Fig. 1.
Cyclization reactions involved in (B)Chl biosynthesis. Shown is isocyclic ring formation via O2-dependent and -independent routes, catalyzed by AcsF and Ycf54 (solid arrow) and BchE (dashed arrow), respectively. Here x denotes the as-yet unassigned subunit required for the O2-dependent reaction. International Union of Pure and Applied Chemistry numbering of the relevant macrocycle carbons is indicated, and formation of the ring E is highlighted. The oxygen sources for the O2-dependent and -independent enzymes are molecular oxygen and water, respectively.