Peroxisomes

•Peroxisomes are small (0.1–1 μm), single-membrane-enclosed organelles

•contain enzymes involved in a variety of metabolic reactions, including several aspects of energy metabolism.

•Most human cells contain 100–1000 peroxisomes, depending on the metabolic activity of the cell.

•Peroxisomes do not have their own genomes and all their proteins are synthesized from the nuclear genome.

•proteins are synthesized on free ribosomes and then imported into peroxisomes as completed polypeptide chains

•However, some membrane proteins are transferred to peroxisomes from the endoplasmic reticulum

•Also like mitochondria and chloroplasts, peroxisomes can replicate by division.

•However, unlike those organelles, peroxisomes can be rapidly regenerated even if entirely lost to the cell.

•the proteins of peroxisomes are typical eukaryotic proteins.

•The assembly of peroxisomes is mediated by an intersection of proteins synthesized on free cytosolic ribosomes and proteins transported to peroxisomes from the ER.

•These transmembrane proteins include proteins involved in the transport of metabolites as well as proteins called peroxins or Pex proteins, which are involved in peroxisome assembly

•Mutations in peroxins-peroxisome biogenesis disorders result from defective peroxisome assembly

•Most peroxisome transmembrane proteins are first translocated into the ER and inserted into the ER membrane

•These proteins then bud in vesicles from a specialized region of the ER called the peroxisomal ER

•Formation of a functional peroxisome requires the fusion of two different types of vesicles containing different classes of peroxins, both of which are needed for the import of matrix proteins.

Functions:

•Human peroxisomes contain more than 50 different enzymes that play critical roles in several metabolic pathways, some of which are specific to particular tissues.

•Peroxisomes originally were defined as organelles that carry out oxidation reactions leading to the production of hydrogen peroxide.

•Because hydrogen peroxide is harmful to the cell, peroxisomes also contain the enzyme catalase, which decomposes hydrogen peroxide either by converting it to water or by using it to oxidize another organic compound.

•A variety of substrates are broken down by such oxidative reactions in peroxisomes, including uric acid, amino acids, purines, methanol, and fatty acids. The oxidation of fatty acids  is a particularly important

•example, since it provides a major source of metabolic energy. In animal cells, fatty acids are oxidized in both peroxisomes and mitochondria, but in yeasts and plants, fatty acid oxidation is restricted to peroxisomes.

•In mammals, peroxisomes are involved particularly in the catabolism of very long chain and branched fatty acids.

•In addition to providing a compartment for oxidation reactions, peroxisomes are involved in biosynthesis of lipids. In animal cells, cholesterol and dolichol are synthesized in peroxisomes as well as in the ER.

• In the liver, peroxisomes are also involved in the synthesis of bile acids, which are derived from cholesterol.

•In addition, peroxisomes contain enzymes required for the synthesis of plasmalogens—a family of phospholipids in which one of the hydrocarbon chains is joined to glycerol by an ether bond rather than by an ester bond

•Plasmalogens are important membrane components in some tissues, particularly heart and brain, although they are absent in others.

•Peroxisomes play two particularly important roles in plants. First, peroxisomes in seeds are responsible for the conversion of stored fatty acids to carbohydrates, which is critical to providing energy and raw materials for growth of the germinating plant.

•Second, peroxisomes in leaves function coordinately with chloroplasts to metabolize a side product of the Calvin cycle, through which CO2 is converted to carbohydrates during photosynthesis (C2 cycle)