Wang D, Fu A
The Key Laboratory of Western Resources Biology and Biological Technology, College of Life Sciences, Northwest University, Xian, China; Shaanxi Province Key Laboratory of Biotechnology, College of Life Sciences, Northwest University, Xian, China.
The Key Laboratory of Western Resources Biology and Biological Technology, College of Life Sciences, Northwest University, Xian, China; Shaanxi Province Key Laboratory of Biotechnology, College of Life Sciences, Northwest University, Xian, China.
Enzymes. 2016;40:143-171. doi: 10.1016/bs.enz.2016.09.002. Epub 2016 Oct 12.
Mitochondria possess oxygen-consuming respiratory electron transfer chains (RETCs), and the oxygen-evolving photosynthetic electron transfer chain (PETC) resides in chloroplasts. Evolutionarily mitochondria and chloroplasts are derived from ancient α-proteobacteria and cyanobacteria, respectively. However, cyanobacteria harbor both RETC and PETC on their thylakoid membranes. It is proposed that chloroplasts could possess a RETC on the thylakoid membrane, in addition to PETC. Identification of a plastid terminal oxidase (PTOX) in the chloroplast from the Arabidopsis variegation mutant immutans (im) demonstrated the presence of a RETC in chloroplasts, and the PTOX is the committed oxidase. PTOX is distantly related to the mitochondrial alternative oxidase (AOX), which is responsible for the CN-insensitive alternative RETC. Similar to AOX, an ubiquinol (UQH2) oxidase, PTOX is a plastoquinol (PQH2) oxidase on the chloroplast thylakoid membrane. Lack of PTOX, Arabidopsis im showed a light-dependent variegation phenotype; and mutant plants will not survive the mediocre light intensity during its early development stage. PTOX is very important for carotenoid biosynthesis, since the phytoene desaturation, a key step in the carotenoid biosynthesis, is blocked in the white sectors of Arabidopsis im mutant. PTOX is found to be a stress-related protein in numerous research instances. It is generally believed that PTOX can protect plants from various environmental stresses, especially high light stress. PTOX also plays significant roles in chloroplast development and plant morphogenesis. Global physiological roles played by PTOX could be a direct or indirect consequence of its PQH2 oxidase activity to maintain the PQ pool redox state on the thylakoid membrane. The PTOX-dependent chloroplast RETC (so-called chlororespiration) does not contribute significantly when chloroplast PETC is normally developed and functions well. However, PTOX-mediated RETC could be the major force to regulate the PQ pool redox balance in the darkness, under conditions of stress, in nonphotosynthetic plastids, especially in the early development from proplastids to chloroplasts.
线粒体拥有消耗氧气的呼吸电子传递链（RETCs），而放氧的光合电子传递链（PETC）存在于叶绿体中。从进化角度来看，线粒体和叶绿体分别起源于古代的α-变形菌和蓝细菌。然而，蓝细菌在其类囊体膜上同时拥有RETC和PETC。有人提出，除了PETC外，叶绿体在类囊体膜上可能还拥有一条RETC。从拟南芥斑驳突变体immutans（im）的叶绿体中鉴定出质体末端氧化酶（PTOX），证明了叶绿体中存在RETC，且PTOX是关键氧化酶。PTOX与线粒体交替氧化酶（AOX）关系较远，AOX负责对氰化物不敏感的交替RETC。与AOX类似，AOX是一种泛醇（UQH2）氧化酶，PTOX是叶绿体类囊体膜上的质体醌（PQH2）氧化酶。缺乏PTOX时，拟南芥im表现出光依赖的斑驳表型；突变植株在早期发育阶段无法在中等光照强度下存活。PTOX对类胡萝卜素生物合成非常重要，因为类胡萝卜素生物合成中的关键步骤——八氢番茄红素去饱和作用，在拟南芥im突变体的白色区域被阻断。在众多研究实例中，PTOX被发现是一种与胁迫相关的蛋白。一般认为，PTOX可以保护植物免受各种环境胁迫，尤其是高光胁迫。PTOX在叶绿体发育和植物形态发生中也起着重要作用。PTOX所发挥的整体生理作用可能是其PQH2氧化酶活性维持类囊体膜上PQ库氧化还原状态的直接或间接结果。当叶绿体PETC正常发育且功能良好时，依赖PTOX的叶绿体RETC（即所谓的叶绿体呼吸作用）贡献不大。然而，在黑暗中、胁迫条件下、非光合质体中，尤其是在从前质体到叶绿体的早期发育过程中，PTOX介导的RETC可能是调节PQ库氧化还原平衡的主要力量。
