Figure 5. Models of enzyme-product complexes.

(a) Model of product fusicoccadiene bound in the active site of PaFS₃₄₄. The three-dimensional active site contour is represented by black meshwork, and fusicoccadiene was manually fit into this meshwork. The position of the Mg²⁺₃-PP_i cluster is modeled after the Mg²⁺₃-bisphosphonate cluster in the PaFS₃₄₄-Mg²⁺₃-pamidronate complex. Two perpendicular views are shown; the backbone carbonyl of V192 at the helix G break (δ⁻) is oriented toward the location of a proposed carbocation intermediate in the PaFS mechanism. (b) Cut-away view of the active site pocket of PaFS₃₄₄, showing how the surface contour (black meshwork) is defined by residues lining the pocket. (c) Ophiobolin F docked in the active site of ophiobolin F synthase modeled after the PaFS₃₄₄-fusicoccadiene complex in (b). The extra active site volume resulting from the W225L and V228A substitutions readily accommodate the larger C₂₅ sesterterpene.