Peroxisome Structure and Function and Peroxisome associated diseases

Peroxisomes (Microbodies)

Cell is a factory where thousands of chemical reactions take place. Cellular metabolism generates toxic by-products especially free radicals and peroxides that should be cleared or neutralised from the premise at the earliest. Peroxisomes are the organelle assigned with the duty of dealing with these dangerous chemicals.  

Definition: are single membrane bound cytoplasmic vesicles that carry out a number of reactions including oxidation of urate, glycolate and aminoacids, generating H₂O₂. This organelle contain more than 50 enzymes for degradation of fatty acids and amino acids. Peroxisomes are present in both plants and animals.

As you know D-amino acids are absent in mammalian cells. Naturally occurring D-amino acids are present only in bacterial cell wall (cross linking in peptidoglycan cell wall). Interestingly, D-amino acid oxidase, an enzyme for oxidation of D-amino acids is present in the peroxisomes of mammalian liver and kidney, what is it for as we don’t have D-amino acids?

Lysis or death of intestinal bacteria may result in release of bacterial cell wall that has D-amino acids. It is believed that D-amino acid oxidase is involved in the clearing of bacterial cell wall debris.

Origin:

According to one hypothesis, peroxisomes are believed to be an ancient organelle that carried out all oxygen metabolisms in primitive eukaryotic cells.

Another hypothesis suggests the formation of peroxisome from endoplasmic reticulam membranes by budding. But now it is clear that peroxisomes always arise from pre-existing ones similar to that of mitochondria and chloroplast

Functions:  

H₂O₂ scavenger:

Peroxisomes detoxify various toxic substances that enter the blood stream-including alcohols, phenols etc. Metabolic activities in the cell often release H₂O₂ as by product which is toxic to the cell. Catalase enzyme in the peroxisome converts this H₂O₂ into H₂O and O_2.

2 H₂O₂ -> H₂O + O₂ (by catalase enzyme). Catalase accounts nearly 40% of total peroxisome protein.

How the cells are protected from free radicals?

"fire fighters of the cell"

Free radicals are highly reactive atom or molecule that has a single unpaired electron and can cause DNA damage

Free radical generation during metabolic activities of cell is thought to be the major reason for aging. Mitochondria and ER membranes are the major sites of H₂O₂ release. Catalase is capable of disarming H₂O₂ released by peroxisome. Cytosolic hydroperoxides (like H₂O₂) are cleared from the system by glutathione peroxidase. Superoxide dismutase (SOD) is the other enzyme dealing with superoxide anions (O₂-).

Other functions include photorespiration and beta oxidation of fatty acids.

Peroxisome Associated Disease

• Zellweger Syndrome: a rare inherited disease where peroxisomes are absent in live and renal cells. 
• Adrenoleukodystrophy (ALD): a single enzyme deficiency disease. Normally very long chain fatty acids (VLCFA, 24-26 C-atoms) are transported by a membrane protein and metabolized in peroxisome. Defect in this membrane protein lead to accumulation of VLCFA in the brain and destroy the myelien sheath, the insulator of nerve cells. 

Extra points:

Luciferase enzyme, that generates the light emitted by fireflies, is a peroxisomal enzyme.

Plasmalogens: are unusual class of phospholipids where one of the fatty acids is linked to glycerol by an ether linkage rather than an ester linkage and are abundant in myelien sheaths. Plasmalogens are synthesised by peroxisomes. Abnormalities in Plasmalogens synthesis may lead to severe neurological dysfunction.