Coenzyme Q₁₀

Description

Coenzyme Q₁₀ is a fat-soluble nutrient also known as CoQ₁₀, vitamin Q₁₀, ubidecarenone, or ubiquinone. It is a natural product of the human body that is primarily found in the mitochondria, which are the cellular organelles that produce energy. It occurs in most tissues of the human body; however, the highest concentrations are found in the heart, liver, kidneys, and pancreas. Ubiquinone takes its name from a combination of the word ubiquitous, meaning something that is found everywhere, and quinone 10. Quinones are substances found in all plants and animals. The variety found in humans has a 10-unit side chain in its molecular structure. Apart from the important process that provides energy, CoQ₁₀ also stabilizes cell membranes and acts as an antioxidant. In this capacity, it destroys free radicals, which are unstable molecules that can damage normal cells.

General use

CoQ₁₀ is used extensively in Canada, Western Europe, Japan, and Russia to treat congestive heart failure. It is available as a prescription medication almost everywhere it is sold, although it is sold over-the-counter as a nutritional supplement in the United States. Some studies have shown it to be effective for as many as 70% of patients with congestive heart failure. It appears to improve patient health and wellbeing, and to increase cardiac efficiency. The dosage generally recommended for this condition is 100–300 mg a day, preferably in divided doses. According to Dr. Karl Folkers in Prevention's Healing with Vitamins, it takes one to three months to achieve desired results from supplementation, and as long as six months to attain maximum benefit.

CoQ₁₀ may also help people with some forms of cardiomyopathy. Patients should consult their physician about the possible benefits of supplementation for this condition.

The usefulness of CoQ₁₀ in lowering blood pressure is not well documented. One study suggests that the supplement is helpful for hypertension , but the results are in question as it was not a double-blind, controlled research project. The dose recommended is 200–250 mg a day, with results taking several months to appear. It is possible that some patients with essential hypertension who are initially low in CoQ₁₀ may eventually be able to decrease the amount of their other blood pressure medications. This must be done under the care of a health care provider.

Oral supplementation of CoQ₁₀ has been shown to improve periodontal disease, as it decreases the size of abnormally deep pockets in the gums, and also reduces the extent of bacterial contamination. Other possible benefits of CoQ₁₀ are to decrease angina symptoms, improve immune function in patients with AIDS and other immune deficiencies, improve control of blood sugar, lower cholesterol , improve physical stamina, and help people with muscular dystrophy and Huntington's disease. A group of researchers at the University of California at San Diego reported in 2002 that coenzyme Q₁₀ appears to slow the progress of Parkinson's disease , Friedreich's ataxia, and other conditions marked by degeneration of the central nervous system. The supplement can also reduce the toxicity of some types of chemotherapy. Doxorubicin, a chemotherapeutic agent, is known to sometimes damage the heart. Concomitant supplementation seems to reduce this toxic effect. The possible benefits of CoQ₁₀ should be discussed with a nutritionally-oriented health care provider.

Since 1961, when it was first noticed that cancer patients in Sweden and the United States had low levels of the enzyme, coenzyme Q₁₀ has been studied as a possible cancer treatment. Researchers believe that coenzyme Q₁₀ may protect against cancer by stimulating the immune system, and functioning as an antioxidant. Although animal studies have been conducted, as of early 2004 no report of a randomized clinical trial involving human subjects whose survival times were lengthened by using coenzyme Q₁₀ in addition to a traditional cancer treatment has been reported in a peer-reviewed medical journal.

Deficiency

Patients with certain conditions tend to have lower levels of CoQ₁₀, and may benefit from supplements. Some diseases that are associated with decreased amounts of this nutrient are AIDS, chronic fatigue , congestive heart failure, cardiomyopathy, and inflammatory gum disease . Levels of CoQ₁₀ tend to decrease with age; tests for its presence in the body are not widely available. Adverse effects from this supplement are rare and mild, so anyone suffering from one of the listed conditions should consider discussing supplementation with a health care provider.

