Figure 4: The various states of the 90-loop/substrate-binding loop captured in the four actives sites of the H53D+FAD+dUMP complex. a. Active site of chain A. b. Active site of chain B. c. Active site of chain C. d. Active site of chain D. The 2Fo-Fc electron density around the ligand atoms are contoured at 1.0 sigma. Ribbon drawings for the protein chains and stick representation for FAD (SO4) and dUMP (PO4) molecules. The SO4 or PO4 density observed in the FAD and dUMP binding sites are labeled (naming convention similar to Figure 3). The Asp53 and Arg90 are labeled with their chain numbers. The open and closed conformations of the substrate-binding loop are colored in green and red. The views from a to d show movement of the 90-loop/substratebinding loop from the closed state (chain A) to the open state (chain D). Please refer to table 2 for additional information.