Ubiquinone

Ubiquinone, also called coenzyme Q, is a lipid-soluble molecule that transfers electrons and protons within biological membranes, especially in mitochondrial respiration.

Ubiquinol: Ubiquinol is the reduced form of ubiquinone, carrying two electrons and two protons. Ubiquinone becomes ubiquinol when it accepts electrons and protons.
Quinone: A quinone is a class of cyclic compounds with two carbonyl groups conjugated with carbon–carbon double bonds. Ubiquinone’s reactive ring belongs to this chemical family.
Oxidative phosphorylation: Oxidative phosphorylation is the process by which electron transport establishes a proton gradient that powers ATP synthesis. Ubiquinone connects electron flow to the gradient used for ATP production.
Cytochrome c: Cytochrome c is a small heme protein that carries single electrons between respiratory complexes on the outer surface of the inner mitochondrial membrane. Unlike ubiquinone, it is a water-soluble protein carrier that transfers electrons but not protons.
Q cycle: The Q cycle is a mechanism that couples quinone oxidation and reduction to proton translocation in respiratory and photosynthetic complexes. It shows how ubiquinone transfers electrons while contributing to proton gradients.
Isoprenoid: An isoprenoid is a molecule built from repeating five-carbon isoprene units or their biochemical derivatives. Ubiquinone’s long hydrophobic tail is assembled from isoprenoid units.
Mitochondrial disease: Mitochondrial disease is a group of disorders caused by impaired mitochondrial structure or function. Defects in ubiquinone synthesis can disrupt mitochondrial energy production.
Plastoquinone: Plastoquinone is a lipid-soluble quinone that transfers electrons and protons in photosynthetic membranes. It is the photosynthetic counterpart to ubiquinone’s role in respiration.
Complex I: Complex I is a respiratory-chain enzyme that transfers electrons from NADH to ubiquinone and pumps protons across the inner mitochondrial membrane. It reduces ubiquinone using electrons from NADH.
Inner mitochondrial membrane: The inner mitochondrial membrane encloses the mitochondrial matrix and hosts the respiratory chain and ATP synthase. Ubiquinone diffuses within this membrane to connect respiratory complexes.

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