n., plural: coenzymes
Definition: small molecule needed by an enzyme to function
In photo: Succinate dehydrogenase complex with cofactors, flavin, iron-sulfur centers, and heme inside the mitochondrion Source: modified by Maria Victoria Gonzaga, from the works of Richard Wheeler, CC BY-SA 3.0.
Enzymes can break down complicated large molecules into simpler smaller ones, they can combine small molecules or atoms to form large metabolites. Therefore, enzymes play an important role in biochemical and cellular organization. Enzymes are similar to catalysts in their chemical ability to speed up reactions without themselves being changed or consumed. These biological reactions include carboxyl group transfer, peptide linkage hydrolysis, breaking carbon bonds, and the conversion of substances to their optical isomers. In these reactions, enzymes may or may not function alone, enzymes may need the assistance of a cofactor. A holoenzyme or an active enzyme is a complex that consists of two parts: the protein part or apoenzyme, and the cofactor part. The protein part or the apoenzyme cannot function alone and needs to be activated by the cofactor. A cofactor could be an activator, which is usually a cation. It may also be an organic molecule of a complicated structure, which is referred to as a coenzyme. The catalytic activity of enzymes mostly depends on the presence of non-protein compounds called coenzymes. Cofactors are highly bound to apoenzymes; therefore, coenzymes cannot be isolated from apoenzymes without denaturation of the enzyme proteins.
Coenzymes play a vital role in several biochemical pathways such as breaking down macronutrients into smaller molecules (Catabolism) or the formation of new biological compounds in the body (Anabolism).
What is a coenzyme? Sometimes, a coenzyme is referred to as a co-substrate because it binds to the enzyme along with the substrate at the beginning of a chemical reaction and they leave the enzyme altered at the end of the reaction. However, they are called coenzymes because they bind to the enzyme before other substrates will. Moreover, coenzymes are reconverted by other enzymes found in the cell to their original form to be reused. A coenzyme is usually a form of activated vitamin that is essential for biochemical pathways. Coenzymes form complexes with enzymes. These complexes convert nutrients into useful forms of energy. They produce biomolecules that are considered to be the basis of our life.
Some nutrients act as cofactors and coenzymes. Others are being broken down by the help of coenzymes. Therefore, it is essential to maintain the dietary intake of trace elements to produce the energy required for life.
Enzymes that require the presence of coenzymes to function will not be able to maintain the normal metabolic processes or to maintain the activity of the natural biochemical processes that keep the normal functions of the cell activated such as cell growth, differentiation, division, and repair.
Additionally, coenzymes function to keep the integrity of some regulatory proteins and hormones structures.
Some vitamins act as coenzymes participating in the biochemical processes such as catabolism, anabolism, and production of energy. Vitamins A and K are two fat-soluble vitamins that act as coenzymes or cofactors, while all the water-soluble enzymes can act as cofactors or coenzymes. In addition to their action as cofactors, vitamins have a critical role in several vital processes such as the production of hormones, the integrity of collagen in bones, blood coagulation, and proper vision.
Examples of Coenzymes
Coenzymes are not specific to substrates, instead, they act as a carrier to the reaction products. Coenzymes are regenerated to be reused. An important example of coenzymes is nicotinamide adenine dinucleotide (NAD) which is used to activate lactic dehydrogenase enzyme.
In the dehydrogenation of pyruvate to lactate, NAD itself is reduced by accepting hydrogen atom for catalytic reactions, whereas some enzymes require the nicotinamide adenine dinucleotide phosphate (NADP) phosphate which is likewise reduced .
For the synthesis of steroids, NADP coenzyme is required. The reduced enzyme is, then, re-oxidized by transferring the introduced hydrogen along a hydrogen acceptors chain to be combined with molecular oxygen forming a water molecule.
NAD+ is the first molecule that binds to the enzyme and it is the last molecule to be unbound from the complex. Therefore, it is the rate-limiting step of the biochemical reaction. As such, it is considered to be a coenzyme not a substrate.
Nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) help enzymes that remove hydrogen (dehydrogenases) to assist in the catabolic process of amino acids, fats, and carbohydrates as well as the enzymes participating in the synthesis of steroids, fats, and other metabolites.
Types of Coenzymes
Some enzymes contain a ‘built-in’ cofactor called prosthetic groups such as flavoproteins and some pyridoxine- and biotin-containing enzymes. Flavoproteins are enzymes that contain metal. They transfer hydrogen atoms to their prosthetic group from their coenzymes, such as the reduced NAD. In such cases, the flavin adenine dinucleotide (FAD), which is a derivative from the riboflavin, acts as a prosthetic group when accepting hydrogen. Then, coenzyme Q re-oxidizes the flavin to proceed in the electron transport chain to produce a water molecule. Biotin has a role in fatty acids synthesis; therefore, it is expected to have a function in the fatty acid-derived hormones, such as prostaglandin.
