Functional and molecular characterization of plastid terminal oxidase from rice (Oryza sativa).
The plastid terminal oxidase (PTOX) is a plastohydroquinone:oxygen oxidoreductase that shares structural similarities with alternative oxidases (AOX). Multiple roles have been attributed to PTOX, such as involvement in carotene desaturation, a safety valve function, participation in the processes of chlororespiration and setting the redox poise for cyclic electron transport. We have investigated a homogenously pure MBP fusion of PTOX. The protein forms a homo-tetrameric complex containing 2 Fe per monomer and is very specific for the plastoquinone head-group. The reaction kinetics were investigated in a soluble monophasic system using chemically reduced decyl-plastoquinone (DPQ) as the model substrate and, in addition, in a biphasic (liposomal) system in which DPQ was reduced with DT-diaphorase. While PTOX did not detectably produce reactive oxygen species in the monophasic system, their formation was observed by room temperature EPR in the biphasic system in a [DPQH₂] and pH-dependent manner. This is probably the result of the higher concentration of DPQ achieved within the partial volume of the lipid bilayer and a higher Km observed with PTOX-membrane associates which is ≈47mM compared to the monophasic system where a Km of ≈74μM was determined. With liposomes and at the basic stromal pH of photosynthetically active chloroplasts, PTOX was antioxidant at low [DPQH₂] gaining prooxidant properties with increasing quinol concentrations. It is concluded that in vivo, PTOX can act as a safety valve when the steady state [PQH₂] is low while a certain amount of ROS is formed at high light intensities.
Faculty of Biology, University of Freiburg, D-79104 Freiburg, Germany.
Qiuju
Q
Yu
Commissariat à l'Energie Atomique (CEA) Saclay, iBiTec-S, CNRS UMR 8221, Service de Bioénergétique, Biologie Structurale et Mécanisme, 91191 Gif-sur-Yvette Cedex, France.
Kathleen
K
Feilke
Commissariat à l'Energie Atomique (CEA) Saclay, iBiTec-S, CNRS UMR 8221, Service de Bioénergétique, Biologie Structurale et Mécanisme, 91191 Gif-sur-Yvette Cedex, France.
Anja
A
Krieger-Liszkay
Faculty of Biology, University of Freiburg, D-79104 Freiburg, Germany. Electronic address: peter.beyer@biologie.uni-freiburg.de.
Peter
P
Beyer
D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics; D002736:Chloroplasts / Q000201:enzymology* / Q000235:genetics; D004579:Electron Transport / Q000235:genetics; D007700:Kinetics; D008027:Light; D012275:Oryza / Q000201:enzymology*; D010088:Oxidoreductases / Q000737:chemistry / Q000235:genetics; D010788:Photosynthesis; D018087:Plastids / Q000201:enzymology*; D017382:Reactive Oxygen Species / Q000378:metabolism; D020524:Thylakoids / Q000201:enzymology
D029681:Arabidopsis Proteins; D010088:Oxidoreductases; D017382:Reactive Oxygen Species; C405742:IM protein, Arabidopsis
Carotene desaturation; PTOX; Photosynthesis; Plastid terminal oxidase; Plastoquinone; Reactive oxygen species