Coenzymes are a variety of small-molecule organic, free-diffusion and non-protein molecules. They are loosely related to the enzyme activity and are essential to it. Coenzymes are sometimes called co-substrates. These molecules are enzyme substrates and do not form a permanent part of the enzyme structure. Most coenzymes are vitamins or are derived from vitamins. Although vitamins do have other functions in the human body, acting as a coenzyme in the body is the main role of vitamins.

Cofactors are present in ~30% of mature proteins. They are frequently incorporated into an enzyme as it is folded and are involved in the enzyme’s catalytic activity. Magnesium is an essential cofactor for over 300 enzymes in the human body, including DNA polymerase. In this case, the magnesium ion aids in the formation of the phosphodiester bond on the DNA backbone. Iron, copper, cobalt, and manganese are other common cofactors.

Many vitamins are coenzymes, as they are nonprotein, organic helper molecules for enzymes. For example, biotin—a type of B vitamin—is important in a variety of enzymes that transfer carbon dioxide from one molecule to another. Biotin, vitamin A and other vitamins must be ingested in our diet, as they cannot be made by human cells.

Vitamins are essential compounds that must be acquired through the diet because the body can't synthesize them. One of the reasons vitamins are needed is because they play an indirect role in catalysis, in which enzymes speed up chemical reactions. However, most vitamins can't help enzymes on their own. In order to participate in catalytic reactions, most vitamins have to change into coenzymes that are small "co-pilot" molecules that pair up with enzymes. These coenzymes are extremely useful because they stay the same after catalysis, so they're recycled and reused multiple times.

Most vitamins have to be converted into coenzymes before they can pair up with enzymes. These changes add small functional groups like phosphates to the vitamin structure, or they involve reduction-oxidation, or redox, reactions where electrons are either added or removed. For example, Vitamin B2 has to grab and bind to a phosphate group, PO3-, to form the coenzyme FMN. Folate is a vitamin that goes through a redox reaction and reduces two of its bonds by gaining electrons and it gets four hydrogens to form the coenzyme THF.

Coenzymes help enzymes by transferring electrons in redox reactions, or adding functional groups to substrates, which are converted into the final product by the enzyme. The functional groups that coenzymes add to the substrate are relatively small: the coenzyme PLP adds an amine group, -NH2, for example. Coenzymes also perform redox reactions. They either take electrons from the substrate or give electrons to it. These reactions are reversible and depend on the concentrations of both oxidized and reduced forms of the coenzyme. The more oxidized coenzymes are, the more reduction there will be, and vice versa.

Coenzymes carry out fairly simple chemical reactions, but these reactions have a major impact on metabolic functions. Vitamin K prevents blood clotting by speeding up the synthesis of gamma-carboxyglutamate, a molecule that binds to free-floating calcium ions. There is a lot less calcium buildup in the arteries, and a lower risk of heart disease. Energy is also stored in coenzymes during cellular respiration, during which cells obtain energy from breaking down food. This energy is released later on by oxidizing the stored coenzymes.

One of the primary characteristics of a coenzyme is that it isn't permanently changed by catalysis. Any changes in the coenzyme's structure are reversed before it's recycled. Coenzymes that participate in redox reactions, like FAD and NAD+, are converted back to their previous form by losing electrons. Not all coenzymes are changed back this quickly, especially coenzymes that transfer functional groups. For example, THF binds to a CH2 group and is converted to DHF after the reaction is finished. DHF is reduced to THF and the enzyme is reused.

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