Structural Biochemistry/Cell Organelles/Peroxisome

Background Information


They are theorized to have evolved from bacteria that formed a symbiotic relationship with their host cell. It was believed that the development of this relationship over generations led to bacteria evolving as an organelle inside the body. This view has recently been countered since it was found that cells without peroxisomes can restore these peroxisomes with a simple gene introduction. Therefore, the previous theory was challenged since the introduction of peroxisomes is not an evolutionary process and can easily be replicated.

Peroxisome Structure
Analyzing the structures of peroxisomes has helped figure their function and role in the biological world. Peroxisomes are not derived from the endoplasmic reticulum and therefore are not a part of endomembrane system. They replicate by fission. This organelle is surrounded by a lipid bilayer membrane which encloses the crystalloid core. The bilayer is a plasma membrane which regulates what enters and exits the peroxisome. There are at least 32 known peroxisomal proteins, called peroxins, which carry out peroxisomal function inside the organelle.



Peroxisome Function
The main function of peroxisomes is to break down long fatty acid chains through beta-oxidation and synthesize necessary phospholipids (such as plasmologen) that are critical for proper brain and lung function. Furthermore, they aid certain enzymes with energy metabolism in many eukaryotic cells as well with cholesterol synthesis in animals. Peroxisomes are also involved in germinating seeds in the glyoxylate cycle, photosynthesis in leaves, and oxidation of amines in various yeasts.

During catabolism of fatty acid chains in animal cells, peroxisomes break down long fatty acids into medium fatty acids which are then transported to mitochondria where the majority of catabolism happens. However, in yeast and plant cells, catabolism of fatty acid chains happens only in the peroxisome and the mitochondria is not involved. The catabolism of fats and fat-soluble vitamins, such as vitamin A and vitamin K, as well as the production of bile acids also takes place in peroxisomes. If this catabolism is not done properly, genetic disorders or skin disorders often result.

The synthesis of plasmalogens in animal cells also takes place in peroxisomes. These organelles are very important to the cell because production of plasmalogens is critical to proper functioning of the nervous system since a lack of plasmalogens causes abnormalities in the myelination of nerve cells.

Peroxisomal Disorders
Peroxisomal disorders may result due to abnormalities in single enzymes or groups of proteins that are necessary for normal peroxisome function. These disorders can affect a range of organ systems, but problems with the nervous system are the most commonly observed. Along with brain damage, many of these disorders also lead to skeletal and craniofacial dysmorphism, liver dysfunction, progressive sensorineural hearing loss, and retinopathy. Some examples of peroxisomal disorders: 1. Adrenoleukodystrophy: a fatal inherited X-linked disorder that leads to extensive brain damage and adrenal gland failure. 2. Zellweger syndrome spectrum (PBD-ZSD): a rare cerebrohepatorenal syndrome evolving due to a lack of functional peroxisomes. 3. Rhizomelic chondrodysplasia punctata type 1 (RCDP1): a rare brain disorder evolving due to shortening of the proximal bones.

Peroxisomal Evolution
Due to peroxisome's ability to "divide and import proteins post-translationally," it is suggested that it is similar to the mitochondria where an endosymbiotic relationship was formed in its origin.

Studies have shown that: (39–58%) of peroxisome are of eukaryotic origin (13–18%) are enzymes from the mitochondria

This lead to the conclusion that it did not have a endosymbiotic origin, but rather it used proteins from other eukaryotic cells.