Fig. 14.

Proposed biosynthetic pathway with GGGPS, DGGGPS and CarS acting as (isotopically labeled) feeding substrate gatekeepers. The pathway for endogenous GGPP is colored green, while the experimental feeding substrate pathways are indicated in blue (Poulter et al. 1980) and red (Eguchi et al. 2003). Dashed arrows indicate biochemically uncharacterized reactions. Crosses indicate reactions that are not possible (red) or thought impossible on the basis of enzyme characterization experiments or the feeding study result (orange). *GGK and GGPK have not been shown to be active on (partially) saturated isoprenoids. ** GGGPS can utilize phytyl-PP as a prenyl donor, but this has not been demonstrated for DGGGPS. *** Due to the substrate promiscuity of GGR (toward headgroups) and headgroup diversification enzymes (toward the lipid backbone and radyl tails), isoprenyl saturation and headgroup modification could occur in parallel