Figure 1.

Comparison of the X-ray crystal structures for (A) unliganded ScOMPDC (Protein Databank (PDB) entry 3GDL) and (B) the complex between ScOMPDC and the tight-binding substrate analog 6-azauridine 5′-monophosphate (6-azaUMP) (PDB entry 3GDK). The hypothetical 6-azaUMP ligand has been inserted in panel A to mimic the position of the ligand at panel B. (B) shows the ligand-driven movement of the phosphodianion gripper loop (P202 to 220, shaded blue) toward the pyrimidine umbrella loop (A151 to T165, shaded green) and of R235 toward the phosphodianion. The −CH₂OH side chain of S154 bridges these loops and holds Q215 near the dianion by donating an H-bond to the amide side chain of Q215 and accepting an H-bond from the C-5 pyrimidine −NH (dashed red circle).