Figure 3. CTD dephosphorylation and close-up views of the Fcp1 active site.

A) Two-step Fcp1 reaction pathway. Step 1 is the attack of the active-site aspartate on the phosphorus of phospho-CTD to form an acylphosphate intermediate. Step 2 entails a nucleophilic attack by water on the acylphosphate, resulting in formation of the inorganic phosphate product. B) Stereo view of the active site of Fcp1-Mg-BeF₃ complex that mimics the acylphosphate intermediate. C) Stereo view of the Fcp1-Mg-AlF₄ ⁻ complex that mimics the step 2 transition state. Side chains are depicted in stick representation; Mg and water are denoted by blue and red spheres, respectively. The proposed nucleophilic water is labeled Wat*. Atomic contacts indicated by dashed lines.