Have you ever seen people using hydrogen peroxide over wounds and cuts as an antiseptic? Hydrogen is a reactive oxygen species which decomposes rapidly when exposed to light. It is not only used as a mild antiseptic, but also as a bleaching agent, in the production of other organic peroxides, in sewage treatment to oxidise organic wastes, etc. But did you know that your body also produces hydrogen peroxide in small quantities? Our cells have a special organelle known as a peroxisome which produces hydrogen peroxide through certain oxidative metabolic reactions. Peroxisomes are classified as a special category of organelles known as microbodies which are single membrane bound organelles but are different from the ones that are part of the endomembrane system. In this article we will discuss different types of microbodies, their location, structure and function.
Table of Contents
Microbodies are the eukaryotic cell organelles that are bound by a single membrane. These are absent in prokaryotic cells and are filled with different types of oxidative enzymes and their main functions involve detoxification of peroxides, oxidative metabolism and photorespiration in plants, etc.
The major types of microbodies found in higher eukaryotes are -
Apart from this, some protozoans (trypanosomes) have glycosomes which are microbodies containing glycolytic enzymes and some anaerobic ciliates, flagellates and fungi have hydrogenosomes which are microbodies that help in the production of ATP and molecular hydrogen in anaerobic conditions.
Fig: Types of microbodies
These are the microbodies that are involved in the synthesis of hydrogen peroxide during their metabolism. These organelles were named peroxisomes by de Duve in 1969 and were first seen in rat kidney and liver cells with the help of fractionation technique.
Both plant and animal cells contain peroxisomes in their cytoplasm, lying close to the mitochondria, ER and chloroplast (in plant cells). These are derived from the endoplasmic reticulum and are present in numbers of 70-100 in photosynthetic cells.
The average diameter of peroxisomes ranges from around 0.5-1 micron. They are bound by a single phospholipid bilayer membrane enclosing a granular matrix. A central core known as a nucleoid, which is crystalline or fibrous in nature, is present in the matrix. Multiple oxidative enzymes such as catalase, peroxidase, uric acid oxidase, b-amino acid oxidase, b-hydroxy acid oxidase, D-amino acid oxidase, etc are found in peroxisomes.
Fig: Structure of peroxisomes
Peroxisomes help in oxidative detoxification of toxic substances such as phenols, nitrites, formaldehyde, methanol, formic acid, etc and 25% of the consumed alcohol.
They also help in the breakdown of xenobiotics which cannot be metabolised by other digestive enzymes in the body. Peroxisomes are responsible for metabolism of long chain fatty acids in animal cells.
In photosynthetic plant cells peroxisomes help in photorespiration. They pick up the glycolate generated in the chloroplasts and oxidise it to form glyoxylate, thereby releasing hydrogen peroxide as a byproduct. The glyoxylate is eventually converted to amino acid glycine.
Peroxisomes also help in conversion of nitrogen fixed in the root nodules to acyl derivatives of urea or ureids, catalysis of condensation reactions, and the removal of amine groups from amino acids by oxidation.
These microbodies have similarities to the peroxisomes as they contain similar enzymes in addition to the enzymes of the glyoxylate cycle. They were first explained in 1967 by R. W. Breidenbach.
Glyoxysomes are seen in the cytoplasm of plant cells. Germinating seeds of watermelon, peanuts, castor, etc generally show the presence of numerous glyoxysomes.
The glyoxysomes are bound by a single membrane and their matrix consists of a crystalloid core. These organelles mostly contain enzymes such as malate synthetase, isocitrate lyase and many enzymes of the Krebs cycle.
Glyoxysomes help in synthesising carbohydrates from the lipids present in the endosperm of germinating seeds through the glyoxylate cycle. They help in the oxidation of fatty acids and produce acetyl coA in the process. This acetyl coA is further metabolised through the glyoxylate cycle to form carbohydrates.
