Photoprotective role of NADPH:protochlorophyllide oxidoreductase A.
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 alpha-helices and seven beta-strands. The model predicts the presence of two functionally distinctive Pchlide binding sites where the pigment is coordinated by cysteine 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.
Buhr
Frank
F
Lehrstuhl für Pflanzenphysiologie, Universität Bayreuth, Universitätsstrasse 30, D-95447 Bayreuth, Germany.
El Bakkouri
Majida
M
Valdez
Oscar
O
Pollmann
Stephan
S
Lebedev
Nikolai
N
Reinbothe
Steffen
S
Reinbothe
Christiane
C
D001665:Binding Sites; D004735:Energy Transfer / Q000502:physiology; D001467:Hordeum / Q000201:enzymology*; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D009247:NADH, NADPH Oxidoreductases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D010777:Photochemistry; D010940:Plant Proteins / Q000502:physiology; D011521:Protochlorophyllide / Q000378:metabolism; D040681:Structural Homology, Protein
D045342:Light-Harvesting Protein Complexes; D010940:Plant Proteins; D011521:Protochlorophyllide; D009247:NADH, NADPH Oxidoreductases