What is Microbodies | Peroxisomes | Glyoxisomes | Structure & Types | Function | B-Oxidation | Glycolate cycle | Cell

Microbodies : [ Structure & Types ]

Microbodies

Microbodies are spherical or oblate in form.

They are bounded by a single membrane and have an interior or matrix which is amorphous or granular.

Microbodies are most easily distinguished from other cell organelles by their content of catalase enzyme.

Recent biochemical studies have distinguished two types of microbodies, namely :

(1) Peroxisomes

(2) Glyoxisomes

These two organelles differ both in their enzyme complement and in the type of tissue in which they are found.

The cell

Peroxisomes :

Peroxisomes are found in animal cells and in the leaves of higher plants.

They contain oxidase and catalase (e.g. D-amino oxidase and Urate oxidase )

In both they participate in the oxidation of substrates, producing hydrogen peroxide which is subsequently destroyed by catalase activity :

(1)

 (2)

In-plant cells, peroxisomes remain associated with ER, chloroplast and mitochondria and are involved in photorespiration.

Glyoxisomes :

Glyoxisomes occur only in plant cells and are particularly abundant in germinating seeds that store fats as a reserve food material.

They contain enzymes of the glyoxylate cycle beside the catalases and oxidases.

Peroxisomes :

Peroxisomes occur in many animal cells and in a wide range of plants.

They are present in all photosynthetic cells of higher plants in etiolated leaf tissue, in coleoptiles and hypocotyls, in tobacco stem and callus, in ripening pear fruits and also in Euglenophyta, protozoa, brown algae, fungi, liverworts, mosses, and ferns.

Structure :

Peroxisomes are variable in size and shape, but usually, appear circular in cross-section having a diameter between 0.2 and 1.5µm ( 0.15 to 0.25 µm diameter in most mammalian tissues; 0.5 µm in rat liver cells )

They have a single limiting unite membrane of lipid and protein molecules, which encloses their granular matrix.

Functions of Peroxisomes :

Peroxisomes are found to perform the following two types of biochemical activities :

[1] Hydrogen Peroxide ( H₂O₂ ) metabolism :

Peroxisomes are so-called because they usually contain one or more enzymes ( i.e. D-amino acid oxidase and Urate oxidase ) that use molecular oxygen to remove hydrogen atoms from a specific organic substrate (R) in or the oxidative reaction that produces hydrogen peroxide ( H₂O₂ ) :

RH₂ + O₂  gives R + H₂O₂

Catalase ( which forms 40% of total peroxisomes protein ) utilizes the H₂O₂ generated by other enzymes in the organelle to oxidize a variety of other substances including alcohol, phenol, formic acid, formaldehyde- by the peroxidative reaction :

H₂O₂ + R’ H₂   gives   R’ + 2H₂O

This type of oxidative reaction is particularly important in liver and kidney cells, whose peroxisomes detoxify various toxic molecules that enter the bloodstream.

Almost half of alcohol one drinks is oxidized to acetaldehyde in this way.

However when excess H₂O₂ accumulates in the cell, catalase converts H₂O₂ to H₂O :

H₂O₂   gives  2H₂O + O₂

[2] Glycolate cycle :

Peroxisomes of plant leaves contain catalase together with the enzyme of glycolate pathway, as glycolate oxidase, glutamate, glyoxylate, serine-glyoxylate and aspartate-d ketoglutarate aminotransferases, hydroxy pyruvate reductase and malic dehydrogenase.

They also contain FAD, NAD and  NADP coenzymes.

The glycolate cycle is thought to bring about the formation of the amino acids- glycine and serine – from the non-phosphorylate intermediates of photosynthetic carbon reduction cycle, i.e. glycerate to serine, or glycolate to glycine and serine in a sequence of reaction which involves chloroplasts, peroxisomes mitochondria and cytosol.

The glycolate pathway also generates C₁ compound and serves as the generator of precursors for nucleic acid biosynthesis.

Photorespiration :

In green leaves, there are peroxisomes that carry out a process called photorespiration which is a light-stimulated production of CO₂ by mitochondria in the dark.

[3] β-Oxidation :

Peroxisomes of rat liver cells contain enzymes of β-oxidation for the metabolism of fatty acids.

They are capable of oxidizing palmitoyl-CoA ( or fatty acyl-CoA ) to acetyl-CoA, using molecular oxygen and NAD as electron acceptors.

[4] Other Function :

Mammalian cells do not contain D-amino acids, but the peroxisomes of mammalian liver and kidney contain D-amino acid oxidase.

Thus, the presumed role of this enzyme is to initiate the degradation of D-amino acid that may arise from breakdown and absorption of peptidoglycan material of intestinal bacteria.

Uric acid oxidase ( uricase ) is important in the catabolic pathway that degrades purines.

Glyoxisomes :

Glyoxisomes are found to occur in the cells of yeast, Neurospora and oil-rich seeds of many higher plants.

They resemble with peroxisomes in morphological details, except that, their crystalloid care consists of dense rods of 6.0 µm diameter.

They have enzymes for fatty acid metabolism and gluconeogenesis, i.e. conversion of stored lipid molecules of spherosomes of germinating seeds into the molecules of carbohydrates.

Functions of Glyoxisomes :

Glyoxisomes perform following the biochemical activity of plant cells :

[1] Fatty acid metabolism :

During germination of oily seeds, the stored lipid molecules of spherosomes are hydrolyzed by the enzyme lipase ( glycerol ester hydrolase ) to glycerol and fatty acids.

The phospholipid molecules are hydrolyzed by the enzyme phospholipase.

The long-chain fatty acids which are released by the hydrolysis are then broken down by the successive removal of two carbon or C₂ fragments in the process of β-oxidation.

Β-Oxidation :

During β-oxidation process, the fatty acid is first activated by enzyme fatty acid thiokinase to a fatty  acyl-CoA which is oxidized by a FAD-linked enzyme fatty acyl-CoA dehydrogenase into trans-2-enoyl-CoA dehydrogenase into trans-2-enoyl-CoA.

Trans-2-enoyl-CoA is hydrated by an enzyme enoyl hydratase or crotonase to produce the L-3-hydroxy acyl-CoA, which is oxidized by a NAD linked L-3-hydroxy acyl-CoA dehydrogenase to produce 3-keto acyl-CoA.

The  3-keto acyl-CoA losses a two-carbon fragment under the action of the enzyme thiolase or β-keto thiolase to generate an acetyl-CoA and a new fatty acyl-CoA with two fewer carbon atoms than the origin.

This new fatty acyl-CoA is then recycled through the same series of reactions until the final two molecules of acetyl-CoA are produced.

The complete the β-oxidation chain can be represented as follows:

B-Oxidation

[2] Glyoxylate cycle :

The glyoxylate pathway occurs in glyoxisomes and it involves some of the reaction of the Krebs cycle in which citrate is formed from oxaloacetate and acetyl-CoA under the action of citrate synthetase enzyme.

The citrate is subsequently converted into isocitrate by aconitase enzyme.

The cycle then involves the enzymatic conversion of isocitrate to glyoxylate and succinate by isocitrate enzyme.

The glyoxylate an another mole of acetyl-co. A form a mole of malate by malate synthesis:

This malate is converted to oxaloacetate by malate dehydrogenase for the cycle to be completed,

Thus, overall, the glyoxylate pathway involves:

Succinate is the end of the glyoxysomal metabolism of fatty acid and is not further metabolized within this organelle.