Once the stored lipids in the plant cells are mobilised by the glyoxylate cycle in glyoxysomes, they stop acting as glyoxysomes and become peroxisomes. They reappear in senescent plant tissues to help in degradation and mobilisation of lipids.
These microbodies are ovoid or spherical in shape and help in lipid and protein metabolism. Perner discovered spherosomes in 1953.
Plant cells mainly involved with storage of lipids, such as cells of endosperm tissue of oilseeds, show the presence of spherosomes which arise from the endoplasmic reticulum.
Fig: Spherosomes in plant cells
Spherosomes are bound by a phospholipid monolayer which forms a half-unit membrane. The hydrophilic polar heads of the phospholipids are arranged towards the outer side while the hydrophobic nonpolar tails are arranged to face the inner side. 98% of the composition of spherosomes consist of lipids while the remaining 2% consist of proteins.
Spherosomes help in the storage and synthesis of lipids. Limited variety of hydrolytic activity, similar to the one shown by lysosomes, is shown by the spherosomes of endosperm tissue of tobacco and root tip cells of maize. This is because their spherosomes contain hydrolytic enzymes.
Solution: Spherosomes are bound by a phospholipid monolayer which forms a half-unit membrane. The hydrophilic polar heads of the phospholipids are arranged towards the outer side while the hydrophobic nonpolar tails are arranged to face the inner side. Thus, the correct option is c.
3. The glyoxysomes convert ____ to ____ in the endosperm of watermelon.
Solution: Glyoxysomes help in synthesising carbohydrates from the lipids present in the endosperm of germinating seeds through the glyoxylate cycle. They help in the oxidation of fatty acids and produce acetyl coA in the process. This acetyl coA is further metabolised through the glyoxylate cycle to form carbohydrates. Thus, the correct option is b.
3. Which of the following enzymes is not present in a peroxisome?
Solution: Multiple oxidative enzymes such as catalase, peroxidase, uric acid oxidase, b-amino acid oxidase, b-hydroxy acid oxidase, D-amino acid oxidase, etc are found in peroxisomes. Isocitrate lyase is present in glyoxysomes. Thus, the correct option is a.
4. Why are glyoxysomes considered to be specialised peroxisomes?
Answer: Glyoxysomes are similar to peroxisomes as they contain peroxisomal enzymes in addition to enzymes of the glyoxylate cycle which they use to convert stored lipids into carbohydrates. Once the stored lipids in the plant cells are mobilised by the glyoxylate cycle in glyoxysomes, they stop acting as glyoxysomes and become peroxisomes. They reappear in senescent plant tissues to help in degradation and mobilisation of lipids.
Answer: Xenobiotics include compounds that are completely alien to living organisms and are not expected to be found in them. These would include certain drugs, pesticides, environmental pollutants, etc.
Answer: Peroxisomes and lysosomes differ in the enzymes they contain and the functions they perform. Lysosomes are mainly involved with breakdown and digestion of lipids, proteins, nucleic acids and polysaccharides due to the presence of hydrolytic enzymes. Peroxisomes on the other hand consist of oxidative enzymes which help in oxidation of compounds and produce peroxides as byproducts.
Answer: The assembly of peroxisomes is done in a process similar to that of assembly of mitochondria and chloroplasts. The proteins comprising the peroxisomes are synthesised on the free ribosomes in the cytosol and then transported to the peroxisomes. Special protein importing pathways help in targeting the proteins to the interior of the peroxisomes. Even the phospholipids forming the peroxisome membrane are synthesised in the ER and transported to the peroxisomes via phospholipid transfer proteins.
Apart from this, peroxisomes divide and multiply just like mitochondria and chloroplasts.
Answer: The Zellweger syndrome is a lethal disorder that affects one within the first ten years of one’s life. It is caused due to the mutations in a minimum of ten different genes which influence protein import in peroxisomes. One of these genes is the one which codes for the receptor which binds to the PTS1 targeting signal on the proteins targeted for import into the peroxisomes. This signal is a simple amino acid sequence ( Ser-Lys-Leu) at the C terminal end of the proteins.