Peroxidases

Peroxidases
Identifier
Gene name (s)	CAT , DUOX1 , DUOX2 , MPO , TPO , GPX1 , GPX2 , GPX3 , GPX4 , GPX5 , GPX6 , GPX7 , GPX8 , EPX , LPO , PXDN , PXDNL
Enzyme classification
EC, category	1.11.1.- ,  oxidoreductase
Response type	Redox reaction
Substrate	Peroxide + red. Donor
Products	Water / alcohols + ox. Donor
Occurrence
Parent taxon	Creature

Peroxidases are enzymes that catalyze the reduction of peroxides (mostly hydrogen peroxide ) . The reaction is mainly used for removal of toxic peroxides, but also for the oxidation of iodide in the synthesis of thyroid hormone thyroxine in man. The reverse reaction of catalase , on the other hand, is used in some peroxidases to generate hydrogen peroxide. Peroxidases can be found in all living things; 17 such enzymes have been discovered in humans, five of which are known to have hereditary enzyme deficiency diseases (DUOX2, MPO, TPO, EPX, CAT).

Various electron donors (added in brackets) serve as reducing agents , according to which the further classification is made: catalase (a second molecule of hydrogen peroxide), cytochrome c peroxidase (red.Cytochrome c ), thyroid peroxidase ( iodide ), glutathione peroxidase ( glutathione ), which is only in Ascorbate peroxidase occurring in plants and the manganese peroxidase ( manganese (II)) and ligninase ( lignin- degrading enzyme) occurring in fungi .

If hydrogen peroxide is to be produced, on the one hand the reversal of the catalase function and on the other hand the reduction of oxygen with NADPH ( dual oxidase ) is used.

Catalyzed reactions

${\displaystyle \mathrm {H_{2}O_{2}+(2e^{-}+2H^{+})\longrightarrow 2\ H_{2}O} }$

Reduction of hydrogen peroxide

${\displaystyle \mathrm {2\ H_{2}O_{2}\rightleftharpoons O_{2}+2\ H_{2}O} }$

Reduction of hydrogen peroxide. Specifically: catalase

${\displaystyle \mathrm {ROOR'+(2e^{-}+2H^{+})\longrightarrow ROH+R'OH} }$

Reduction of organic peroxides

${\displaystyle \mathrm {O_{2}+NADPH+H^{+}\longrightarrow H_{2}O_{2}+NADP^{+}} }$

Generation of hydrogen peroxide with the dual peroxidase

The substrate specificity of the peroxidases is very different. For example, in cytochrome c peroxidase, the substances that the electrons transfer are very specific. The horseradish peroxidase (Engl. Horseradish peroxidase, HRP), however, a variety of donors and acceptors use. Therefore, HRP is also used as a marker enzyme in certain molecular biology techniques for the detection of proteins and nucleic acids (e.g. Western blot , Southern blot ). Peroxidases are also used as histological markers - see also Immunohistochemistry .

Many peroxidases have a heme as a prosthetic group . But there are also other prosthetic groups, e.g. B. Selenocysteine in glutathione peroxidase . This enzyme is part of the antioxidant protective system of the human organism.

Function in humans

In peroxisomes , peroxisomal oxidases constantly produce H ₂ O ₂ , which as a cell toxin must be eliminated quickly (RH ₂ + O ₂ → R + H ₂ O ₂ ). Peroxidases solve this by catalyzing a further (detoxifying) oxidation with H ₂ O ₂ as electron acceptor (RH ₂ + H ₂ O ₂ → R + 2 H ₂ O). A special case is catalase (a peroxidase), which catalyzes two H ₂ O ₂ to form two H ₂ O and O ₂ .

The thyroid peroxidase catalyses the first reaction step in the synthesis of the thyroid hormones from the amino acid tyrosine . Antibodies against thyroid peroxidase play an important role in autoimmune diseases that involve the thyroid gland. More details in the article Thyroid Peroxidase .

Individual evidence

1. ↑ UniProt search result peroxidases in humans
2. ↑ UniProt search result peroxidases by EC number
3. ↑ PROSITE entry haemperoxidases
4. ↑ PROSITE entry glutathione peroxidases
5. ↑ PROSITE entry catalase
6. ↑ PROSITE entry fungal peroxidases
7. ^ Matthias Schott, Werner A Scherbaum: Autoimmune thyroid diseases . In: Deutsches Ärzteblatt . tape 103 , no. 45 , November 10, 2006, p. A 3023-32 .

Web links

Wikibooks: Biochemistry and Pathobiochemistry: Tyrosine Metabolism  - Learning and Teaching Materials