Figure 4.

Superposition of helices D and H of A. terreus aristolochene synthase complexed with PP_i (yellow), E. coli farnesyl diphosphate synthase complexed with isopentenyl diphosphate and dimethylallyl S-thiolodiphosphate (green), and DCS complexed with 2F-FPP (blue), based on the superposition of their trinuclear metal clusters. For clarity, only residues interacting with Mg²⁺_A, Mg²⁺_B, and Mg²⁺_C are shown on helices D, H, and Hα-1. The constellation of three magnesium ions is identical regardless of whether Mg²⁺_B is chelated by an aspartate-rich motif or the NSE/DTE motif, and regardless of whether the enzyme catalyzes an isoprenoid chain elongation reaction or a cyclization reaction. Coordination of Mg²⁺_B by an NSE/DTE motif requires the additional helix Hα-1.