In this essay we will discuss about:- 1. Introduction to Peroxisomes 2. Morphology of Peroxisomes 3. Structure of Peroxisomes 4. Functions of Peroxisomes 5. Peroxisomal Disorders.
Essay # 1. Introduction to Peroxisomes:
Peroxisomes are small membrane-bound, self-replicating organelle found in nearly all eukaryotic cells. They contain oxidative enzymes, such as D-amino acid oxidase, ureate oxidase, and catalase. They are a major site of oxygen utilization and prevent the body from toxic substances like hydrogen peroxide, or other metabolites. They resemble lysosome; however, they are not formed in the Golgi complex. Peroxisomes are distinguished by a crystalline structure inside a sac which also contains amorphous gray material. They enlarge and bud to produce new per-oxisomes.
Peroxisomes were first described by Mollenhauer, Moore and Kelly in 1966. Peroxisomes were first observed as small, single-membrane bound organelles (micro-bodies) in kidney cells of the mouse. After centrifugation of cell homogenate peroxisomes were found in the crude organellar pellet, together with mitochondria and lysosomes. But, they could be separated out from other organelles by subsequent equilibrium density centrifugation, due to their remarkable high density. Characterization of this fraction showed the presence of H2O2 producing oxidases (among which enzymes to degrade fatty acids) and catalase to decompose H2O2 (hence the name peroxisomes).
Since they are rich in enzymes like per-oxidase, oxidizer and catalases, they are called as peroxisomes. Those peroxisomes contain the enzymes of glyoxylate cycle (i.e., glycolate metabolism), are called as glyoxysomes. They have been isolated from algae, yeasts, leaves and seeds of higher plants, protozoans, amphibians, birds and mammals.
In all probability the peroxisomes arise from the E.R. The E.R. membrane dilates and finally the peroxisomes are pinched off. The peroxisomal enzymes are synthesized on ribosomes on rough E.R. membrane.
Essay # 2. Morphology of Peroxisomes:
They are oblate or spheroidal in shape measuring 0.2-1.2 μm in diameter. The average diameter of peroxisome occurring in rat liver cells ranges between 0.6-0.7μm. They are bound by a single lipoproteinaceous unit membrane of about 60Å thickness. They contain granular contents condensing in the centre. Their membrane is permeable to amino acids, uric acids, hydroxyl acids etc., but shows controlled permeability to pyridine nucleotides.
Essay # 3. Structure of Peroxisomes:
Peroxisomes have a single limiting unit membrane of lipid and protein molecules, which encloses their granular matrix. In some cases the matrix contains numerous threads or fibrils, while in others they are observed to contain either an amorphous nucleoid or a dense inner core which in many species shows a regular crystalloid structure. Peroxisomes are similar in appearance to lysosomes, but the two have very different origins. Lysosomes are generally formed in the Golgi complex, whereas peroxisomes self-replicate. Unlike self-replicating mitochondria, however, peroxisomes do not have their own internal DNA molecules. Consequently, the organelles must import the proteins they need to make copies of themselves from the surrounding cytosol.
The enzymes and other proteins destined for peroxisomes are synthesized in the cytosol. Each contains a peroxisomal targeting signal (PTS) that binds to a receptor molecule that takes the protein into the peroxisome. Two peroxisomal targeting signals have been identified- a 9-amino acid sequence at the N-terminal of the protein and a tri-peptide at the C- terminal (Fig. 4.59).
Essay # 4. Functions of Peroxisomes:
Peroxisomes contain a variety of enzymes, which primarily function together to remove toxic substances, and in particular, hydrogen peroxide (a common by-product of cellular metabolism) from the cell. These organelles contain enzymes that convert the hydrogen peroxide to water, rendering the potentially toxic substance safe for release back into the cell. Some types of peroxisomes, such as those in liver cells, detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to molecules of oxygen (a process termed oxidation).
They are also known to be essential in many vital pathways, including: metabolism of free oxygen radicals, synthesis of cholesterol and ether lipids, bile acid formation, catabolism of long chain fatty acids, catabolism of purines, prostaglandins, leucotriens, and alcohol detoxification in liver and metabolism of estradiol. Peroxisomes are also present in plant cells where they participate in symbiotic nitrogen fixation and photorespiration.
The glyoxysomes show glyoxylate cycle. The phenomenon of photorespiration can be seen in C3 – plants. It is the chief site of photo respiration. Three organelles are found associated with the phenomenon of photorespiration, namely peroxisomes, chloroplasts and mitochondria. The peroxisomes also participate in β- oxidation.
Essay # 5. Peroxisomal Disorders:
Two major categories of metabolic disorders have been discovered to be caused by molecular defects in peroxisomes. The first category consists of disorders of per-oxisome biogenesis in which the organelle fails to develop normally, causing defects in numerous peroxisomal proteins. The second category involves defects of single per-oxisomal enzymes. The most serious of these disorders is Zellweger syndrome, which is characterized by an absence or reduced number of peroxisomes in the cells. This disorder results from the inheritance of two mutant genes for one of the receptors (PXR1) needed to import proteins into the peroxisome.