FIGURE 6.

Proposed model for the DXDD+His/Arg catalytic tetrad, with speculative hydrogen bonding interactions among the catalytic residues and the natural GGPP substrate. A, productive binding in the active site of CPS involved in GA biosynthesis. B, inhibitory binding of Mg²⁺ preventing GGPP cyclization in the active site of CPS involved in GA biosynthesis. C, productive binding of GGPP in the class II active site of diterpene synthases involved in more specialized/secondary metabolism. The catalytic aspartates are labeled by their relative position within the DXDD motif.