Preparations

Natural sources

Food products are a good source of CoQ₁₀, and provide approximately half of the body's requirement. Cold-water fish such as mackerel, salmon, sardines, and tuna are particularly high in CoQ₁₀. Vegetable oils and meats also provide good sources. The liver manufactures adequate amounts to fulfill the need not met in the diet. People who are deficient in B vitamins, selenium, vitamin C , and vitamin E may not be able to make as much CoQ₁₀ as they need because these nutrients are required for production. Consumption of foods rich in CoQ₁₀ and production of the nutrient in the liver will not provide the amounts needed to treat heart failure and other conditions that may contribute to a deficiency of this nutrient. In those cases, supplements are required.

Supplemental sources

Supplements of CoQ₁₀ are widely available; however, its cost varies considerably. As of 2004, it is available in the United States, ranging in price from $7.79 for a bottle of 40 30-mg capsules to $38.95 for a bottle of 60 100-mg capsules. It is found in various forms including capsules, gelcaps, liquids, and tablets. The latter may be the best choice, as this form generally includes a source of fat that improves absorption. Vitamin E is a helpful stabilizing additive as well. Most of the CoQ₁₀ products currently available on the market are manufactured in Japan. Like other supplements, Co₁₀ is best kept in a cool, dry place, out of direct light, and out of the reach of children.

Precautions

As of 2004, the safety of CoQ₁₀ for pregnant or breast-feeding women has not been established, and its use is not recommended under these conditions. It is also not recommended for young children. People diagnosed with heart failure, diabetes, kidney problems, or liver disease should use particular care with this supplement, as the dosage of other medications may require adjustment. These individuals should consult a physician before taking coenzyme Q₁₀.

Side effects

Reported adverse effects related to supplemental CoQ₁₀ use include diarrhea , irritation of the stomach, poor appetite, and nausea . These effects are rarely reported and are mild. CoQ₁₀ is considered extremely safe for most people. If doses over 300 mg per day are taken, liver enzyme levels may be affected, and may need monitoring.

Interactions

It is possible that CoQ₁₀ decreases the action of sodium warfarin (known by the brand name, Coumadin), which is prescribed to prevent the formation of blood clots in patients at risk of heart attack or stroke . Some oral diabetes medications may also interfere with the action of CoQ₁₀. Cholesterol-lowering drugs in the statin group may have this effect as well.

Resources

BOOKS

Bratman, Steven, and David Kroll. Natural Health Bible. Roseville, CA: Prima Publishing, 2000.

Griffith, H. Winter. Vitamins, Herbs, Minerals & Supplements: The Complete Guide. Tucson, AZ: Fisher Books, 2000.

Pressman, Alan H., and Sheila Buff. The Complete Idiot's Guide to Vitamins and Minerals., 2nd ed. Indianapolis: Macmillan General Reference, 2000.

Therapeutic Research Faculty Staff. Natural Medicines Comprehensive Database. Stockton, CA: Therapeutic Research Faculty, 1999.

PERIODICALS

Baker, S. K., and M. A. Tarnopolsky. "Targeting Cellular Energy Production in Neurological Disorders." Expert Opinion on Investigational Drugs 12 (October 2003): 1655–79.

Naini, A., V., J. Lewis, M. Hirano, and S. DiMauro. "Primary Coenzyme Q₁₀ and the Brain." Biofactors 18 (2003): 145–52.

Shults, C. W. "Coenzyme Q₁₀ in Neurodegenerative Diseases." Current Medical Chemistry 10 (October 2003): 1917–21.

Shults, C. W., D. Oakes, K. Kieburtz, et al. "Effects of Coenzyme Q₁₀ in Early Parkinson's Disease: Evidence of Slowing of the Functional Decline." Archives of Neurology 59 (October 2002): 1541–50.

Vedanarayanan, V. V. "Mitochondrial Disorders and Ataxia." Seminars in Pediatric Neurology 10 (September 2003): 200–9.

ORGANIZATIONS

Food and Drug Administration (FDA). 5600 Fishers Lane, Rockville, MD 20857. (888) 463-6332. <http://www.fda.gov>.

National Cancer Institute (NCI). <http://www.nci.nih.gov>.

National Center for Complementary and Alternative Medicine (NCCAM) Clearinghouse. P. O. Box 7923, Gaithersburg, MD 20898-7923. (888) 644-6226. Fax: (866) 464-3615. <http://nccam.nih.gov>.

