Yu Qiuju, Feilke Kathleen, Krieger-Liszkay Anja, Beyer Peter
Faculty of Biology, University of Freiburg, D-79104 Freiburg, Germany.
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.
Biochim Biophys Acta. 2014 Aug;1837(8):1284-92. doi: 10.1016/j.bbabio.2014.04.007. Epub 2014 Apr 26.
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.
质体末端氧化酶（PTOX）是一种质体氢醌：氧氧化还原酶，与交替氧化酶（AOX）具有结构相似性。PTOX具有多种作用，例如参与类胡萝卜素去饱和、安全阀功能、参与叶绿体呼吸过程以及为循环电子传递设定氧化还原平衡。我们研究了一种均一纯的PTOX的MBP融合蛋白。该蛋白形成一个同四聚体复合物，每个单体含有2个铁，并且对质体醌头部基团具有高度特异性。使用化学还原的癸基质体醌（DPQ）作为模型底物，在可溶性单相系统中研究了反应动力学，此外，还在双相（脂质体）系统中进行了研究，其中DPQ用DT-黄递酶还原。虽然PTOX在单相系统中未检测到产生活性氧，但在双相系统中通过室温EPR观察到活性氧的形成，其形成方式依赖于[DPQH₂]和pH。这可能是由于脂质双层部分体积内DPQ浓度较高以及与单相系统相比，PTOX-膜缔合物观察到的较高Km（≈47mM）导致的，在单相系统中测定的Km约为74μM。在脂质体和光合活性叶绿体的碱性基质pH条件下，低[DPQH₂]时PTOX具有抗氧化作用，随着对苯二酚浓度增加获得促氧化特性。得出的结论是，在体内，当稳态[PQH₂]较低时，PTOX可作为安全阀，而在高光强下会形成一定量的活性氧。
