title: Dark-operative protochlorophyllide oxidoreductase generates substrate radicals by an iron-sulphur cluster in bacteriochlorophyll biosynthesis.
abstract: 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.
authors: 1] Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan [2].
authors: Jiro
authors: J
authors: Nomata
authors: 1] Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan [2].
authors: Toru
authors: T
authors: Kondo
authors: Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan.
authors: Tadashi
authors: T
authors: Mizoguchi
authors: Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan.
authors: Hitoshi
authors: H
authors: Tamiaki
authors: 1] Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan [2].
authors: Shigeru
authors: S
authors: Itoh
authors: Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
authors: Yuichi
authors: Y
authors: Fujita
mesh_terms: D025541:Bacteriochlorophyll A / Q000096:biosynthesis / Q000138:chemical synthesis*; D004789:Enzyme Activation / Q000528:radiation effects; D005609:Free Radicals; D007501:Iron / Q000737:chemistry* / Q000528:radiation effects; D008027:Light; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry* / Q000528:radiation effects; D010788:Photosynthesis / Q000502:physiology / Q000528:radiation effects; D011521:Protochlorophyllide / Q000737:chemistry* / Q000528:radiation effects; D013379:Substrate Specificity; D013455:Sulfur / Q000737:chemistry* / Q000528:radiation effects
chemical_list: D025541:Bacteriochlorophyll A; D005609:Free Radicals; D011521:Protochlorophyllide; D013455:Sulfur; D007501:Iron; D044925:Oxidoreductases Acting on CH-CH Group Donors; C023886:protochlorophyllide reductase