OTHER

National Cancer Institute (NCI). Complementary and Alternative Medicine (CAM) Information Summary: Coenzyme Q₁₀. Bethesda, MD: NCI, 2003. [cited June 3, 2004]. <http:/www.nci.nih.gov/cancerinfo/pdq/cam/coenzymeQ10>.

National Institute of Neurological Disorders and Stroke (NINDS). "Study Suggests Coenzyme Q₁₀ Slows Functional Decline in Parkinson's Disease." NINDS press release, 14 October 2002. [cited June 3, 2004]. <http://www.ninds.nih.gov/news_and_events/pressrelease_parkinsons_coenzymeq10_10140>.

Rebecca J. Frey, PhD

coenzyme , any one of a group of relatively small organic molecules required for the catalytic function of certain enzymes . A coenzyme may either be attached by covalent bonds to a particular enzyme or exist freely in solution, but in either case it participates intimately in the chemical reactions catalyzed by the enzyme. Often a coenzyme is structurally altered in the course of these reactions, but it is always restored to its original form in subsequent reactions catalyzed by other enzyme systems.

Adenosine triphosphate (ATP) is a coenzyme of vast importance in the transfer of chemical energy derived from biochemical oxidations. Other nucleotides (formed from uracil , cytosine , guanine , and inosine) have also been found to act as coenzymes. For example, uridine triphosphate—a derivative of uracil—has been demonstrated to be of great importance in the metabolism of carbohydrates, as in the biosynthesis of glycogen and sucrose.

Those coenzymes that have been found to be necessary in the diet are vitamins . One such compound, biotin, is a member of the B complex; it was first isolated in 1935 from dried egg yolk, and its structure was established in 1942. Biotin is usually found attached to a lysine residue in certain enzymes, where it participates in reactions involving the transfer of carboxyl (-COOH) groups; one such reaction is essential for the synthesis of fatty acids.

Another group of coenzymes is the cobalamin family; one member, cyanocobalamin (vitamin B ₁₂ ) is known to be essential in the diet, although its role in metabolism remains obscure. Closely related cobalamins seem to be involved in the biosynthesis of methionine and methane. The complicated cyanocobalamin molecule was reported in 1973 to have been synthesized; it was first isolated from liver some 25 years prior to that date.

Coenzyme A has been shown to participate in a variety of biochemical reactions, all involving acyl groups such as the acetyl unit; it is, for instance, associated with the pivotal first step of the Krebs cycle , in which an acetyl unit (the breakdown product of carbohydrates) is introduced into the cycle to be converted eventually into carbon dioxide, water, and chemical energy. Coenzyme A is derived from adenine, ribose, and pantothenic acid (a vitamin of the B complex).

The two flavin coenzymes, riboflavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), occur universally in living organisms and play important roles in biochemical oxidations and reductions. They are usually found tightly bound to certain enzymes (flavoproteins) and are derived from riboflavin (vitamin B ₂ ).

Glutathione, a tripeptide consisting of residues of glutamic acid, cysteine, and glycine, is known to act as a coenzyme in a few enzymatic reactions, but its importance may lie in its role as a nonspecific reducing agent within the cell. It is hypothesized that glutathione serves to maintain the biological activity of certain proteins by keeping selected cysteine sidechains in the reduced thiol form, thereby not allowing these residues to oxidize and cross-link with one another to form cystine residues. (Unnecessary cross-links often result in distortions of protein structure.)

Heme, a complicated molecule containing iron in the ferrous state, serves as a coenzyme in a variety of biochemical processes. It forms an essential part of the structure of hemoglobin and participates intimately in the uptake and release of oxygen by this protein. (In this case the use of the word coenzyme may be inappropriate in that often hemoglobin is not considered to be an enzyme, since it does not catalyze a chemical reaction.) Heme is an important part of the cytochromes , enzymes that catalyze the biochemical oxidations and reductions involved in the production of chemical energy in the form of ATP; heme is also associated with the various enzymes that catalyze the cleavage of peroxides.

Lipoic acid seems to be involved in the removal of carboxyl groups from α-keto acids and in the transfer of the remaining acyl groups to various acceptors. Lipoic acid in fact transfers the acetyl group of pyruvic acid to coenzyme A. Like biotin, lipoic acid is commonly found attached to lysine residues within certain enzymes. It was first reported to have been purified and isolated in crystalline form in 1953.

