Figure 12–27 An electron micrograph of three peroxisomes in a rat liver cell. The paracrystalline, electron-dense inclusions are composed primarily of the enzyme urate oxidase. (Courtesy of Daniel s. friend.)
Figure 12–28 The structure of a plasmalogen. plasmalogens are very abundant in the myelin sheaths that insulate the axons of nerve cells. They make up some 80–90% of the myelin membrane phospholipids. in addition to an ethanolamine head group and a long-chain fatty acid attached to the same glycerol phosphate backbone used for phospholipids, plasmalogens contain an unusual fatty alcohol that is attached through an ether linkage (bottom left).
Figure 12–29 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. The vacuole in plant cells is equivalent to the lysosome in animal cells. (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 droplets that store fat, reflecting their central role in fat mobilization and gluconeogenesis during seed germination. (a, from s.e. frederick and e.h. newcomb, J. Cell Biol. 43:343–353, 1969. With permission from The rockefeller press; B, from W.p. Wergin, p.J. Gruber and e.h. newcomb, J. Ultrastruct. Res. 30:533–557, 1970. With permission from academic press.) delivering its cargo to the peroxisome lumen, Pex5 undergoes ubiquitylation. This modification is required to release Pex5 back into the cytosol, where the ubiquitin is removed. An ATPase composed of Pex1 and Pex6 harnesses the energy of ATP hydrolysis to help release Pex5 from peroxisomes.
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 profound peroxisomal deficiency. These individuals, whose cells contain “empty” peroxisomes, have severe abnormalities in their brain, liver, and kidneys, and they die soon after birth. A mutation in the gene encoding peroxin Pex5 causes one form of the disease. A defect in Pex7, the receptor for the N-terminal import signal, causes a milder peroxisomal disease.
It has long been debated whether new peroxisomes arise from preexisting ones by organelle growth and fission—as mentioned earlier for mitochondria and plastids—or whether they derive as a specialized compartment from the endoplasmic reticulum (ER). Aspects of both views are true (Figure 12–30). Most peroxisomal membrane proteins are made in the cytosol and insert into the membrane of