Plastid terminal oxidase (PTOX) - its physiological role and biological significance
Author: Mahesh Kumar Samota

Plastid Terminal Oxidase or Plastoquinol Terminal Oxidase (PTOX) is an enzyme that resides on the thylakoid membrane. PTOX catalyzes the oxidation of the plastoquinone pool which exerts a variety of effects on the development and functioning of plant chloroplasts. The enzyme is important for carotenoid biosynthesis and prevents the over-reduction of the plastoquinone pool. The lack of PTOX indirectly causes photo damage because protective carotenoids are not synthesized without the oxidase. The most confirmed function of PTOX in developed chloroplasts is its role in chlororespiration. The ptox1 mutant accumulated phytoene in white leaf sectors with a corresponding deficiency in Beta-carotene consistent with the expected function of PTOX1 in promoting phytoene desaturase activity. There was also no accumulation of the carotenoid-derived SL ent-2'-epi-5-deoxystrigol in root exudates. Elevated concentrations of auxin were detected in the mutant supporting that SL interaction with auxin is important in shoot branching control. Results in the study demonstrated that PTOX1 is required for both carotenoid and SL synthesis.

Plastids have retained from their cyanobacterial ancestor a fragment of the respiratory electron chain comprising an NADPH dehydrogenase and a diiron oxidase which sustain the so-called chloro-respiration pathway. Despite its very low turnover rates compared with photosynthetic electron flow knocking out the plastid terminal oxidase (PTOX) in plants or micro algae leads to severe phenotypes that encompass developmental and growth defects together with increased photo sensitivity. On the basis of a phylogenetic and structural analysis of the enzyme we discuss its physiological contribution to chloroplast metabolism with an emphasis on its critical function in setting the redox poise of the chloroplast stroma in darkness. The emerging picture of PTOX is that of an enzyme at the crossroads of a variety of metabolic processes such as among others there gulation of cyclic electron transfer and carotenoid biosynthesis which have in common their dependence on there redox state of the plasto quinone pool set largely by the activity of PTOX in darkness. The plastid terminal oxidase (PTOX) has a complex evolutionary history with several independent duplication events. PTOX and the alternative oxidase(AOX) are structurally similar as expected from their homologous function but they display profound differences from both a biochemical and enzymatic standpoint. The carotenoid biosynthesis hypothesis straight forwardly relates the lack of PTOX to a defect in chloroplast biogenesis but fails to account for the fact that some plastids apparently cope with the lack of plasto quinone(PQ) oxidase as evidenced by a variegated phenotype. Because chlororespiration controls the redox state of the PQ pool in darkness PTOX is a key enzyme in the bioenergetics of the photosynthetic cell. Together with NADH dehydrogenase-like (NDH) /NAD(P)H dehydrogenase-like (NDA) PTOX sets the redox poise in the dark not only of the PQ pool but also of the entire stroma and thereby potentially impacts to different degrees all the processes that either involve redox reactions or are redox controlled.

Photo-protection by PTOX

PTOX may consume up to 10% of photo chemically produced oxygen via the astaxanthin biosynthesis pathway which would lower the oxygen partial pressure in the cell and consequently decrease ROS production . One could also argue that PTOX contributes to PSII photoprotection by shifting the site of ROS production from the appressed membranes where the majority of PSII resides to the non-appressed membranes where PTOX would be located.

The Dual Role of the Plastid Terminal Oxidase (PTOX)

At normal condition PTOX cannot operate because it has no access to its substrate PQ while at saturating light intensities proton gradient and plastoquinol concentration increases.The stroma gets more alkaline allowing PTOX to associate with membrane and production of ROS trigger a ROS signalling pathway and thereby a stress acclimation response

Role in oxidative stress

• Astaxanthin protecting microalgae against oxidative stress

• Reduces sub-cellular oxygen levels

• Converts photosynthetically evolved oxygen into water via a coupled electron transport from carotenoid desaturation steps

• Serves as a "sunshade" to functions as a powerful antioxidant against reactive oxygen species

References:

1. Bennoun P. 2002. The present model for chlororespiration. Photosynth. Res. 73:273-77

2. Heyno E, Gross CM, Laureau C, Culcasi M, Pietri S, Krieger-Liszkay A. 2009. Plastid alternative oxidase (PTOX) promotes oxidative stress when overexpressed in tobacco. J. Biol. Chem. 284:31174-80


About Author / Additional Info:
I am currently pursuing Ph.D. in BIOCHEMISTRY from IARI New Delhi.