Ubiquinone’s distinctive structure is defined by a polyisoprenoid side chain connected to a benzoquinone ring. It serves multiple roles in plants, functioning as an electron transporter in inner mitochondrial membranes, as well as acting as an antioxidant to protect against free radicals. The biosynthesis of ubiquinone is connected to the biosynthesis of tyrosine, tryptophan, and phenylalanine through the shared compounds L-tyrosine and chorismate. Its biosynthesis also takes place in many organelles, with key steps occurring in the mitochondria, chloroplasts, and peroxisomes of plant cells. The compound L-tyrosine begins in the cytoplasm and is converted to homogentisic acid before it can enter the chloroplast through the transporter homogentisate prenyltransferase. Once in the chloroplast, homogentisic acid can follow one of three different sets of reactions, ultimately forming five different compounds, plastoquinol-9, α-tocopherol, β-tocopherol, α-tocotrienol and β-tocotrienol. Pyrophosphate compounds from reactions early in all three sets are provided as products from terpenoid backbone biosynthesis. Meanwhile, chorismate, which also begins in the cytoplasm, can follow two distinct pathways. The first involves its transfer into the mitochondrion, where it undergoes a series of reactions until it forms ubiquinone. This ubiquinone can be used for oxidative phosphorylation within the mitochondrion. The second pathway chorismate follows brings it into the chloroplast, where multiple PHYLLO enzymes catalyze a series of reactions to form 2-succinyl benzoate. With the addition of coenzyme A, 2-succinyl benzoyl-CoA can be moved out of the chloroplast and into the peroxisome. Through a pair of reactions, this compound is ultimately hydrolyzed to form 1,4-dihydroxy-2-naphthoate, which is transported back into the chloroplast to form phylloquinol. Phylloquinone can also react with a hydrogen ion to form phylloquinol in the cell membrane.
References
Ubiquinone Biosynthesis References
Linka, Nicole & Theodoulou, Frederica. (2013). Metabolite Transporters of the Plant Peroxisomal Membrane: Known and Unknown. Sub-cellular biochemistry. 69. 169-194. 10.1007/978-94-007-6889-5_10.
Liu M, Lu S: Plastoquinone and Ubiquinone in Plants: Biosynthesis, Physiological Function and Metabolic Engineering. Front Plant Sci. 2016 Dec 16;7:1898. doi: 10.3389/fpls.2016.01898. eCollection 2016.
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