Peroxisomes: Definition, Structure, Functions, Enzymes & More

Peroxisomes are tiny vesicles, single membrane-bound organelles located around the eukaryotic cells. They hold digestive enzymes for tearing down toxic materials in the cell and oxidative enzymes for metabolic action. They are a heterogeneous group of organelles and the existence of the marker enzymes differentiated them from other cell organelles. Peroxisomes play a necessary role in lipid production and are also engaged in the transformation of reactive oxygen species such as hydrogen peroxide into safer molecules like water and oxygen.

The peroxisome evolves its name from the phenomenon that many metabolic enzymes that develop hydrogen peroxide as a by-product are segregated here because peroxides are toxic to cells. Within peroxisomes, hydrogen peroxide is discredited by the enzyme catalase to water and oxygen. Peroxisomes are circled by a single membrane and they range in the diameter from 0.1 to 1 mm. They lie either in the form of a network of interconnected tubules (peroxisome reticulum) as in liver cells or as individual micro peroxisomes in alternative cells such as tissue culture fibroblasts.

Definition

Peroxisomes are small, membrane-enclosed cellular organelles consisting of oxidative enzymes that are engaged in a variety of metabolic reactions, including several conditions of energy metabolism. They are recognized as an important type of microbody found in both plants and animal cells.

Peroxisomes Function

Peroxisomes Function

They were diagnosed as organelles by Belgian cytologist Christian de Duve in 1967 after previously been represented. First peroxisomes to be discovered were disengaged from leaf homogenate of spinach. They are most richly found in detoxifying organs such as the liver and kidney cells. However, they can be activated to proliferate in response to metabolic needs.

Structure of Peroxisomes

  • Peroxisomes fluctuate in shape, size, and number depending upon the energy demands of the cell. These are formed of a phospholipid bilayer with many membrane-bound proteins.
  • The enzymes engaged in lipid metabolism are synthesized on independent ribosomes and selectively imported to peroxisomes. These enzymes include one of the two signaling sequences Peroxisome Target Sequence 1 being the most common one.

    Peroxisomes Structure

    Peroxisomes Structure

  • The phospholipids of peroxisomes are generally synthesized in smooth Endoplasmic reticulum. Due to the ingress of proteins and lipids, the peroxisome grows in size and breaks into two organelles.
  • They are membrane-bound spherical figures of 0.2 to 1.5 μm in diameter located in all eukaryotic organisms including both plants and animal cells.
  • They are gotten floating freely in the cytoplasm in close association of ER, mitochondria, or chloroplast within the cell.
  • They are among the smoothest of eukaryotic organelles.
  • They lie either in the form of a network of interconnected tubules called peroxisome reticulum or as individual microperoxisomes.
  • They are irregular in size and shape according to the cell and usually circular in cross-section.
  • They range from 0.2 -1.5 μm in diameter.
  • It consists of a single inhibiting membrane of lipid and protein molecules encircling the granular matrix.
  • The matrix consists of fibrils or a crystalloid structure containing enzymes.

Peroxisomes Enzymes

Almost 60 known enzymes are present in the matrix of peroxisomes. They are accountable to carry out oxidation reactions leading to the production of hydrogen peroxide. The main groups of enzymes include:

  • Urate oxidase
  • D-amino acid oxidase
  • Catalase

Peroxisomes Functions

The main function of peroxisome is the lipid metabolism and the processing of reactive oxygen species. Other peroxisome functions include:

Peroxisomes Functions

Peroxisomes Functions

  • The synthesis of ether glycerolipids of plasmalogens.
  • The formation of bile acids, dolichol, and cholesterol.
  • The catabolism of purines, polyamines, and amino acids, and the detoxification of reactive oxygen species
  • In methylotrophic yeasts, peroxisomes are also involved in the metabolism of methanol and methylamines.

In plants, peroxisomes facilitate photosynthesis and seed germination. They prevent loss of energy during photosynthesis carbon fixation.

β–oxidation

The main reason for the high energy density of fats is the low ratio of oxygen atoms in every fatty acid molecule. For occurrence, palmitic acid, a fatty acid containing 16 carbon atoms and containing a molecular mass of more than 250 gms/mole, has only two oxygen atoms.

While this formes lipid good storage molecules, they cannot be directly burnet as fuel or quickly catabolized in the cytoplasm through glycolysis. They need to be processed before they can be shunted into the mitochondria for complete oxidation through the citric acid cycle and oxidative phosphorylation.

When these molecules need to be oxidized to release ATP, they need to be first broken down into smaller molecules before they can be processed in the mitochondria. Within peroxisomes, long-chain fatty acids are progressively broken down to generate acetyl coenzyme A (acetyl CoA) in a process called β–oxidation.

Acetyl CoA then combines with oxaloacetate to form citrate. While most carbohydrates enter the citric acid cycle as a three-carbon molecule called pyruvate which is then decarboxylated to form acetyl CoA, peroxisomal β–oxidation allows fatty acids to access the citric acid cycle directly.

