Peroxisomes



 Peroxisomes:

These were discovered by Baudhin et .al ( 1953) .These are also spherical in shape about 1_ 5 ųm  in size. There occur in fungi ( yeast and Neurospom) and plant cells and are abundant in germinating seeds which store fats as a reserve food material. The stored lipid molecules are hydrolysed by lipase to glycerol and fatty acids. These fatty acids are then  broken down in the process of ß _ oxidation. 



Peroxisomes:

They contain enzymes for peroxide biosynthesis .They are found in both plant and animal cells, generally in close association with endoplasmic reticulum, mitochondria and chloroplasts. Despite the absence of DNA, Peroxisomes are believed to be able to replicate like mitochondrion and plastids. They are believed to be vestige of an ancient organelle present in protoeucaryotes which performed all oxidation reactions prior to evolution of mitochondria. Recent research suggests that peroxisomes have actinobacterial origin. They contain special proteins called peroxins for obtaining materials from cytosol and endoplasmic reticulum. Peroxisomes occur in all Eukaryotic cells.

They are quite abundant in liver and kidney cells .A photosynthetic cell may have 70_ 100 peroxisomes. Peroxisomes are believed to develop from endoplasmic reticulum with a variable size ( diameter approx.0.5_ ųm) and  shape.They are covered over by a single membrane and the interior contains a matrix which may be granular or have fibrils arranged variously. In some cases the matrix has a central dense, crystalline or fibrous core which is called nucleoid.



The peroxisomes contain one or more peroxide producing enzymes like urate oxidase ( uricase), D_ amino acid oxidase, æ _ hydroxy acid oxidase and ß _ hydroxy acid oxidase. These enzymes produce H2O2 .Peroxisomes also contain peroxide destroying catalase, for metabolizing hydrogen peroxide. 

(a) Peroxisomes in animal cells take part in oxidation of a number of biochemicals including extra amino acids, alcohol and toxins e.g., about 50% of the alcohol consumed by a person is detoxified by Peroxisomes inside liver cells. The microbodies may also take part in lipid metabolism. 

(b) Plant peroxisomes found in photosynthetic cells are associated with the chloroplasts and they perform photorespiration. Peroxisomes pick up glycolate from chloroplasts which is oxidised with the help of oxygen to produce glyoxylate. The latter is changed to amino acid glycine which condenses to produce amino acid serine and carbon dioxide. 


(c) Unusual substances or xenobiotics ( e.g., D_ amino acids,  alkanes) which cannot be metabolised by normal enzymes are broken down  inside Peroxisomes.

(d) Urate produced during catabolism of nucleic acids and some proteins is changed into allontoin inside Peroxisomes. 

(e) Long chain ( e.g. prostaglandins) and branched chain fatty acids  are initially broken down by Peroxisomes. 

(f) In root nodules, they convert fixed nitrogen in ureids for transport.

(ii) Glyoxisomes: They contain enzymes for ß _ oxidation of fatty acids and glyoxylate pathway.They have been recorded only in plant cells and are quite common in germinating oil seeds and the cells of some fungi. Glyoxisomes have a single convering acetyl which is metabolised  in glyoxylate cycle to produce carbohydrates

After completion of their function, glyoxisomes are believed to be changed into Peroxisomes. They reappear in senescent plant tissues for degradation of lipids and mobilisation  of degradation products. 



Peroxisomes:

☆ The Peroxisome is a specialized metabolic compartment bounded by a single membrane. 

☆ Peroxisomes contain enzymes  that remove hydrogen atoms from various substrates and transfer them to oxygen (O2), producing hydrogen peroxide as a by_ product. 

☆ Peroxisomes in the liver detoxify alcohol and other harmful compounds. 

☆ Specialized Peroxisomes called glyoxisomes are found in the fat_ storing tissues of plant seeds.

■ These organelles contain enzymes that initiate the conversion of fatty acids to sugar, which the emerging seeding uses as a source of energy and carbon init it can produce its own sugar by photosynthesis. 


Where is Peroxisomes functions and Structure ?




Functions:

Peroxisomes are membrane-bound organelles found in the cytoplasm of eukaryotic cells. They have a characteristic spherical or oval shape and vary in size, ranging from 0.1 to 1.0 micrometers in diameter. Peroxisomes are present in almost all eukaryotic cells, including plants and animals.

Functionally, peroxisomes are involved in several important metabolic processes, including:

1): Beta_ oxidation of fatty acids:Peroxisomes play a crucial role in breaking down long-chain fatty acids into acetyl-CoA molecules, which can then be used for energy production.

2): Detoxification of harmful compounds:They are involved in detoxifying substances like ethanol, formaldehyde, and other toxic compounds. They do this by various enzymes, such as catalase, which decomposes hydrogen peroxide, a toxic byproduct generated during these detoxification processes.

3): Synthesis and degradation  of specific lipids: Peroxisomes are involved in the synthesis of certain lipids, such as plasmalogens, which are important components of cell membranes. They also participate in the breakdown of certain lipids, such as bile acids.

 4):Metabolism of amino acids:Certain enzymatic reactions related to the metabolism of specific amino acids, such as the synthesis of glycine and the oxidation of some amino acids, occur within peroxisomes.

The specific enzymes and metabolic pathways present within peroxisomes vary depending on the cell type and the organism. However, the primary function of peroxisomes is to carry out these metabolic processes efficiently, often in collaboration with other organelles like mitochondria and endoplasmic reticulum.

Structure:

In terms of their structure, peroxisomes consist of a single membrane enclosing the organelle. The membrane is composed of phospholipids, proteins, and transporters. Within the peroxisome, various enzymes are present, which are essential for their metabolic functions. These enzymes are imported into the peroxisome matrix from the cytoplasm and are either synthesized within the peroxisome or imported from the cytoplasm. The matrix of the peroxisome contains the enzymes and substrates necessary for the metabolic reactions to occur.

In summary, peroxisomes are important organelles involved in various metabolic processes, including fatty acid metabolism, detoxification, lipid synthesis and degradation, and amino acid metabolism. They have a specific structure that enables the efficient functioning of these metabolic pathways.























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