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 of Peroxisomes
Peroxisomes are variable in size and shape, but usually appear circular in cross-sections 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 unit membrane of lipid and protein molecules, which encloses their granular matrix.
In some cases (e.g., in the festuciod grasses) the matrix contains numerous threads or fibrils, while in others they are observed to contain either an amorphous nucleoid of a dense inner core which in many species shows a regular crystalloid structure.
Little is known about the function of the core, except that it is the site of the enzyme urate oxidase in rat liver peroxisomes and much of the catalase in some plants.
Functions of Peroxisomes
Peroxisomes are found to perform the following two types of biochemical activities:
A. Hydrogen peroxide 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 specific organic substrates (R) in an oxidative reaction that produces hydrogen peroxide (H2O2):
RH2+O2 → R + H2O2
Catalase (which forms 40% of total peroxisome protein) utilizes the H2O2 generated by other enzymes in the organelle to oxidize a variety of other substances—including alcohols, phenols, formic acid, and formaldehyde—by the “peroxidative” reaction:
H2O2 + R′ H2 → 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 the alcohol one drinks is oxidized to acetaldehyde in this way. However, when excess H2O2 accumulates in the cell, catalase converts H2O2 to H2O:
2H2O2 → 2H2O + O2
H 2O2 and aging. Most cytosolic H 2O2 is produced by mitochondria and membranes of the endoplasmic reticulum, although there are also H2O2 -producing enzymes localized in the cytoplasmic matrix. Catalase acts as a “safety valve” for dealing with the large amounts of H2O2 generated by peroxisomes;
However, other enzymes such as glutathione peroxidase; are capable of metabolizing organic hydroperoxides and also H2O2, in the cytosol (cytoplasmic matrix) and mitochondria.
The production of superoxide anion (O2–) in mitochondria and cytosol (cytoplasmic matrix) is regulated mainly by the enzyme superoxide dismutase. All of these protective enzymes are present at high levels in aerobic tissues.
Recently, a possible relationship has been stressed between peroxides and free radicals (such as superoxide anion O2–) with the process of aging. These radicals may act on DNA molecules to produce mutations altering the transcription into mRNA and the translation into proteins.
In addition, free radicals and peroxides can affect the membranes by causing the peroxidation of lipids and proteins. For these reasons reducing compounds such as vitamin E or enzymes such as superoxide dismutase could play a role in keeping the healthy state of a cell.
B. Glycolate cycle.
Peroxisomes of plant leaves contain catalase together with the enzymes of the glycolate pathway, as glycolate oxidase, glutamate glyoxylate, serine-glyoxylate, and asparate-α- 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-phosphorylated intermediates of photosynthetic carbon reduction cycle, i.e., glycerate to serine, or glycolate to glycine and serine in a sequence of reactions which involve chloroplasts, peroxisomes, mitochondria, and cytosol.
The glycolate pathway also generates C1 compounds and serves as the generator of precursors for nucleic acid biosynthesis.
In green leaves, there are peroxisomes that carry out a process called photorespiration which is a light-stimulated production of CO2 that is different from the generation of CO2 by mitochondria in the dark.
In photorespiration, glycolic acid (glycolate), a two-carbon product of photosynthesis is released from chloroplasts and oxidized into glyoxylate and H2O2 by a peroxisomal enzyme called glycolic acid oxidase. Later on, glyoxylate is oxidized into CO2 and formate:
CH2OH. COOH + O2 → CHO – COOH + H2O2
CHO — COOH + H2O2 → HCOOH + CO2 + H2O
Photorespiration is so-called because light induces the synthesis of glycolic acid in chloroplasts. The entire process involves the intervention of two basic organelles: chloroplasts and peroxisomes.
Lastly, photorespiration is driven by atmospheric conditions in which the O2 tension is high and the CO2 tension low. Apparently, O2 competes with CO2 for the enzyme ribulose diphosphate carboxylase which normally is the key enzyme in CO2 fixation during photosynthesis.
When O2 is used by the enzyme, an unstable intermediate is formed which breaks down into 3-phosphoglycerate and phosphoglycolate. The latter tends to increase the glycolate concentration by removal of its phosphate group and, therefore, more glycolate is available for additional oxidation and CO2 release.
Photorespiration is a wasteful process for the plant cell, since, it significantly reduces the efficiency of the process of photosynthesis (i.e., it returns a portion of fixed CO2 to the atmosphere). It is a particular problem in C3 plants that are more readily affected by low CO2 tensions; C4 plants are much more efficient in this regard.
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.
The acetyl- CoA formed by this process is, eventually, transported to the mitochondria where it enters into the citric acid cycle. If alternatively, acetyl-CoA remains in the cytosol, it is reconverted into fatty acids and ultimately to neutral fats. The β-oxidation pathway of the peroxisomes is very similar to the one that occurs in mitochondria with one very important exception.
In mitochondria, the flavin dehydrogenase donates its electrons to the respiratory chain. It does not react with molecular oxygen. In peroxisomes, then dehydrogenase reacts directly with O2 and in so doing generates H2O2. Mitochondria contain no catalase and, therefore, cannot deal with the formation of toxic hydrogen peroxide. For peroxisomes, this is not a problem.
D. Other functions.
Mammalian cells do not contain D-amino acids, but the peroxisomes of the mammalian liver and kidneys contain D-amino acid oxidase. It is suggested that this enzyme is meant for D-amino acids that are found in the cell wall of the bacteria.
Thus, the presumed role of this enzyme is to initiate the degradation of D-amino acid that may arise from the breakdown and absorption of peptidoglycan material of intestinal bacteria. Uric acid oxidase (uricase) is important in the catabolic pathway that degrades purines.
Thus, peroxisomes are unusually diverse organelles, and even in different cells of a single organism may contain very different sets of enzymes. They can also adapt remarkably to changing conditions. For example, yeast cells grown on sugar have tiny peroxisomes.
But when some yeasts are grown on methanol, they develop large-sized peroxisomes that oxidize methanol; when grown on fatty acids, they develop large peroxisomes that break down fatty acids to acetyl-CoA.