MICROBODIES

MICROBODIES

INTRODUCTION 

          

Ø  A microbody is a type of organelle that is found in the cells of plants ,protozoa, and animals.

Ø  The types of microbody includes Peroxisomes , Glyoxysomes , Glycosomes and Hydrogenosomes.

Ø   In vertebrates , microbodies are especially found in the liver and kidney organs.

HISTORY

Ø  First discovered and named in 1954 by Rhodin.

Ø  In 1956Rouiller and Bernhard presented the first worldwide accepted images of microbodies in liver cell.

Ø  In 1956, ChristiandeDuve and coworkers isolated microbodies from the liver of a rat.

Ø  De Duve also believed that the name microbody was too general and chose peroxisome because of its relationship with hydrogen peroxide.

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In 1967, Breidenbach and Beevers were the first to isolate microbodies from plants, which they named glyoxysomes because they were found to contain enzymes of the glyoxylate cycles.

STRUCTURE

Ø  Microbody is usually a vesicle with a spherical shape, ranging from 0.2-1.5 micrometers in diameter.

Ø  Found in cytoplasm of cell but only visible with the help of an electron microscope.

Ø  Surrounded by a single phospholipid bilayer membrane and contain a matrix of intracellular material including enzymes and other proteins.

Ø  Do not contain any genetic material for self replication.

Ø  the contents of such structures may be amorphous, crystalline or fibrillar in nature.

 

FUNCTION

Ø  Contain enzymes that participate in the prepratory or intermediate stages of biochemical reactions within the cell.

Ø  This facilitates the breakdown of fats, alcohols and amino acids.

Ø  Generally microbodies are involved in detoxification of peroxides and in photorespiration in plants.

Ø  Import proteins into the organelles and aid in proliferation.

Types of Microbodies

·         Peroxisomes

·         Glyoxysomes

·         Hydrogenosomes

·         Glycosomes

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Peroxisomes

Ø  Characterised by their peroxidase activity.

Ø  More or less dense spherical bodies bounded by a single unit membrane.

Ø  Size is about 0.6mm-0.8mm.

Ø  Almost every species in the plant kingdom contain peroxisomes in most of the cell.

Ø  Found on in highervertebrates among animals.

Ø  In higherplants , leaves of C4 plants also contain micro bodies.

Ø  They are mostly associated with mitochondria and plastids and they are mainly responsible for photo-respiration.

Ø  Protein receptors PLX5 and PEX7 with cargo proteins contain PTS1 and PTS2 amino acid sequences.

Enzyme content

Ø  The most important component of the cell organelle is peroxidase or catalase or both.

Ø  Also contains Glycollate oxylate,Glutamate glyoxylase,Transaminases, other enzymes required for the breakdown of fatty acids.

Ø  Coenzymes are FAD,NAD AND NADP.

Ø  Atlast there are 32 known peroxisome proteins.

Ø  They assemble from derivative of ER vesicles and they replicate by fission.

Ø  Peroxisomal proteins translated contain specific peroxisomal targeting sequences PTS at the C-end of proteins (PTS1) or at N-terminal called PTS2 .They are important in unfolded condition.

Ø  Protein receptors PLX5 and PEX7 with cargo proteins contain PTS1 and PTS2 amino acid sequences.Once the proteins are delivered they return to cytoplasm.

Ø  They are characterized by containing H2O2- producing oxidases and catalase.

Ø  The name peroxisome was applied because this organelle is specifically involved in the formation and decomposition of hydrogen peroxide in the cell.

FUNCTION

Ø  Mainly involved in photorespiration in C3 plants like dicots.

Ø  When the intracellular concentration of oxygen is high;RUBP carboxylase present in chloroplasts acts as oxygenase and splits  ribulose diphosphate into phosphoglycerate and phosphoglycollate .The latter is transported into peroxisomes, where phospho glycollate is dephosphorylatated and converted to glyoxalate.

Ø  Then it is converted to Glycine in turn converted to serine in Mitochondria by decarboxylation process.

Ø  As oxygen is utilized this process it is called photorespiration which is insensitive to cyanide ;thus it is distinct from respiratory oxidation.

GLYOXYSOMES

Ø  Distinct from peroxisomes due to different function.

Ø  Found in fungi and other higher plants.

Ø  Storage organs rich in oil globules contain greater number of glyoxysomes.