There are many other examples of coenzymes involved in several biochemical reactions. Another example is the coenzymes that are involved in the removal of carbon dioxide (decarboxylation) from a compound to assist in the breakdown of carbohydrates for the production of energy, such as the active form of vitamin B1, thiamin. Others carry hydrogen to serve in oxidation reactions that produce energy from high energy nutrients. Vitamin B12 coenzyme forms called pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP) act as cofactors for about 120 enzymes, e.g. synthetases, racemases, cleavage enzymes, decarboxylases, and transaminases. PLP and PMP participate in different amino acids metabolic processes.
Coenzyme A is essential for the metabolism of fatty acid, amino acids, carbohydrates, and other biological molecules. It contains pantothenic acid (PA), which is a form of vitamin B. PA also participates in the fatty acid synthesis as an acyl-carrier protein cofactor. Vitamin B12 coenzyme forms participate in the synthesis of methionine (amino acid).
Biocytin is the coenzyme of biotin. It assists in several carboxylation reactions of fatty acids and amino acids to facilitate their metabolism. Furthermore, biocytin has a role in the formation of urea. The coenzyme form of folate carries one-carbon unit that is required for the conversion of amino acid to pyrimidine and purine bases required for the formation of DNA and RNA.
Ascorbic acid is a cofactor of hydroxylases. They hydroxylase lysine and proline to keep the integrity of collagen structure; moreover, they hydroxylase cholesterols for the formation of bile acids, as well as the tyrosine hydroxylation to form the hormone noradrenaline.
Vitamin A aldehyde form, retinol, serves as a cofactor for apoproteins found in the eye. Apoproteins are responsible for the vision in dim light. They are also involved in the bright light and color vision in the retina.
Table 1: Vitamins as examples of coenzymes.
|Water-Soluble Vitamins||Coenzyme||Coenzyme Function|
|thiamine pyrophosphate||decarboxylation reactions|
|flavin mononucleotide or flavin adenine dinucleotide||oxidation-reduction reactions involving two hydrogen atoms|
|nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate||oxidation-reduction reactions involving the hydride ion (H−)|
|pyridoxal phosphate||variety of reactions including the transfer of amino groups|
|methylcobalamin or deoxyadenoxylcobalamin||intramolecular rearrangement reactions|
|folic acid||tetrahydrofolate||carrier of one-carbon units such as the formyl group|
|pantothenic Acid||coenzyme A||carrier of acyl groups|
|vitamin C (ascorbic acid)||none||antioxidant; formation of collagen, a protein found in tendons, ligaments, and bone|
Functions of Coenzymes
Minerals and vitamins play an important role in the anabolic and catabolic pathways that lead to the synthesis of biomolecules such as lipids, nucleic acids, proteins, and carbohydrates as coenzymes or cofactors.
- Vitamins as coenzymes: Vitamin A metabolite form, retinoic acid, functions as genes regulator, therefore, it is very important for the normal development of cells. Vitamin K is a coenzyme for enzymes that move —CO2 groups (g-carboxylases). The released carboxylic group binds to calcium, this step is important for the formation of osteocalcin, an important protein for bone remodelling. Additionally, it is important for the formation of prothrombin, which plays a crucial role in blood coagulation.
- Minerals as cofactors and catalysts: Minerals can function in biological processes as cofactors and catalysts. When minerals act as catalysts they do not integrate with an enzyme or its substrate. However, they accelerate the biochemical reaction between the enzyme and its substrate. Alternatively, when minerals act as cofactors, they become a part of the enzyme or protein structure that is essential for the biochemical reaction to proceed. Minerals that act as cofactors include manganese, selenium, magnesium, and molybdenum. Some minerals, such as cobalt, iodine, calcium, and phosphorus, act as cofactors for certain non-enzymatic proteins. Others, like copper, zinc, and iron, act as cofactors for both non-enzymatic and enzymatic proteins.
In normal conditions, the rate of reaction is directly proportional to the enzyme concentration. Therefore, the high concentration of the substrate and the enzyme results in a high rate of product turnover, similar to catalyzed chemical reactions, enzymatic reactions are reversible. However, in the normal conditions, enzymatic reactions proceed in one way only since the products are regularly consumed by the following enzyme in the pathway of the biochemical reactions. In the case of vitamin deficiencies, coenzymes required for biochemical reactions are missing, therefore, the products of reactions buildup in the body and may lead to the reversal of the reaction.
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