A major function of the oxidation reactions performed in peroxisomes is the breakdown of fatty acid molecules. The process, called β oxidation, shortens the alkyl chains of fatty acids sequentially in blocks of two carbon atoms at a time, thereby converting the fatty acids to acetyl CoA. The peroxisomes then export the acetyl CoA to the cytosol for use 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 12–28). Plasmalogen deficiencies cause profound abnormalities in the myelination of nerve-cell axons, 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 also adapt remarkably to changing conditions. Yeasts grown on sugar, for example, have few small peroxisomes. But when some yeasts are grown on methanol, numerous large peroxisomes are formed that oxidize methanol; and when grown on fatty acids, they develop numerous large peroxisomes that break down fatty acids to acetyl CoA by β oxidation.
Peroxisomes are also important in plants. Two types of plant peroxisomes have been studied extensively. One is present in leaves, where it participates in photo-respiration (discussed in Chapter 14) (Figure 12–29A). The other type of peroxisome is present in germinating seeds, where it converts 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 12–29B). 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 in the cytosol. 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 (Ser–Lys–Leu) located at the C-terminus of many peroxisomal proteins functions as an import signal (see Table 12–3, p. 648). Other peroxisomal proteins contain a signal sequence near the N-terminus. If either sequence is attached to a cytosolic protein, the protein is imported into peroxisomes. The import signals are first recognized by soluble receptor proteins in the cytosol. Numerous distinct proteins, called peroxins, participate in the import process, which is driven by ATP hydrolysis. A complex of at least six different peroxins forms a protein translocator in the peroxisome membrane. Even oligomeric proteins do not have to unfold to be imported. To allow the passage of such compactly folded cargo molecules, the pore formed by the transporter is thought to be dynamic in its dimensions, adapting in size to the particular cargo molecules to be transported. In this respect, the mechanism differs from that used by mitochondria and chloroplasts. One soluble import receptor, the peroxin Pex5 recognizes the C-terminal peroxisomal import signal. It accompanies its cargo all the way into peroxisomes and, after cargo release, cycles back to the cytosol. After