Microbodies: Structure, Functions, and Importance

Back in 1954, biologist Rhodin reported microbodies in the proximal convoluted tubule of the kidney of a mouse. It was found at the ultrastructural level of the tubule. Later, in 1958, Porter and Caulfield reported the same in plants.

Microbodies are now identified as ubiquitous subcellular respiratory organelles present in eukaryotic cells. Morphologically, microbodies from all tissues appear the same and share the same types of enzymatic properties, but depending upon the tissue they vary in their metabolic pathways happening inside this subcellular compartment. Microbodies (peroxisomes and glyoxysomes) were just recognized as subcellular elements, but it was at the end of the 1960s biologists could establish their significance.

Definition:

A microbody can thus be defined as a cytoplasmic organelle that is more or less globular in shape. They are degradative enzymes that are encapsulated within a single membrane. They are considered as containers for metabolic activity. There are multiple types of microbodies. Some of them include peroxisomes, glyoxysomes, glycosomes, and Woronin bodies. 

Structure:

These microbodies are the bodies present in the cytosol of the cell. These are also called cytosomes. A microbody is usually a vesicle with a spherical shape, ranging from 0.2-1.5 micrometers in diameter. Microbodies can be seen in the cytoplasm of a cell, but they will be visible only through an electron microscope. A single phospholipid bilayer membrane surrounds them and they have a matrix of intracellular material that includes enzymes and other proteins, but they do not seem to contain any genetic material to allow them to self-replicate

Classification:

The enzymes present in microbodies take part in the preparatory or intermediate stages of various biochemical reactions happening within the cell. Breakdown of fats, alcohols, and amino acids are facilitated by these enzymes. Generally, detoxification of peroxides and photorespiration in plants are major functions of microbodies. Depending upon the functionality, microbodies are classified. Some important microbodies include i) Glyoxysomes ii) Peroxisomes iii) Glycosomes and iv)Woronin bodies

1. Peroxisomes: 

Peroxisomes are the organelles from the microbody family present in almost all eukaryotic cells. They are mainly involved in the metabolism of fatty acids along with other metabolites. A couple of enzymes present in peroxisomes help the cells to get rid of toxic peroxides. Peroxisomes are single membrane-bound bodies. The membrane separates the enzymes of peroxides from remaining cytosol. The membrane proteins are critical for various functions including import of proteins into the organelles and proliferation. Peroxisomes can replicate also.

2. Glyoxysomes:

Glyoxysomes are mainly found in the plant kingdom. Be it germinating seeds of plants or filamentous fungi, these microbodies will be there. Glyoxysomes are those kinds of peroxisomes performing the function of the glyoxylate cycle. Peroxisomes are responsible for f b-oxidation (break down fats and produce Acetyl-CoA) and other important pathways including amino acid and bile acid metabolism, along with oxidation or detoxification of various harmful compounds in the liver like alcohol.

Visibility and Distinguishing Factors

Although all classes share the common characteristics, yet their metabolic roles are specified depending upon the developmental stage and type of cell. Catalase, found in these organelles, can be stained black in a way that the organelles become prominently visible under electron micrographs. The proteins present in this type of organelle are found as a granular matrix denser than that of the cytosol. This type of organelle is devoid of any internal membranous structures but some of their matrices may include a striking proteinaceous crystal or dense aggregate. This protein structure can be viewed under electron microscopy. Peroxisomes or glyoxysomes become visible under fluorescence microscope by the use of antibodies specific to any one of their proteins, like catalase. Due to the relatively simple structure of the internal matrix of these microbodies, they can be easily distinguished from chloroplasts or mitochondria. Both chloroplast and mitochondria have internal folded or stacked membranes.

Glyoxysomes and Peroxisomes in Plants

Thus, we can say that both peroxisomes and glyoxysomes are membrane-bound microbodies containing oxidative enzymes. The enzymes found within microbodies are transported from the cytosol by a process called peroxisomal targeting sequences (PTS). 

These organelles can be viewed under the electron. In higher plants all classes of peroxisomes have the following characteristics: (1) They all are single membrane-bound organelles; (2) their equilibrium density is high; and (3) granular matrix or internal content.

In the photosynthetic cells of leaves, peroxisomes, mitochondria, and chloroplast interact with one another at the time of photorespiration. During germination when fatty acids get converted to carbohydrate (sugars), the enzyme Glyoxysomes are found in contact with lipid bodies of cotyledons or endosperm.

In plant cells, these microbodies may appear to be dividing or may be found in an interconnected or tubular state. Production and destruction of the toxic agent called hydrogen peroxide (H₂O₂) are the major functions of all the microbodies of plant cells. They follow the reaction:

2H₂O₂ = 2H₂O + O₂

This reaction happens in the animal as well. It is found to occur to remove this toxic chemical from the blood.

The process called glyoxylate cycle helps in the mobilization of storage lipids in growing seedlings. Succinate produced in glyoxysomes gets converted to sucrose in the cytosol. Photosynthetically active tissues such as green leaves, cotyledons etc contain leaf peroxisomes with enzymes essential for photorespiration. Peroxisomes in root nodules and legumes are responsible for nitrogen metabolism. For tropical legumes, nitrogen gets transported in the form of ureides. Reactions Of Ureide Biosynthesis occur in multiple subcellular compartments. One of the last steps of this pathway involves the conversion of urate to allantoin, which is then catalyzed by urate oxidase in peroxisomes. Unspecialized Peroxisomes are found in plant tissues but they are not active photosynthetically and also are devoid of lipids. They are found in the roots of most plants. These peroxisomes are smaller in size and have low frequency 

Some other Functions of Peroxisome and Glyoxysomes:

1. Oxidative enzymes, like catalase, D-amino acid oxidase, and uric acid oxidase(except in humans) are found in peroxisomes. Due to the absence of uric acid oxidase in humans, the accumulation of uric acid occurs leading to the disease called gout.

