Coenzymes
A Coenzyme' (cofactor) is a low molecular weight non-protein structure attached to the protein chain of an enzyme. Coenzymes play an important role in the transfer of hydrogen atoms, electrons, or groups of atoms during an enzyme-catalyzed reaction. According to the method of binding to the protein part of the enzyme, we distinguish:
- dissociable coenzymes
- prosthetic groups
1. Formation of complex 1
2. Regeneration of substrate + transfer of x to coenzyme
3. Formation of complex 2
4. Regeneration of coenzyme + formation of product
Dissociable coenzymes[edit | edit source]
They are kept in contact with the enzyme by a non-covalent interaction. They can easily separate from the original enzyme molecule and bind to another. These include, for example, NAD+ or NADP+.
Prosthetic groups[edit | edit source]
They are structures firmly bound to the enzyme mainly covalent bonds. The character of this bond makes the prosthetic group a stable part of the enzyme. Examples include FMN, FAD and lipoic acid.
Action of coenzymes[edit | edit source]
It is conditioned by two reactions. In the course of these two reactions, on the one hand, reaction catalysis takes place, on the other hand, simultaneous regeneration of the coenzyme. As mentioned above, we distinguish two types of coenzymes.
The two types of coenzymes differ in the way they are regenerated.
Catalysis and regeneration of dissociable coenzymes[edit | edit source]
First, dissociable coenzyme K, bound to apoenzyme E1, accepts the transferred group X from the substrate S1. Next, the K−X complex binds to the second apoenzyme E2 and transfers the transferred X group in this form to the S2 product. At the same time, coenzyme K is regenerated
Catalysis and regeneration of prosthetic groups[edit | edit source]
The course of the reaction with prosthetic groups is different due to their strong binding to the apoenzyme. The main difference is that only one apoenzyme participates in the catalysis of both reactions.
Functions of coenzymes[edit | edit source]
Coenzymes are important for the functions of enzymes that catalyze oxidation-reduction events (oxidoreductases), or enzymes catalyzing the transfer of groups (transferases).
Coenzymes of oxidoreductases[edit | edit source]
For more information see Redox Enzymes.These coenzymes are involved in the transfer of a hydrogen atom or electrons.
| Coenzyme | Note |
|---|---|
| NAD+ | transfer of reducing equivalents (hydride ions: 2 e-, 1 H+) from catabolic events to the respiratory chain |
| NADPH | reducing agent of biosynthetic processes, is formed mainly in the pentose cycle |
| FMN | transfer of reducing equivalents (2 e-, 2 H+) using nitrogen atoms |
| FAD | transfer of reducing equivalents (2 e-, 2 H+) using nitrogen atoms |
| Coenzyme Q (ubiquinone) | part of the mitochondrial respiratory chain |
| Lipoic acid | bound by an amide bond to the side chain of lysine (referred to as liponamide), contains an intramolecular disulfide bond that has an oxidoreductive effect and changes to a dithiol upon reduction |
| Hem | only transfers electrons, e.g. mitochondrial cytochromes of the respiratory chain, cytochrome P450 |
| Glutathione | antioxidant in erythrocytes, contains a redox-active intramolecular disulfide bond - similar to lipoic acid |
| [[Vitamin C|Template:L-ascorbic acid]] | coenzyme of monoxygenases and dioxygenases, participation in the hydroxylation of proline and lysine residues in the synthesis of collagen, synthesis of catecholamines and bile acids |
| Tetrahydrobiopterin (BH4, THB) | coenzyme involved in the hydroxylation of tyrosineu, phenylalanineu, tryptophanu, etc. |
Transferase coenzymes[edit | edit source]
These coenzymes allow the transfer of groups.
| Coenzyme | Note |
|---|---|
| Nucleoside triphosphates: ATP, GTP, UTP, CTP | they mostly carry the rest of phosphoric acid, part of kinases |
| Coenzyme A | contains a -SH group, to which a carboxylic acid residue can be attached via a thioester bond, e.g. acetyl coenzyme A |
| Tetrahydrofolate (THF) | transfers one-carbon residues (methyl, formyl, methylene) |
| Pyridoxal Phosphate (PLP) | coenzyme important for metabolism AMK (transamination, decarboxylation) |
| Phosphoadenosine phosphosulfate (PAPS) | carries sulfate |
| S-adenosylmethionine (SAM) | transfers methyl e.g. during conversion of noradrenaline to adrenaline or during creatine synthesis, DNA methylation |
| Cobalamins | coenzymes of methyltransferases – e.g. methylation of homocysteine to methionine |
Coenzymes of carboxylation and decarboxylation reactions[edit | edit source]
| Coenzyme | Note |
|---|---|
| Pyridoxal Phosphate (PLP) | coenzyme important for metabolism AMK (transamination, decarboxylation) |
| Lipoic acid | bound by an amide bond to the lysine side chain of the respective apoenzyme → liponamide, which participates in the oxidative decarboxylation of 2-oxoacids |
| Thiamine diphosphate (TPP) | coenzyme of oxidative decarboxylation of 2-oxoacids and transketolases |
| Biotin | coenzyme of all carboxylases, reacts with bicarbonate (HCO3-) to form carboxybiotin, which transfers CO2 to other molecules and thus creates carboxyl groups in them (COOH) – an example can be the synthesis of malonyl-CoA from acetyl-CoA or oxaloacetate from pyruvate |
Links[edit | edit source]
Related Articles[edit | edit source]
References[edit | edit source]
- MATOUŠ, Bohuslav, et al. Basics of medical chemistry and biochemistry. 1. edition. Prague : Galen, 2010. 540 pp. ISBN 978-80-7262-702-8.
- KOOLMAN, Jan – RÖHM, Klaus-Heinrich. Color Atlas of Biochemistry. 1. edition. Prague : Grada, 2012. 512 pp. ISBN 978-80-247-2977-0.
- MASOPUSTOVÁ, Nina, NÁDĚJOVÁ, Barbora. Coenzymes.
- Wikipedia: the free encyclopedia [online]
