Coenzyme Definition, functions, Types, and Examples

Coenzyme Definition

coenzyme is a non-protein organic compound that binds with an enzyme to catalyze a reaction. Coenzymes are often widely called cofactors, but they are chemically different. A coenzyme cannot work on its own, but it can be reused multiple times when combined with an enzyme.

Coenzyme functions

An enzyme without a coenzyme is called apoenzyme. Without coenzymes or cofactors, enzymes cannot catalyze reactions effectively. The enzyme may not work at all. If the reactions cannot occur at the normal catalyzed rate, then an organism will have difficulty sustaining life.

When an enzyme gains a coenzyme, it becomes a holoenzyme or active enzyme. Active enzymes transform substrates into products that an organism needs to perform essential functions, whether chemical or physiological. Coenzymes, like enzymes, can be reused and recycled without changing reaction rate or effectiveness. They adhere to a portion of the active site in an enzyme, allowing the catalyzed reaction to occur. When an enzyme is denatured by extreme temperature or pH, the coenzyme can no longer bind to the active site.

Coenzyme Definition, functions, Types, and Examples Cofactors

Types of enzymes

Cofactors are molecules that bind to an enzyme during chemical reactions. In general, all the compounds that help enzymes are called cofactors. However, cofactors can be divided into three subgroups according to chemical composition and function:


These are reusable non-protein molecules that contain (organic) carbon. They bind freely to an enzyme at the active site to help catalyze reactions. Most are vitamins, vitamin derivatives, or nucleotide forms.


Unlike coenzymes, true cofactors are reusable, non-carbon (inorganic) non-protein molecules. Cofactors are typically metal ions such as iron, zinc, cobalt, and copper that bind freely to the active site of an enzyme. They must also be supplemented in the diet since most organisms do not naturally synthesize metal ions.

Prosthetic groups

These can be organic vitamins, sugars, lipids, or inorganic metal ions. However, unlike coenzymes or cofactors, these groups bind very tightly or covalently to an enzyme to help catalyze reactions. These groups are often used in cellular respiration and photosynthesis.

Examples of coenzymes

Most organisms cannot naturally produce coenzymes in quantities large enough to be effective. Instead, they are presented to an organism in two ways:

  • Examples of coenzymes are nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), and flavin adenine dinucleotide (FAD). 
  • These three coenzymes are involved in the oxidation of hydrogen transfer. Another is coenzyme A (COA) which is involved in the transfer of acyl groups.


Many, but not all, coenzymes are vitamins or are derived from vitamins. If the vitamin intake is too low, then an organism will not have the necessary coenzymes to catalyze the reactions. The water-soluble vitamins, which include all the B vitamins and vitamin C, lead to the production of coenzymes. Two of the most important and widespread vitamin-derived coenzymes are nicotinamide adenine dinucleotide (NAD) and coenzyme A.

NAD is derived from vitamin B3 and functions as one of a cell’s most important coenzymes when converted to its two alternative forms. When NAD loses an electron, the low-energy coenzyme called NAD + is formed. When NAD gains an electron, a high-energy coenzyme called NADH is formed.

NAD + primarily transfers the electrons necessary for redox reactions, especially those involved in parts of the citric acid cycle (TAC). TAC produces other coenzymes, such as ATP. If an organism is deficient in NAD +, the mitochondria become less functional and provide less energy for cellular functions.

When NAD + gains electrons through a redox reaction, NADH is formed. NADH, often called coenzyme 1, has numerous functions. It is considered the number one coenzyme in the human body because it is necessary for many different things. This coenzyme mainly transports electrons for reactions and produces energy from food. For example, the electron transport chain can only start with electron delivery from NADH. Lack of NADH causes energy deficits in cells, resulting in generalized fatigue. Furthermore, this coenzyme is recognized as the most powerful biological antioxidant to protect cells against harmful or harmful substances.

Coenzyme Definition, functions, Types, and Examples Cofactors

Coenzyme A, also known as acetyl-CoA, is naturally derived from vitamin B5. This coenzyme has several different functions. First, it is responsible for initiating the production of fatty acids within cells. Fatty acids form the phospholipid bilayer that comprises the cell membrane, A facility necessary for life. Coenzyme A also initiates the citric acid cycle, resulting in the production of ATP.


Non-vitamin coenzymes usually aid in chemical transfer to enzymes. They ensure physiological functions, such as blood clots and metabolism, that occur in an organism. These coenzymes can be formed from nucleotides such as adenosine, uracil, guanine, or inosine.

Adenosine triphosphate (ATP) is an example of an essential non-vitamin coenzyme. It is the most widely distributed coenzyme in the human body. It transfers substances and supplies the necessary energy for essential chemical reactions and muscle contractions. To do this, ATP carries both phosphate and energy to different locations within a cell. When phosphate is removed, energy is also released. This process is a consequence of the electron transport chain. Without coenzyme ATP, very little energy would be available at the cellular level and normal life functions may not occur.

Coenzyme Definition, functions, Types, and Examples

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Coenzyme Definition, functions, Types, and Examples

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