The nicotinamide nucleotides were the first coenzymes to be detected (1904) in extracts of a living organism. Nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are derived from adenine, ribose, and nicotinic acid or niacin (a vitamin of the B complex) and are important intermediates in the biochemical oxidations and reductions that provide chemical energy within the cell. Both NAD and NADP can be reduced by accepting a hydride ion (H ^- , a proton with two electrons) from an appropriate donor; the resulting NADH and NADPH can then be oxidized back to their original states by transferring their hydride ions to various acceptors. In this fashion electron pairs (and protons) are shuttled about in the cell from high-energy donors to lower-energy acceptors. As a general rule, NADPH donates its hydride ions to biosynthetic processes, such as the fixing of carbon dioxide to make carbohydrates during the dark reaction of photosynthesis. NADH, on the other hand, donates its hydride ions to systems such as the cytochromes, which eventually donate them to oxygen to make (with the addition of a proton) water, producing chemical energy in the form of ATP as a byproduct; the process is not yet completely understood.

Pyridoxal phosphate is a coenzyme that is essential for many enzymatic reactions, almost all of which are associated with amino acid metabolism. It is, for example, involved in the synthesis of tryptophan, a derivative of pyridoxine (another vitamin of the B complex).

The coenzyme tetrahydrofolic acid is derived in humans from the B-complex vitamin folic acid. This coenzyme and its close relatives participate in the transfer of various carbon fragments from one molecule to another; they are, for instance, involved in the synthesis of methionine and thymine.

Thiamine pyrophosphate is derived from another B-complex vitamin, thiamine. This coenzyme often plays a role in the removal of carboxyl (-COOH) groups from organic acids, releasing the carbon and oxygen atoms as carbon dioxide (CO ₂ ). This coenzyme, for example, helps to remove a carboxyl group from pyruvic acid, leaving behind an acetyl group, which it donates to lipoic acid; the lipoic acid then transfers the acetyl group to coenzyme A, which finally inserts it into the beginning of the Krebs cycle. This important three-step enzymatic process requires the participation of three coenzymes; hundreds of other biochemical reactions require coenzymes as well, and this serves to explain the great significance of those molecules in the functioning of living organisms. In the case of human beings, it also serves to explain the importance of proper dietary intake of vitamins, which provide the only source of certain "building blocks" for several of these coenzymes.

Coenzyme

Coenzymes are small organic molecules that link to enzymes and whose presence is essential to the activity of those enzymes. Coenzymes belong to the larger group called cofactors, which also includes metal ions; cofactor is the more general term for small molecules required for the activity of their associated enzymes. The relationship between these two terms is as follows

I. Cofactors

• Essential ions
• Loosely bound (forming metal-activated enzymes)
• Tightly bound (forming metalloenzymes
• Coenzymes
• Tightly bound prosthetic groups
• 2 Loosely bound cosubstrates

Many coenzymes are derived from vitamins . Table 1 lists vitamins, the coenzymes derived from them, the type of reactions in which they participate, and the class of coenzyme.

Prosthetic groups are tightly bound to enzymes and participate in the catalytic cycles of enzymes. Like any catalyst , an enzyme–prosthetic group complex undergoes changes during the reaction, but before it can catalyze another reaction, it must return to its original state.

Flavin adenine dinucleotide (FAD) is a prosthetic group that participates in several intracellular oxidation -reduction reactions. During the catalytic cycle of the enzyme succinate dehydrogenase, FAD accepts two electrons from succinate, yielding fumarate as a product. Because FAD is tightly bound to the enzyme, the reaction is sometimes shown this way

succinate + E–FAD → fumarate + E–FADH₂

where E–FAD stands for the enzyme tightly bound to the FAD prosthetic group. In this reaction the coenzyme FAD is reduced to FADH₂ and remains tightly bound to the enzyme throughout. Before the enzyme can catalyze the oxidation of another succinate molecule, the two electrons now belonging to E–FADH₂ must be transferred to another electron acceptor, ubiquinone. The regenerated E–FAD complex can then oxidize another succinate molecule.

