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Photoprotective role of NADPH:protochlorophyllide oxidoreductase A

Buhr, Frank, El Bakkouri, Majida, Valdez, Oscar, Pollmann, Stephan, Lebedev, Nikolai, Reinbothe, Steffen, Reinbothe, Christiane
Proceedings of the National Academy of Sciences of the United States of America 2008 v.105 no.34 pp. 12629-12634
Hordeum vulgare, barley, oxidoreductases, photostability, pigments, binding capacity, energy transfer, protein conformation
A homology model of NADPH:protochlorophyllide (Pchlide) oxidoreductase A (POR; E.C. of barley is developed and verified by site-directed mutagenesis. PORA is considered a globular protein consisting of nine α-helices and seven β-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.