sets the average distance of the Cα coordinates from two structures is calculated accordingly.