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.
Because we do not discuss peroxisomes elsewhere, we shall digress to consider some of the functions of this diverse family of organelles, before discussing their biosynthesis. Peroxisomes are found in all eucaryotic 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 (Figure 12-31).
Like mitochondria, peroxisomes are major sites of oxygen utilization. One hypothesis is that peroxisomes are a vestige of an ancient organelle that performed all the oxygen metabolism in the primitive ancestors of eucaryotic cells. When the oxygen produced by photosynthetic bacteria first began to accumulate in the atmosphere, it would have been highly toxic to most cells. Peroxisomes might have served to lower the intracellular concentration of oxygen, while also exploiting its chemical reactivity to perform useful oxidative reactions. According to this view, the later development of mitochondria rendered peroxisomes largely obsolete because many of the same reactions—which had formerly been carried out in peroxisomes without producing energy—were now coupled to ATP formation by means of oxidative phosphorylation. The oxidative reactions performed by peroxisomes in present-day cells would therefore be those that have important functions not taken over by mitochondria.
Peroxisomes Use Molecular Oxygen and Hydrogen Peroxide to Perform Oxidative Reactions
Peroxisomes are so named because they usually contain one or more enzymes that use molecular oxygen to remove hydrogen atoms from specific organic substrates (designated here as R) in an oxidative reaction that produces hydrogen peroxide (H2O2):
Catalase utilizes the H2O2 generated by other enzymes in the organelle to oxidize a variety of other substrates—including phenols, formic acid, formaldehyde, and alcohol—by the “peroxidative” reaction: H2O2 + R′ H2 → R′ + 2H2O. This type of oxidative reaction is particularly important in liver and kidney cells, where the peroxisomes detoxify various toxic molecules that enter the bloodstream. About 25% of the ethanol we drink is oxidized to acetaldehyde in this way. In addition, when excess H2O2 accumulates in the cell, catalase converts it to H2O through the reaction:
A major function of the oxidative reactions performed in peroxisomes is the breakdown of fatty acid molecules. 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. In mammalian cells, β oxidation occurs in both mitochondria and peroxisomes; in yeast and plant cells, however, this essential reaction occurs exclusively in peroxisomes.
An essential biosynthetic function of animal peroxisomes is to catalyze the first reactions in the formation of plasmalogens, which are the most abundant class of phospholipids in myelin (Figure 12-32). Deficiency of plasmalogens causes profound abnormalities in the myelination of nerve cells, which is one reason why many peroxisomal disorders lead to neurological disease.
Peroxisomes are unusually diverse organelles, and even in the various cell types of a single organism they may contain different sets of enzymes. They can also adapt remarkably to changing conditions. Yeast cells grown on sugar, for example, have small peroxisomes. But when some yeasts are grown on methanol, they develop large peroxisomes that oxidize methanol; and when grown on fatty acids, they develop large peroxisomes that break down fatty acids to acetyl CoA by β oxidation.
Peroxisomes are also 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 (Figure 12-33A). As discussed in Chapter 14, 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 (Figure 12-33B). In the glyoxylate cycle, two molecules of acetyl CoA produced by fatty acidbreakdown 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.
A Short Signal Sequence Directs the Import of Proteins into Peroxisomes
A specific sequence of three amino acids located at the C terminus of many peroxisomal proteins functions as an import signal (see Table 12-3). Other peroxisomal proteins contain a signal sequence near the N terminus. If either of these sequences is experimentally attached to a cytosolic protein, the protein is imported into peroxisomes. The import process is still poorly understood, although it is known to involve soluble receptor proteins in the cytosol that recognize the targeting signals, as well as docking proteins on the cytosolic surface of the peroxisome. At least 23 distinct proteins, called peroxins, participate as components in the process, which is driven by ATP hydrolysis. Oligomeric proteins do not have to unfold to be imported into peroxisomes, indicating that the mechanism is distinct from that used by mitochondria and chloroplasts and at least one soluble import receptor, the peroxin Pex5, accompanies its cargo all the way into peroxisomes and, after cargo release, cycles back out into the cytosol. These aspects of peroxisomal protein import resemble protein tranport into the nucleus.
The importance of this import process and of peroxisomes is demonstrated by the inherited human disease Zellweger syndrome, in which a defect in importing proteins into peroxisomes leads to a severe peroxisomal deficiency. These individuals, whose cells contain “empty” peroxisomes, have severe abnormalities in their brain, liver, and kidneys, and they die soon after birth. One form of this disease has been shown to be due to a mutation in the gene encoding a peroxisomal integral membrane protein, the peroxin Pex2, involved in protein import. A milder inherited peroxisomal disease is caused by a defective receptor for the N-terminal import signal.
Most peroxisomal membrane proteins are made in the cytosol and then insert into the membrane of preexisting peroxisomes. Thus, new peroxisomes are thought to arise from preexisting ones, by organelle growth and fission—as mentioned earlier for mitochondria and plastids, and as we describe below for the ER (Figure 12-34).
Peroxisomes are specialized for carrying out oxidative reactions using molecular oxygen. They generate hydrogen peroxide, which they use for oxidative purposes—destroying the excess by means of the catalase they contain. Peroxisomes also have an important role in the synthesis of specialized phospholipids required for nerve cellmyelination. Like mitochondria and plastids, peroxisomes are thought to be self-replicating organelles. Because they contain no DNA or ribosomes, however, they have to import their proteins from the cytosol. A specific sequence of three amino acids near the C terminus of many of these proteins functions as a peroxisomal import signal. The mechanism of protein import is distinct from that of mitochondria and chloroplasts, and oligomeric proteins can be transported into peroxisomes without unfolding.
By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.