Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 105, Issue 34, Pages 12629-12634Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0803950105
Keywords
chlorophyll biosynthesis; chloroplast development; greening; reactive oxygen species
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Funding
- Chaire d'Excellence program of the French Ministry of Research
- Deutsche Forschungsgemeinschaft Research Project [DFG-RE1465/1-1, DFG-RE1465/1-2, DFG-RE1465/1-3]
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A homology model of NADPH:protochlorophyllide (Pchlide) oxidoreductase A (POR; E.C. 1.3.33.1) of barley is developed and verified by site-directed mutagenesis. PORA is considered a globular protein consisting of nine a-helices and seven P-strands. The model predicts the presence of two functionally distinctive Pchlide binding sites where the pigment is coordinated by cystein residues. The pigment bound to the first, high-affinity Pchlide binding site is used for the formation of the photoactive state of the enzyme. The pigment bound to the second, low-affinity Pchlide binding site is involved in the PORA:PORB interaction, allowing for resonance energy transfer between the neighboring PORs in the complex. In the in vitro reconstituted light-harvesting POR:Pchlide complex (LHPP), light absorbed by PORA-bound Pchlide b is transferred to PORB-bound Pchlide a. That induces the conversion of Pchlide a to chlorophyllide (Chlide) a. This energy transfer eliminates the possibility of Pchlide b photoreduction and prevents that excited triplet states of either Pchlides a or b accumulate and provoke singlet oxygen production. Together, our results provide a photoprotective role of PORA during greening.
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