Peroxisome

Peroxisomes, also Microbodies (obsolete) called, are cell organelles in eukaryotic cells, which are surrounded by a cell membrane. You consume in a variety of metabolic functions of oxygen and therefore are considered the first detoxification apparatus, which were necessary to the occurrence of an oxygen-containing atmosphere.

Construction

It is in peroxisomes small (100 - 1000 nm diameter), coated with a single membrane vesicles which are located in the cytoplasm of a cell. In these spatially separated areas ( cellular compartments ) can be protected by the membrane, reactions take place, which would be dangerous for the cell, it would take place in the cytoplasm. This is an example of the importance of the cell compartmentalization. Peroxisomes contain enzymes for the metabolism of hydrogen peroxide ( H2O2), which is why the term " peroxisome " established. Morphologically, they were formerly known as " micro Bodies".

Number, size and proteome of peroxisomes are dependent on cell type and growth conditions. So you have, for example, in baker's yeast (S. cerevisiae) made ​​the observation that good glucose supply only a few, small peroxisomes are present. In contrast, when the yeasts were treated with long-chain fatty acids, is 20 to 25 large organelles were formed.

Molecular oxygen is often used as the co- substrate is formed from the then hydrogen peroxide ( H2O2). The hydrogen peroxide -degrading peroxidase owe the peroxisomes her name.

Functions

In the peroxisomes are about 60 monooxygenases and oxidases called enzymes that catalyze the oxidative degradation of fatty acids, ethanol and other compounds. These enzymes use molecular oxygen as a co- substrate, so as to form hydrogen peroxide for cell function. Hydrogen peroxide is a cell toxin in the cytoplasm and can destroy many important biomolecules.

Hydrogen peroxide may be reduced by two ways. One way to detox consists in its immediate implementation by catalase in a disproportionation reaction in which water and oxygen is formed:

Peroxisomes also possess the eponymous peroxidase. For their function, the hydrogen peroxide is consumed, according to:

Often the enzyme concentrations are so high that they form crystalline aggregates ( Nucleoide ).

After Endosymbiontenhypothese bacteria- like organisms were taken up by the " Urkaryonten " ( precursor of eukaryotes ) cells, which already had a meaningful utilization of oxygen apparatus ( citric acid cycle, together with the respiratory chain ) and thus capable of ATP synthesis by way of oxidative phosphorylation in the course of evolution were. These were the forerunners of the "modern" mitochondria.

The peroxisomes were not superfluous, but they were involved in the catabolism; the link was the ( high-energy ) acetyl -CoA. To be The figure shows an example of how ethanol is used not only to detoxify hydrogen peroxide, but also even in a metabolite ( acetyl -CoA) are of general importance in the catabolism ( energy gain ) and anabolism transferred ( synthesis of fatty acids, cholesterol etc. ). Peroxisomes thus contribute to the metabolism of ethanol.

In addition, they catalyze important steps in the biosynthesis of lipids ( plasmalogens ) of the myelin sheath of nerves (hence their function disorders often go with neurological damage associated ). The specific metabolic pathways which occur exclusively in peroxisomes,

• The α - oxidation of phytanic acid
• The β - oxidation of very long chain polyunsaturated fatty acids
• The biosynthesis of plasmalogens
• The conjugation of cholic acid in the bile acid synthesis

Other forms

Glyoxysomes (also glyoxisomes ) are specialized peroxisomes that are found in the endosperm and the storage tissues of fatty sperm. They got their name because they are involved in the glyoxylate cycle. The enzymes contained in the glyoxysomes enable the use of fats to the structure of biopolymers (sugar, proteins), which are necessary for plant growth.

In photosynthetically active plant peroxisomes also take on the photorespiration part - there also in cooperation with mitochondria. They are referred to as foliar- peroxisomes. Herbal glyoxysomes and leaf peroxisomes can transform themselves within each other.

Formation

The origin of peroxisomes was highly controversial in recent years. It is now known that peroxisomes can multiply by division analog mitochondria within the cell. The de novo formation of new peroxisomes is a multistep process that begins with the pinch of Vorläufervesikel from the ER. Probably then merge the small Vorläuferversikel to a mature peroxisome. For the biogenesis is pEX3, an integral membrane protein in yeast essential.

Protein transport

Since peroxisomes do not contain ribosomes, all enzymes must be synthesized in the cytosol and then transported into the peroxisome. These proteins have been post-translationally in the folded state into the peroxisome. There are known two ways. Most proteins need to C -terminal signal sequence, the so-called peroxisome targeting signal ( peroxisome targeting signal ) PTS1. This signal sequence is shorter than that of proteins that are to be brought into the mitochondrion or into the ER; There is often this only from the three amino acids serine -lysine - leucine ( SKL ). The signal sequence that " PTS1 proteins " is detected in the cytosol of Pex5p and led to the peroxisome, where they are transported through a membrane protein complex into the interior of Peroxisoms. Here, the protein - Pex5p complex docks to the integral membrane protein of Pex14. The translocation of the complex occurs then through the Pex2/Pex10/Pex12-Komplex.

The second transport path and an N-terminal signal peptide is made longer by the protein membrane complex of Pex7p Peroxisoms. This signal sequence is also referred to as PTS2, proteins are transported as a result PTS2 proteins. Next Pex7p a spliced ​​form of Pex5p used in mammalian cells. For transport in the matrix of the Peroxisoms the signal peptide is then cut off.

Diseases

Disorders in which peroxisomes play a role:

1 Peroxisomendefekte

• Zellweger syndrome
• Chondrodysplasia punctata rhizomelic type 1 ( mutation of the gene PEX7 )
• Neonatal adrenoleukodystrophy
• Infantile Refsum disease

2 peroxisomal enzyme defect

• Pseudo- Zellweger syndrome (mutation of the acyl -CoA oxidase)
• X - linked adrenoleukodystrophy (sec. by peroxisomal transporter protein defect for VLCFA -CoA synthetase )
• Rhizomelic Chrondrodysplasia punctata type 2 ( mutation from DHAPAT gene)