In metabolism, coenzymes are involved in both group-transfer reactions, for example coenzyme A and adenosine triphosphate (ATP), and redox reactions, such as coenzyme Q10 and nicotinamide adenine dinucleotide (NAD+). Coenzymes are consumed and recycled continuously in metabolism, with one set of enzymes adding a chemical group to the coenzyme and another set removing it. For example, enzymes such as ATP synthase continuously phosphorylate adenosine diphosphate (ADP), converting it into ATP, while enzymes such as kinases dephosphorylate the ATP and convert it back to ADP.
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In metabolism, coenzymes are involved in both group-transfer reactions, for example coenzyme A and adenosine triphosphate (ATP), and redox reactions, such as coenzyme Q10 and nicotinamide adenine dinucleotide (NAD+). Coenzymes are consumed and recycled continuously in metabolism, with one set of enzymes adding a chemical group to the coenzyme and another set removing it. For example, enzymes such as ATP synthase continuously phosphorylate adenosine diphosphate (ADP), converting it into ATP, while enzymes such as kinases dephosphorylate the ATP and convert it back to ADP.
Coenzymes molecules are often vitamins or are made from vitamins. Many coenzymes contain the nucleotide adenosine as part of their structures, such as ATP, coenzyme A and NAD+. This common structure may reflect a common evolutionary origin as part of ribozymes in an ancient RNA world.
Coenzymes as metabolic intermediates
Metabolism involves a vast array of chemical reactions, but most fall under a few basic types of reactions that involve the transfer of functional groups. This common chemistry allows cells to use a small set of metabolic intermediates to carry chemical groups between different reactions. These group-transfer intermediates are the coenzymes.
Each class of group-transfer reaction is carried out by a particular coenzyme, which is the substrate for a set of enzymes that produce it, and a set of enzymes that consume it. An example of this are the dehydrogenases that use nicotinamide adenine dinucleotide (NADH) as a cofactor. Here, hundreds of separate types of enzymes remove electrons from their substrates and reduce NAD+ to NADH. This reduced coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates.
Coenzymes are therefore continuously recycled as part of metabolism. As an example, the total quantity of ATP in the human body is about 0.1 mole. This ATP is constantly being broken down into ADP, and then converted back into ATP. Thus, at any given time, the total amount of ATP + ADP remains fairly constant. The energy used by human cells requires the hydrolysis of 100 to 150 moles of ATP daily which is around 50 to 75 kg. Typically, a human will use up their body weight of ATP over the course of the day.Di Carlo, S. E. and Coliins, H. L. (2001) "Estimating ATP resynthesis during a marathon run: a method to introduce metabolism" Advan. Physiol. Edu. 25: 70-71. This means that each ATP molecule is recycled 1000 to 1500 times daily.
Types
Acting as coenzymes in organisms is the major role of vitamins, although vitamins do have other functions in the body. Coenzymes are also commonly made from nucleotides: such as adenosine triphosphate, the biochemical carrier of phosphate groups, or coenzyme A, the coenzyme that carries acyl groups. Most coenzymes are found in a huge variety of species, and some are universal to all forms of life. An exception to this wide distribution is a group of unique coenzymes that evolved in methanogens, which are restricted to this group of archaea.
