Lipid Catabolism :-

Plant Peroxisomes

&

Glyoxisomes

INTRODUCTION

Peroxisomes are present in almost all eukaryotic cells. They participate in the metabolism of fatty acids and many other metabolites. Peroxisomes harbor enzymes that rid the cell of toxic peroxides. Peroxisomes are bound by a single membrane that separates their contents from the cytosol (the internal fluid of the cell) and contain membrane proteins critical for various functions, such as importing proteins into the organelles and aiding in proliferation. Peroxisomes can replicate by enlarging and then dividing. Peroxisomes were identified as organelles by the Belgian cytologist Christian de Duve in 1967.

Basic structure of peroxisomes

Peroxisomes differ from mitochondria and chloroplasts in many ways. Most notably, they are surrounded by only a single membrane, and they do not contain DNA or ribosomes. Like mitochondria and chloroplasts, however, peroxisomes are thought to acquire their proteins by selective import from the cytosol. But because they have no genome, all of their proteins must be imported. Peroxisomes thus resemble the ER in being a self-replicating, membrane-enclosed organelle that exists without a genome of its own.Peroxisomes are found in all eukaryotic cells. They contain oxidative enzymes, such as catalase and urate oxidase, at such high concentrations that in some cells the peroxisomes stand out in electron micrographs because of the presence of a crystalloid core.

FUNCTIONA major function of the peroxisome is the breakdown of fatty acid molecules, in a process called beta-oxidation. In this process, the fatty acids are broken down two carbons at a time, converted to Acetyl-CoA, which is then transported back to the cytosol for further use. In animal cells, beta-oxidation can also occur in the mitochondria. However, in plant cells this process is exclusive for the peroxisome.

The oxidation of a fatty acid is accompanied by the production of hydrogen peroxide (H2O2) from oxygen. The hydrogen peroxide is decomposed by catalase, either by conversion to water or by oxidation of another organic compound (designated AH2).

Fatty acid oxidation also occurs in peroxisomes, when the fatty acid chains are too long to be handled by the mitochondria. However, the oxidation ceases at octanyl CoA. It is believed that very long chain (greater than C-22) fatty acids undergo initial oxidation in peroxisomes which is followed by mitochondrial oxidation.

In a process called β oxidation, the alkyl chains of fatty acids are shortened sequentially by blocks of two carbon atoms at a time, thereby converting the fatty acids to acetyl CoA. The acetyl CoA is then exported from the peroxisomes to the cytosol for reuse in biosynthetic reactions.

One significant difference is that oxidation in peroxisomes is not coupled to ATP synthesis. Instead, the high-potential electrons are transferred to O2, which yields H2O2. The enzyme catalase, found exclusively in peroxisomes, converts the hydrogen peroxide into water and oxygen.

Peroxisomal β-oxidation also requires enzymes specific to the peroxisome and to very long fatty acids. There are three key differences between the enzymes used for mitochondrial and peroxisomal β-oxidation:i) β-oxidation in the peroxisome requires the use of a peroxisomal carnitine acyltransferase (instead of carnitine acyltransferase I and II used by the mitochondria) for transport of the activated acyl group into the peroxisome.ii) The first oxidation step in the peroxisome is catalyzed by the enzyme acyl CoA oxidase.iii) The β- ketothiolase used in peroxisomal β-oxidation has an altered substrate specificity, different from the mitochondrial β-ketothiolase.

In higher plants, peroxisomes contain also a complex battery of antioxidative enzymes such as superoxide dismutase, the components of the ascorbate-glutathione cycle, and the NADP- dehydrogenases of the pentose-phosphate pathway. It has been demonstrated the generation of superoxide (O2

•-) and nitric oxide (•NO) radicals.

Peroxisomes are very important in plants. Two different types have been studied extensively. One type is present in leaves, where it catalyzes the oxidation of a side product of the crucial reaction that fixes CO2 in carbohydrate . This process is called photorespiration because it uses up O2 and liberates CO2. The other type of peroxisome is present in germinating seeds, where it has an essential role in converting the fatty acids stored in seed lipids into the sugars needed for the growth of the young plant. Because this conversion of fats to sugars is accomplished by a series of reactions known as the glyoxylate cycle, these peroxisomes are also called glyoxysomes . In the glyoxylate cycle, two molecules of acetyl CoA produced by fatty acid breakdown in the peroxisome are used to make succinic acid, which then leaves the peroxisome and is converted into glucose. The glyoxylate cycle does not occur in animal cells, and animals are therefore unable to convert the fatty acids in fats into carbohydrates.

Electron micrographs of two types of peroxisomes found in plant cells(A) A peroxisome with a paracrystalline core in a tobacco leaf mesophyll cell. Its close association with chloroplasts is thought to facilitate the exchange of materials between these organelles during photorespiration. (B) Peroxisomes in a fat-storing cotyledon cell of a tomato seed 4 days after germination. Here, the peroxisomes (glyoxysomes) are associated with the lipid bodies where fat is stored, reflecting their central role in fat mobilization and gluconeogenesis during seed germination. 

Cell-wall containing organisms, such as plants, fungi, and bacteria, require very large amounts of carbohydrates during growth for the biosynthesis of complex structural polysaccharides, such as cellulose, glucans, and chitin. In these organisms, in the absence of available carbohydrates (for example, in certain microbial environments or during seed germination in plants), the glyoxylate cycle permits the synthesis of glucose from lipids via acetate generated in fatty acid β-oxidation.

The glyoxylate cycle bypasses the steps in the citric acid cycle where carbon is lost in the form of CO2. The two initial steps of the glyoxylate cycle are identical to those in the citric acid cycle: acetate → citrate → isocitrate. In the next step, catalyzed by the first glyoxylate cycle enzyme, isocitrate lyase, isocitrate undergoes cleavage into succinate and glyoxylate (the latter gives the cycle its name). Glyoxylate condenses with acetyl-CoA (a step catalyzed by malate synthase), yielding malate. Both malate and oxaloacetate can be converted into phosphoenol pyruvate, which is the substrate of phosphoenol pyruvate carboxykinase, the first enzyme in gluconeogenesis. The net result of the glyoxylate cycle is therefore the production of glucose from fatty acids. Succinate generated in the first step can enter into the citric acid cycle to eventually form oxaloacetate.

Glyoxylate Cycle

References

•en.wikipedia.org/wiki/Peroxisome

•http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?

book=mboc4&part=A2194

•http://www.cytochemistry.net/Cell-biology/lysosome.htm

•en.wikipedia.org/wiki/Glyoxylate_cycle