Plastid terminal oxidase catalyzes the
oxidation of the
plastoquinone pool, which exerts a variety of effects on the development and functioning of
plant chloroplasts.
Carotenoid biosynthesis and plastid development The enzyme is important for
carotenoid biosynthesis during chloroplast
biogenesis. In developing
plastids, its activity prevents the over-reduction of the plastoquinone pool. Knockout plants for PTOX exhibit
phenotypes of variegated leaves with white patches. Without the enzyme, the carotenoid synthesis pathway slows down due to the lack of oxidized plastoquinone with which to oxidize
phytoene, a carotenoid intermediate. The colorless compound
phytoene accumulates in the leaves, resulting in white patches of cells. PTOX is also thought to determine the redox poise of the developing photosynthetic apparatus and without it, plants fail to assemble organized internal membrane structures in chloroplasts when exposed to high light during early development.
Photoprotection Plants deficient in the
IMMUTANS gene that encodes the oxidase are especially susceptible to
photooxidative stress during early
plastid development. The knockout plants exhibit a
phenotype of variegated leaves with white patches that indicate a lack of pigmentation or photodamage. This effect is enhanced with increased light and temperature during plant development. The lack of plastid terminal oxidase indirectly causes photodamage during plastid development because protective carotenoids are not synthesized without the oxidase. The enzyme is also thought to act as a safety valve for stress conditions in the
photosynthetic apparatus. By providing an electron sink when the plastoquinone pool is over-reduced, the oxidase is thought to protect
photosystem II from oxidative damage. Knockouts for
Rubisco and photosystem II complexes, which would experience more photodamage than normal, exhibit an upregulation of plastid terminal oxidase. This effect is not universal because it requires plants to have additional PTOX regulation mechanisms. While many studies agree with the stress-protective role of the enzyme, one study showed that overexpression of
PTOX in tobacco (
Nicotiana tabacum) increases the production of
reactive oxygen species and causes more photodamage than normal. This finding suggests that an efficient antioxidant system is required for the oxidase to function as a safety valve for stress conditions and that it is more important during chloroplast biogenesis than in the regular functioning of the chloroplast.
Chlororespiration and electron flux The most confirmed function of plastid terminal oxidase in developed chloroplasts is its role in
chlororespiration. In this process,
NADPH dehydrogenase (NDH) reduces the quinone pool and the terminal oxidase oxidizes it, serving the same function as
cytochrome c oxidase from
mitochondrial electron transport. In
Chlamydomonas, there are two copies of the gene for the oxidase.
PTOX2 significantly contributes to the flux of electrons through chlororespiration in the dark. There is also evidence from experiments with
tobacco that it functions in plant chlororespiration as well. In fully developed chloroplasts, prolonged exposure to light increases the activity of the oxidase. Because the enzyme acts at the plastoquinone pool in between
photosystem II and
photosystem I, it may play a role in controlling electron flow through
photosynthesis by acting as an alternative electron sink. Similar to its role in
carotenoid synthesis, its oxidase activity may prevent the over-reduction of
photosystem I electron acceptors and damage by photoinhibition. A recent analysis of electron flux through the photosynthetic pathway shows that even when activated, the electron
flux plastid terminal oxidase diverts is two orders of magnitude less than the total flux through photosynthetic
electron transport. This suggests that the protein may play less of a role than previously thought in relieving the
oxidative stress in photosynthesis. ==Structure==