Ubiquinol

Coenzyme Q₁₀: Structure, Redox States, and Biological Functions

Ubiquinol structure

Coenzyme Q₁₀ (CoQ₁₀), known for its central role in cellular energy production and as a potent antioxidant, is a ubiquitous component found in the mitochondria of eukaryotic cells. Its structure and redox properties enable it to participate in various vital physiological processes.

Chemical Structure

The natural ubiquinol form of CoQ₁₀ is identified chemically as 2,3-dimethoxy-5-methyl-6-poly prenyl-1,4-benzoquinol. A distinguishing feature of this molecule in mammals is its polyprenylated side-chain, which typically consists of 9-10 isoprenyl subunits.

Redox States and Function

CoQ₁₀ can exist in three distinct redox states:

• Fully Oxidized (ubiquinone): This form is devoid of electrons.
• Partially Reduced (semiquinone or ubisemiquinone): This intermediate state has acquired one electron.
• Fully Reduced (ubiquinol): The completely reduced form, endowed with two electrons.

The ability of CoQ₁₀ to oscillate between these states, especially the interchange of two electrons between its oxidized (ubiquinone) and reduced (ubiquinol) forms, underpins its functional significance in the body.

Cellular Energy Production

CoQ₁₀ plays an instrumental role in the electron transport chain in mitochondria. By shuttling electrons between complexes, it facilitates the generation of adenosine triphosphate (ATP), the primary energy currency of cells.

Antioxidant Defense

Ubiquinol, the reduced form of CoQ₁₀, acts as a formidable antioxidant, neutralizing harmful reactive oxygen species (ROS) that can damage cellular structures, including lipids, proteins, and DNA. Its antioxidant capabilities are rooted in its ability to donate electrons, converting harmful oxidants to stable, non-reactive species.

Clinical Implications

Given its pivotal roles in energy metabolism and oxidative stress defense, CoQ₁₀ has been investigated for potential therapeutic applications in various clinical conditions. These range from mitochondrial disorders to neurodegenerative diseases, cardiovascular conditions, and even aging.

Characteristics

Ubiquinol is a lipid-soluble benzoquinol that is found in all cellular systems and in nearly every cell, tissue, and organ in mammals. Ubiquinol is acquired through biosynthesis, supplementation, and, in small amounts, diet. Ubiquinol has an established role as an essential component of the electron transport chain transferring electrons resulting in ATP synthesis. In mammals, ATP production takes place predominantly in mitochondria and to a lesser extent in other organelles such as the Golgi apparatus or endoplasmic reticulum. The mitochondria typically produce nearly 95% of the energy required for cellular growth, development, and healthy metabolism. The antioxidant action of ubiquinol is now considered to be one of the most important functions in cellular systems.

Ubiquinol is the antioxidant form of CoQ10 and is essential for mitochondrial synthesis of energy. It is the only known lipid-soluble antioxidant that is endogenously synthesized, protecting biological membranes against lipid peroxidation as well as regenerating other antioxidants such as Vitamin C and Vitamin E.

Because humans can synthesize ubinquinol, it is not classed as a vitamin.

Bioavailability

It is well-established that CoQ₁₀ is not well absorbed into the body, as has been published in many peer-reviewed scientific journals. Since the ubiquinol form has two additional hydrogens, it results in the conversion of two ketone groups into hydroxyl groups on the active portion of the molecule. This causes an increase in the polarity of the CoQ₁₀ molecule and may be a significant factor behind the observed enhanced bioavailability of ubiquinol. Taken orally, ubiquinol exhibits greater bioavailability than ubiquinone.

However, there are authorities that dispute whether ubiquinol is more bioavailable in practice rather than in theory compared to CoQ₁₀ supplements because those have their CoQ₁₀ molecules dissolved in lipid micelles, which then deliver their cargo to the plasma membrane in the intestinal wall. There they dissolve via simple diffusion in the intestinal cells, then onto the lymphatic vessels, and then into the venous system. Since ubiquinol and CoQ₁₀ are redox pairs and can and are rapidly inter-converted in the body, it is not clear that ubiqinol's more hydrophilic nature compared to CoQ₁₀ is of practical significance.

Content in foods

In foods, there are varying amounts of ubiquinol. An analysis of a range of foods found ubiquinol to be present in 66 out of 70 items and accounted for 46% of the total coenzyme Q10 intake. The following chart is a sample of the results.

Food	Ubiquinol (μg/g)	Ubiquinone (μg/g)
Beef (shoulder)	5.36	25
Beef (liver)	40.1	0.4
Pork (shoulder)	25.4	19.6
Pork (thigh)	2.63	11.2
Chicken (breast)	13.8	3.24
Mackerel	0.52	10.1
Tuna (canned)	14.6	0.29
Yellowtail	20.9	12.5
Broccoli	3.83	3.17
Parsley	5.91	1.57
Orange	0.88	0.14

Molecular aspects

Ubiquinol is a benzoquinol and is the reduced product of ubiquinone also called coenzyme Q₁₀. Its tail consists of 10 isoprene units.

Ubiquinol

The reduction of ubiquinone to ubiquinol occurs in Complexes I & II in the electron transfer chain. The Q cycle is a process that occurs in cytochrome b, a component of Complex III in the electron transport chain, and that converts ubiquinol to ubiquinone in a cyclic fashion. When ubiquinol binds to cytochrome b, the pKa of the phenolic group decreases so that the proton ionizes and the phenoxide anion is formed.

If the phenoxide oxygen is oxidized, the semiquinone is formed with the unpaired electron being located on the ring.

Ubiquinol

A page on Proteopedia, Complex III of Electron Transport Chain, contains rotatable 3-D structures of Complex III, which may be used to study the peptide structures of Complex III and the mechanism of the Q cycle.

External links

• ChemSub Online: Ubiquinol

Ubiquinol Resources
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