Cofactor (biochemistry)
Cofactor (also cofactor ) is an umbrella term for various molecules and groups of molecules that are essential for the function of certain enzymes.
The terms coenzyme, coenzyme and cosubstrate are sometimes used interchangeably, but they are usually defined more closely. The following types of cofactors can be distinguished:
Prosthetic group
When cofactors it always is non-protein substances, cofactors are not composed of amino acids. Likewise, compounds which are ubiquitously present (in particular water), although they are often involved in reactions which do not to the cofactors (and not to the substrates ) calculated.
A complex in which an enzyme is bound to a cofactor is known holoenzyme. The actual enzyme, which is the protein portion of a holoenzyme, is called apoenzyme.
- 2.1 Examples 2.1.1 pyridoxal phosphate
- 2.1.2 coenzyme A
- 2.1.3 ubiquinone
- 3.1 Examples
- 7.1 Notes and references
Prosthetic group
As a prosthetic group ( art by ancient Greek word προστίθημι, preceded by ') is called a fixed to a protein (usually covalently) bound non-protein component having a catalytic effect. Since it is often changed from the catalytic seen, it must be regenerated at the enzyme.
Examples
- Biotin in carboxylases
- Heme in hemoglobin, in cytochrome c, cytochrome c oxidase in
- Flavins in flavoproteins
- Moco ( molybdenum cofactor ) in Molybdoenzymen (eg xanthine oxidase )
- FeMo ( molybdenum-iron cofactor, MoFe protein) in nitrogenase
- 13 -trans -retinal in bacteriorhodopsin
- Vitamin B6 ( pyridoxal phosphate ) as a prosthetic group of aminotransferases and the human histidine decarboxylase
Coenzyme
A coenzyme is a low molecular weight organic molecule that non- covalently binds to the enzyme and thus dissociated after catalysis. During the reaction, it takes functional groups, protons, electrons or energy (in the case of ATP ) or enters it (see also donor-acceptor principle). So it is - as the prosthetic group - changed from the reaction shown and must be rebuilt. This distinguishes the coenzyme, for example, by allosteric effectors. Typically done its regeneration in a subsequent reaction. As the coenzyme acts rather as a substrate as such as an enzyme, it is often referred to more accurately as a co-substrate (or co-substrate ). Some coenzymes are derivatives of vitamins.
Examples
Pyridoxal phosphate
A coenzyme is pyridoxal phosphate at the active site of transaminases and decarboxylases. The first step is catalyzed for example the deamination of amino acids to alpha -keto acids, in the second, the amino group is abstracted to another alpha -keto acid transfer (so-called ping - pong -bi -bi mechanism by W. Wallace Cleland ). Pyridoxal phosphate is the same enzyme is regenerated in this case ( of pyridoxamine ), but in two reaction steps. The same also applies for the decarboxylation reaction, which attracts a hydrolysis of the bound to the enzyme intermediate by itself.
Coenzyme A
Another example is coenzyme A, which is involved in free or acetylated form at various points of the citric acid cycle and in fatty acid metabolism. Likewise, in the citric acid cycle, but also in glycolysis the coenzymes FAD, NAD serve primarily as electron and proton acceptors or donators, thus ensuring the transport of these from one reactant to another. Other coenzymes such as ATP transferred whole groups, such as phosphate residues.
Ubiquinone
Another example is the electron carriers ubiquinone (coenzyme Q). Absorption and release of electrons and protons is carried out in the respiratory chain of mitochondria in different protein complexes.
Complete list of the enzyme by the Commission of the International Union of Biochemistry and Molecular Biology ( IUBMB ) recognized coenzymes / cofactors see in the category: coenzyme / cofactor.
Metal ions
Metalloenzyme is the name for enzymes that contain metal ions. Metals may contribute to the stabilization of the enzyme structure, but also serve as an active site in a catalytic reaction. Some metal may be grouped, their presence suggests a specific function. This means that metalloenzymes homologs in other species with altered exercise or in extreme cases, the absence of metal ion, an analogous function. Reason is often the different availability of the respective metals in the environment of the organisms or a different evolutionary development that has led to similar features. An extreme example is the bacterium Borrelia burgdorferi, which dispenses entirely with iron and manganese instead used as a cofactor in the metabolism, both metals, which have a very similar chemical and structural chemistry. Another example is the replacement of iron by copper proteins, proteins in the activation of oxygen. The presence of zinc cation often indicates the function of the Lewis acid through, for example, peptidases, or in so-called " zinc finger ". The presence of the metal ion is therefore often critical for the enzyme function, the absence of the metal, then a limiting factor.
Examples
- Urease contains nickel.
- Glutamatcysteinligase can manganese, magnesium and copper.
- Acireducton synthase in Klebsiella pneumoniae with iron or magnesium is the reaction product of 4 -methylthio -2- ketobutyrate; with nickel, it is 3 - ( methylthio ) propionate.
- Phenylalanine hydroxylase contains iron.
- Leucyl to zinc, cobalt, magnesium or manganese.
- Lipoxygenase also contains iron.
- Superoxide dismutase containing manganese.
- Xanthine molybdenum and iron.
- Cytochrome c oxidase copper and iron.
Alkaline earth metals such as calcium and magnesium, but also partially zinc, are often responsible for the structure and folding of proteins without catalytic effect. These proteins are called metalloproteins.
Enzyme inhibition
Materials that are similar to the cofactor in its binding properties and are able to complex with the enzyme, are competitive inhibitors, they inhibit the enzyme by blocking the binding site of the cofactor competitive (in " competition ").