Cosubstrates are loosely bound coenzymes that are required in stoichiometric amounts by enzymes. The molecule nicotinamide adenine dinucleotide (NAD) acts as a cosubstrate in the oxidation-reduction reaction that is catalyzed by malate dehydrogenase, one of the enzymes of the citric acid cycle.

malate + NAD⁺ → oxaloacetate + NADH

In this reaction, malate and NAD⁺ diffuse into the active site of malate dehydrogenase. Here NAD⁺ accepts two electrons from malate; oxaloacetate and NADH then diffuse out of the active site. The reduced NADH must then be returned to its NAD⁺ form. For each catalytic cycle, a "new" NAD⁺ molecule is needed if the reaction is to occur; thus, stoichiometric quantities of the cosubstrate are needed. The reduced form of this coenzyme (NADH) is converted back to the oxidized form (NAD⁺) via a number of simultaneously occurring processes in the cell, and the regenerated NAD⁺ can then participate in another round of catalysis.

Coenzymes, then, are a type of cofactor. They are small organic molecules that bind tightly (prosthetic groups) or loosely (cosubstrates) to enzymes as they participate in catalysis.

see also Active Site; Cofactor; Enzymes.

Paul A. Craig

Bibliography

Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: W. H. Freeman.

Horton, H. Robert; Moran, Laurence A.; Ochs, Raymond S.; Rawn, David J.; and Scrimgeour, K. Gray (2002). Principles of Biochemistry, 3rd edition. Upper Saddle River, NJ: Prentice Hall.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte (1999). Fundamentals of Biochemistry. New York: Wiley.

coenzymes Organic compounds required for the activity of some enzymes; most are derived from vitamins. Coenzymes that remain tightly bound to the enzyme at all times are sometimes known as prosthetic groups; non‐protein components of the enzyme molecule. Other coenzymes act to transfer groups from one enzyme to another, e.g. coenzyme A transfers acetyl groups between enzymes, NAD transfers hydrogen between enzymes in oxidation and reduction reactions.

An enzyme that requires a tightly bound coenzyme is inactive in the absence of its coenzyme; this can be exploited to assess vitamin B1, B2, and B6 nutritional status, by measuring the activity of enzymes that require coenzymes derived from these vitamins (see enzyme activation Assays).

Coenzyme A (CoA) is derived from the vitamin pantothenic acid; it is required for the transfer and metabolism of acetyl groups (and other fatty acyl groups). Coenzyme Q is ubiquinone.

succinyl coenzyme A An intermediate compound in the conversion of alpha-ketoglutaric acid to succinic acid during the citric-acid cycle, a process linked to the formation of the energy-rich guanosine triphosphate (GTP, see GUANOSINE PHOSPHATE). Succinate is not always the final product as the succinyl coenzyme A may be employed for acylation reactions or to initiate porphyrin synthesis.

coenzyme A (CoA) A complex organic compound that acts in conjunction with enzymes involved in various biochemical reactions, notably the oxidation of pyruvate via the Krebs cycle and fatty-acid oxidation and synthesis (see acetyl coenzyme A). It comprises principally the B vitamin pantothenic acid, the nucleotide adenine, and a ribose–phosphate group (see formula).

succinyl coenzyme A An intermediate compound in the conversion of alpha-ketoglutaric acid to succinic acid during the citric acid cycle, a process linked to the formation of the energy-rich guanosine triphosphate (GTP). Succinate is not always the final product, as the succinyl coenzyme A may be employed for acylation reactions or to initiate porphyrin synthesis.

coenzyme An organic nonprotein molecule that associates with an enzyme molecule in catalysing biochemical reactions. Coenzymes usually participate in the substrate–enzyme interaction by donating or accepting certain chemical groups. Many vitamins are precursors of coenzymes. See also cofactor.

co·en·zyme / kōˈenˌzīm/ • n. Biochem. a nonprotein compound that is necessary for the functioning of an enzyme.

coenzyme (koh-en-zym) n. a nonprotein organic compound that, in the presence of an enzyme, plays an essential role in the reaction that is catalysed by the enzyme.

coenzyme A non-protein, organic substance that acts as a cofactor for an enzyme.

coenzyme An organic substance that acts as a cofactor for an enzyme.

coenzyme Q See ubiquinone.