One of the main byproducts of β–oxidation is hydrogen peroxide which can be harmful to the cell. This molecule is also carefully detoxified by the enzyme catalase within peroxisomes.

Seed Germination

In plants, peroxisomes play important roles in seed germination and photosynthesis. During seed germination, fat stores are mobilized for anabolic reactions that lead to the formation of carbohydrates. This is called the glyoxalate cycle and begins with β–oxidation and the generation of acetyl CoA as well.

In leaves, peroxisomes prevent the loss of energy during photosynthetic carbon fixation through recycling the products of photorespiration. A crucial enzyme called Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is necessary for photosynthesis, catalyzing the carboxylation of ribulose-1,5-bisphosphate (RuBP).

This is the central reaction for the fixation of carbon dioxide to form organic molecules. However, RuBisCO, as the name suggests, can also oxygenate RuBP, using molecular oxygen, releasing carbon dioxide – in effect, reversing the net result of photosynthesis. This is particularly true when the plant is exposed to hot, dry environments and the stomata close in order to prevent transpiration.

When RuBisCO oxidizes RuBP, it generates a 2-carbon molecule called phosphoglycolate. This is captured by peroxisomes where it is oxidized to glycine. Thereafter, it is shuttled between the mitochondria and peroxisomes, undergoing a series of transformations before it is converted into a molecule of glycerate that can be imported into chloroplasts to participate in the Calvin cycle for photosynthesis.

FAQ

What Is The Main Function Of The Peroxisome In A Plant Cell?
Peroxisomes Are Membrane-bound Packets Of Oxidative Enzymes. In-Plant Cells, Peroxisomes Play A Variety Of Roles Including Converting Fatty Acids To Sugar And Assisting Chloroplasts In Photorespiration. In Animal Cells, Peroxisomes Protect The Cell From Its Own Production Of Toxic Hydrogen Peroxide.
Where Is Peroxisome Found?
A Peroxisome Is A Membrane-bound Organelle (Formerly Known As A Micro Body), Found In The Cytoplasm Of Virtually All Eukaryotic Cells. Peroxisomes Are Oxidative Organelles. Frequently, Molecular Oxygen Serves As A Co-substrate, From Which Hydrogen Peroxide (H2O2) Is Then Formed.
Why Is Peroxisome Important?
Peroxisomes Are Organelles In The Cell That Are Membrane-bound And Contain Enzymes Crucial For Metabolic Activity. Peroxisomes Are Important Because They Are: Involved In Lipid Production. Involved In Oxidation Reactions In The Cell.
What Do Peroxisomes Look Like?
Peroxisomes, Sometimes Called Microbodies Are Generally Small (About 0.1 – 1.0 µM In Diameter) Organelles Found In Animal And Plant Cells. They Can Vary In Size Within The Same Organism. Peroxisomes Break Down Organic Molecules By The Process Of Oxidation To Produce Hydrogen Peroxide.
Do Peroxisomes Break Down Alcohol?
Peroxisomes Absorb Nutrients That The Cell Has Acquired. They Are Very Well Known For Digesting Fatty Acids. They Also Play A Part In The Way Organisms Digest Alcohol (Ethanol). Because They Do That Job, You Would Expect Liver Cells To Have More Peroxisomes Than Most Other Cells In A Human Body.
What Would Happen If We Didn't Have Peroxisomes?
Peroxisomes Are Organelles That Help The Cell Metabolize Certain Chemicals And Deal With The Waste The Cell Produces. Without Peroxisomes There Could Be No Way To Produce The Peroxide Necessary For Metabolism, Nor Could There Be Any Way To Get Rid Of It From The Body.
How Peroxisomes Are Formed?
Peroxisomes Are Formed By The Synthesis And Assembly Of Membrane Proteins And Lipids, The Selective Import Of Proteins From The Cytosol, And The Growth And Division Of Resultant Organelles. To Date, 23 Proteins, Called Perox- Ins, Are Known To Participate In These Processes.
What Is The Other Name Of Peroxisomes?
A Peroxisome Is A Membrane-bound Organelle (Formerly Known As A Micro body), Found In The Cytoplasm Of Virtually All Eukaryotic Cells.
How Do Peroxisomes Detoxify?
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). In Order To Carry Out Their Activities, Peroxisomes Use Significant Amounts Of Oxygen.
What Organelles Do Peroxisomes Interact With?
Peroxisomes Interact With Mitochondria In Several Metabolic PathWays, Including B-oxidation Of Fatty Acids And The Metabolism Of Reactive Oxygen Species. Both Organelles Are In Close Contact With The Endoplasmic Reticulum (Er) And Share Several Proteins, Including Organelle Fission Factors.
Are Peroxisomes Self Replicating?
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.
What Do Peroxisomes Contain?
Peroxisomes Contain Enzymes That Oxidize Certain Molecules Normally Found In The Cell, Notably Fatty Acids And Amino Acids. Those Oxidation Reactions Produce Hydrogen Peroxide, Which Is The Basis Of The Name Peroxisome.