Figure 7.

Proposed model for GGPP and Mg²⁺ binding to class II diterpene cyclases. A, Productive binding mode,GGPP-Mg²⁺ complex poised for AtCPS catalyzed cyclization. B, Unproductive Mg²⁺ inhibition mode, binding of Mg²⁺ at the active site interfering with proper binding of GGPP-Mg²⁺ complex. C, Unproductive synergistic substrate inhibition mode, double GGPP-Mg²⁺ complex binding to AtCPS. GG, Geranylgeranyl (either of these hydrocarbon tails may also bind in the hydrophobic portion of the active site). The Z in B and C represents a potential third Mg²⁺ ligand whose presence varies with metabolic function (e.g. is conserved in GA-specific enzymes but not those involved in secondary metabolism).