Cryo-EM structures of Cereibacter sphaeroides ferrochelatase reveal an oligomeric state that occludes the substrate-binding site
Protoporphyrin IX (PPIX) is the last shared intermediate of heme and (bacterio)chlorophyll ((B)Chl) biosynthesis. Insertion of iron into PPIX by the PPIX ferrochelatase (FeCh) PpfC produces heme, whereas magnesium insertion by magnesium chelatase (MgCh) initiates (B)Chl synthesis. Chlorophototrophic organisms must regulate this biosynthetic node to ensure the correct flux of PPIX to each of these crucial tetrapyrrole products, presenting a particularly acute problem for the facultative bacterium Cereibacter (previously Rhodobacter) sphaeroides, which can switch between chemotrophic (respiratory) and phototrophic (photosynthetic) growth modes. To elucidate the structural basis for the catalytic and regulatory functions of C. sphaeroides FeCh (PpfCCs), we used cryogenic electron microscopy to determine its structure, to our knowledge the first such information for a Gram-negative or phototrophic organism. We determined the 2.6 [A] structure of an active site mutant of PpfCCs, which forms a dimer with a monomer-to-monomer interface that coordinates two hemes. We also determined a 2.5 [A] structure of the wild-type enzyme, in which two dimers interact in a homotetrameric arrangement that occludes PPIX binding at the active site of each monomer. We propose that the PpfCCs dimer likely docks onto the inner membrane in a manner analogous to the human enzyme, whereas the tetramer represents a transiently inactive state whereby the hydrophobic lips and active site are both occluded. These novel oligomerization states of FeCh may have evolved to influence the flux of tetrapyrroles through the heme/BChl branchpoint in purple bacteria.