Fig. 6.

Proposed mechanism of PntC. a CMP-AEP electron density (gray mesh: Fo-Fc omit map contoured at 3.5 sigma; blue mesh: 2Fo-Fc map contoured at 5.0 sigma highlighting the heavier Mg and P atoms) for the Tde-PntC:CMP-AEP complex. b Mechanistic interpretation of the crystallographic data. The proposed ternary complex structure (top) with extensive interactions stabilizing the PPi leaving group of CTP (blue). Nucleophilic attack of AEP (teal) generates a pentacoordinate phosphate intermediate or transition state possessing additional negative charge stabilized by Lys25 (middle). In the absence of obvious candidate general base residues, we propose that active site pK_a perturbations may result in the deprotonated AEP substrate shown. Loss of PPi generates the CMP-AEP product bound in the active site (bottom) as seen in the crystal structure in a