The following complexes of T. foetus IMPDH are shown: A. E•SO4-2 (PDB accession number 1ak5), model for apoenzyme 89. B. E•IMP (1me9) 120. C. E•IMP•TAD (1lrt), model for E•IMP•NAD+ 95. D. E•RVP•Na+ (1ME8), model for E-XMP*open 121. F. E•MZP•K+ (1pvn), model for E-XMP*closed 122. G. E•XMP 89. Color key: monomer with active site, blue; adjacent monomer, dark blue; Cys319 loop (residues 313-328), firebrick; flap (residues 412-432), dark magenta; IMP, coral; TAD, green; RVP, pink; MZP, pink; XMP, coral; K+, orange sphere; Na+, gold sphere. Residues 262-268 have been omitted from panels D and G to permit a view of Asp261. In addition, residues 14-27(adjacent monomer) were omitted in panels D, F and G for better visualization of the adenosine subsite. A second K+, unique to T. foetus IMPDH, was also omitted from all panels.