Ø  Such membranous structures loaded with oil globules are also called spherosomes of 0.5-1.5nm size and they are associated with mitrochondria.

Ø  Contains a host of enzymes responsible for b-oxidation of a fatty acids,citrate synthase,isocitrate lyase,malate synthase,hydrogenase urate oxidase, etc.

Ø  Thus it has all  the three enzymes necessary for fatty acid oxidation as well as gloxylate cycle and gluconeogenesis.

Ø  Glyoxysomes (as all peroxisomes) contain enzymes that initiates the breakdown of fatty acids and additionally possess the enzymes to produce intermediate products for the synthesis of sugars by gluconeogenesis.

Ø  The seedlings uses these sugars synthesized from fats until it is mature enough to produce them by photosynthesis.

Ø  In glyoxysomes the fatty acids are hydrolyzed to to acetyl-CoA by peroxisomal B-oxidation enzymes.

Ø  Besides peroxisomal functions, glyoxysomes possess additionally the key enzymes of glyoxylatecycle(Isocitrate lyase and malate synthase) which accomplish the glyoxylate cycle bypass.

Ø  This is the most interactive organelle with other cell organelles such as mitrochondria,cytoplasm and even chloroplasts.

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Functions

Ø  Converts stored lipids into carbohydrates which are required for growth of plant seedling.

Ø  The conversion of lipids into glucose requires coordinated functions of glyoxysomes,mitochondria and proplastids.

Ø  They first convert lipids into glycerol and fatty acids;then the latter is subjected beta-oxidation.

Ø  In germinating seeds ,the acetyl Co-A  produced during b-oxidation is not drawn into kreb’s cycle, but it is utilized in glyoxylate cycle.

Ø  The succinate ,produced in glyoxylate reactions is transported across the membranes into mitochondria where it is further converted into glucose phosphate through phosphoenol pyruvate pathway.

Ø  The last part of the reaction takes place in proplastids.

Ø  Besides oxidation and glyoxylate reactions, glyoxysomes also contain urate oxidase and allantoinase responsible for the conversion of urate into allantoin.This organelle is present only during a short period in the germination of the lipid rich seed and is absent in lipid poor seed such as the pea.

Ø  In seeds rich in lipids such as castor bean and soya beans, glyoxysomes are the sites for breakdown of fatty acid to succinate.

Ø  The enzymes for glyoxylate cycle are present in glyoxysomes.

Hydrogenosomes

Ø A hydrogenosome is a membrane enclosed organelle of some anaerobic ciliates,trichomonads,fungi and animals.

Ø The hydrogenosomes of trichomonads(the most studied of the hydrogenosome containg micro organisms) produce molecular hydrogen , acetate ,carbon dioxide and ATP by the combined actions of pyruvate: ferredoxinj oxido reductase, hydrogenase , acetate: succinate CoA transferase and succinate thiokinase.

Ø Super oxide dismutase, malate dehydrogenase (decarboxylating), ferredoxin ,adenylate kinase and NADH: ferredoxin oxido-reductase are also localized in the hydrogenosome.

Ø It is nearly universally accepted that hydrogenosomes evolved from mitochondria.

Ø IN 2010, scientists reported their discovery of the first known anaerobic metazoans with hydrogenosome-like organelles

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HYDROGENOSOMES INSIDE PROTOZOA

GLYCOSOMES

Ø  The glycosomes is a membrane- enclosed organelle that contains the glycolytic enzymes.

Ø  The term was first used by Scott and Still in 1968 after they realized that the glycogen in the cell was not static but rather a dynamic molecule.

Ø  It is found in a few species of protozoa including the Kinetoplastida which included the suborders Trypanosomatina and Bodonina, most notably in the human pathogenic trypanosomes, which can cause sleeping sickness and Chagas’s disease,, and leishmaniasis.

Ø  The organelle is bounded by a single membrane and contains a dense proteinaceous matrix.

Ø  It is believed to have evolved from the peroxisome.

Ø  This has been verified by work done on Leishmania genetics.

Ø  The glycosome is currently being researched as a possible target for drug therapies

Ø  Glycosomes are unique to kinetoplastids.

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The term glycosome is also used for glycogen containg structures found in hepatocytes responsible for storing sugar, but these are not membrane bound organelles.