2. Catalase of the peroxisome oxidizes another substrate like phenols, formic acid, formaldehyde, and alcohol in the presence of hydrogen peroxide, by the process of the peroxidation reaction:

H₂O₂ + R’H₂ = R’ + 2H₂O

3. Peroxisomes perform beta-oxidation to break fatty acids.

4. Glyoxysomes help in the synthesis of sugar by the process of gluconeogenesis.

3. Glycosome

The membrane-enclosed organelle containing the glycolytic enzymes and a dense proteinaceous matrix is the glycosome. A few species of protozoa, found in the human pathogenic trypanosomes, responsible for sleeping sickness, and Chagas's disease, and Leishmania possess glycosomes. It is considered to have evolved from the peroxisome. Glycosomes possess peroxisomal enzymes and glycolysis enzymes.

Composition and Structure:

Glycogen and proteins are its building blocks. The proteins are the enzymes associated with the metabolism of glycogen. The proteins of glycosomes have their origin in free cytosolic ribosomes. These proteins have a specific sequence that is similar to the alpha-granules of the cytosol. Glycosomes are round to oval in shape with varying size depending on the cell. The membrane is composed of bilayer lipids. The glycogen found within the glycosome is similar to free glycogen found in the cytosol. Glycosomes can be found to be associated or attached to different types of organelles like those of the sarcoplasmic reticulum and its intermediate filaments. The myofibrils and mitochondria, rough endoplasmic reticulum, sarcolemma, polyribosomes, or the Golgi apparatus have various other glycosomes associated with them. Depending upon the attachment of glycosomes their functions may vary. The glycosomes attached to the myofibrils are more prone to serve the myosin. Their job is to provide energy substrates during the generation of ATP through glycolysis. The glycosomes found in the rough and smooth endoplasmic reticulum are known for its use of glycogen synthase and phosphorylase phosphatases.

Woronin Body

The peroxisome derived, dense core microbody protected by a double-layered membrane is the woronin body. It is named after the Russian botanist Mikhail Stepanovich Woronin. It is found near the septae dividing hyphal compartments in filamentous Ascomycota. The main function of these bodies is to plug the septal pores post hyphal wounding. It prohibits the loss of cytoplasm from the sites of injury. The size of the woronin bodies may vary from a range of 100 nm to more than 1 μm. They can be visualized with a light microscope in some species.

Inborn Errors of Peroxisomes/ Peroxisome Biogenesis Disorders in Humans

Peroxisomal disorders are broadly divided into the following two groups:

1. Peroxisome biogenesis disorders (related to the formation of peroxisome within the cell) (abbreviated as PBDs) and

2. Peroxisomal single enzyme deficiencies (functional disorder in peroxisomes). 

The prototype of PBDs was first described in 1964. Typically what happens is that the affected children present a host of multiple major health problems including severe cases of neurological abnormalities (hypotonia, developmental delay, typical facial dysmorphism, epilepsy), skeletal abnormalities (such as abnormal levels of bone calcification), and hepatic dysfunction (enlarged liver and/or abnormal liver functions). In addition to the severe forms, milder ones of this type are grouped together under the name Zellweger spectrum disorders (ZSD). Neonatal adrenoleukodystrophy (NALD), and infantile Refsum disease (IRD) are included in this spectrum. It is observed that in such patients, peroxisome function is destroyed due to mutations in one of the peroxins (PEX). A proper expression of peroxin is essential for normal peroxisome biogenesis. In cases where it falters, ZSD is seen. ZSD is said to primarily originate due to mutations in any of the currently known PEX genes (15 such genes are known as of now).  

Early studies by Moser and his colleagues  in 1982 underlined the important role of peroxisomes in fatty acid β-oxidation. They also successfully discovered the plasmalogens deficiency in patients with ZS in 1983. They also pinned the critical role of peroxisomes in plasmalogen (ether phospholipid) biosynthesis. They also confirmed the vital function of peroxisomes in alpha fatty acid oxidation as there persisted severely elevated levels of phytanic acid in patients with the syndrome.

A growing database of inherited diseases in human beings have been identified  to exist in which there is an impairment in the peroxisomal beta-oxidation. In some cases this occurs due to the apparent lack of peroxisomes which leads to a generalized loss of peroxisomal functions including peroxisomal beta-oxidation. 

In most of these errors, however, peroxisomes are found to be present in a normal state normally but the impairment is still found due to the defective peroxisomal beta-oxidation (which is mostly due to a single or multiple loss of enzymatic activity of enzymes involved in peroxisomal beta-oxidation). In all such cases, there is accumulation of very-long-chain fatty acids (VLFA) in plasma and this assists biochemical diagnosis of patients affected by an inborn error of peroxisomal beta-oxidation. The diagnosis is done with the help of gas-chromatographic analysis of very-long-chain fatty acids present in the cell plasma. However, subsequent enzymic and immunological investigations are needed to identify the precise nature of enzymatic defects in these patients. 

In patients with PBDs, the tissue morphology is different. It usually shows morphologically aberrant peroxisomes called "peroxisomal ghosts". Accumulation of VLCFAs and phytanic acid coupled with deficient plasmalogens are the hallmark significations of peroxisomal biogenesis disorder in a patient.

However, the most unfortunate effect, seen in all inborn errors of peroxisomal beta-oxidation (known till date) there persist multiple abnormalities, especially of the neurological kind and  the patient succumbs to the abnormality within the first decade of life. Prenatal diagnosis of these disorders has only been recently possible and a definite cure is missing. Research on such matters is therefore crucial.