Inhibition of chlorophyll biosynthesis at the ..
Spotlight on Protochlorophyllide Reduction.
Photosynthesis converts solar energy to chemical energy using chlorophylls (Chls). In a late stage of biosynthesis of Chls, dark-operative protochlorophyllide (Pchlide) oxidoreductase (DPOR), a nitrogenase-like enzyme, reduces the C17 = C18 double bond of Pchlide and drastically changes the spectral properties suitable for photosynthesis forming the parental chlorin ring for Chl a. We previously proposed that the spatial arrangement of the proton donors determines the stereospecificity of the Pchlide reduction based on the recently resolved structure of the DPOR catalytic component, NB-protein. However, it was not clear how the two-electron and two-proton transfer events are coordinated in the reaction. In this study, we demonstrate that DPOR initiates a single electron transfer reaction from a [4Fe-4S]-cluster (NB-cluster) to Pchlide, generating Pchlide anion radicals followed by a single proton transfer, and then, further electron/proton transfer steps transform the anion radicals into chlorophyllide (Chlide). Thus, DPOR is a unique iron-sulphur enzyme to form substrate radicals followed by sequential proton- and electron-transfer steps with the protein folding very similar to that of nitrogenase. This novel radical-mediated reaction supports the biosynthesis of Chl in a wide variety of photosynthetic organisms.
13 Chlorophylls and Bilins: Biosynthesis, ..
DPOR has been lost and the reduction step of Pchlide to Chlide has become solely dependent on LPOR in angiosperms, several gymnosperms, and some Pteridophytes (). Based on LPOR and DPOR distribution in the plant, eubacterial, and archaebacterial kingdoms and the oxygen sensitivity of DPOR, a model has been proposed to explain the gene transfer of LPOR and the substitution of LPOR for DPOR during land plant evolution: 1) oxygen-sensitive DPOR initially emerged when the atmosphere of Archaean Earth was anaerobic, presumably from nitrogenase-like genes () and 2) LPOR, which is an oxygen-insensitive and light- and NADPH-dependent enzyme belonging to a short-chain dehydrogenase/reductase superfamily, evolved during the transition from an anaerobic to aerobic atmosphere (, ). Both enzymes seem to have emerged before the endosymbiosis of cyanobacteria, which are believed to be the ancestors of chloroplasts, given that Chlides are biosynthesized using both DPOR and LPOR in modern cyanobacteria (). 3) Upon the establishment of chloroplasts, it is likely that LPOR was transferred to the nucleus, whereas the genes for DPOR were retained in chloroplast genomes. Despite their apparent functional redundancy, both genes have been strictly retained in several algae, lower plants, and gymnosperms. 4) As a final step of “gene replacement,” DPOR loss independently occurred in land plants.