Proteasomes are multiple-protease complexes that digest proteins targeted for destruction by attachment to ubiquitin. Protein degradation is essential to remove excess enzyme and other proteins that become unnecessary to the cell after they perform their normal functions, and also to remove proteins that were incorrectly folded. Protein encoded by virus should also be destroyed. Proteasomes deal primarily with proteins as individual molecules, whereas lysosomes digest bulk material introduced into the cell or whole organelles and vesicles.
The proteasome has a core particle with the shape of a barrel made of four rings stacked on each other. At each end of the core particle is a regulatory particle that contains ATPase and recognizes proteins with ubiquitin molecules attached. Ubiquitin is a small protein (76 amino acids) found in all cells and is highly conserved during evolution—it has virtually the same structure from bacteria to humans. Ubiquitin targets proteins for destruction as follows. A molecule of ubiquitin binds to a lysine residue in the protein to be degraded. Then other ubiquitin molecules attach to the first one; the complex is recognized by the regulatory particle; the protein is unfolded by the ATPases using energy from ATP; and the protein is translocated into the core particle, where it is broken into peptides of about eight amino acids each. These peptides are transferred to the cytosol by a process yet unknown. The ubiquitin molecules are released by the regulatory particles for reuse.
The eight-amino acid peptides may be broken down to amino acids by cytosol enzymes, or they may have other destinations (eg, in some cells they participate in the immune response).
Peroxisomes (peroxide + soma, body) are spherical membrane-limited organelles whose diameter ranges from 0.5 to 1.2 m (see Figure 2–39). Like the mitochondria, they utilize oxygen but do not produce ATP and do not participate directly in cellular metabolism. Peroxisomes oxidize specific organic substrates by removing hydrogen atoms that are transferred to molecular oxygen (O2). This activity produces hydrogen peroxide (H2O2), a substance that is very damaging to the cell. However, H2O2 is eliminated by the enzyme catalase, which is present in peroxisomes. Catalase transfers oxygen atoms from H2O2 to several compounds and also decomposes H2O2 to H2O and O2 (2 H2O2 2 H2O + O2). Catalase activity also has clinical implications. It degrades several toxic molecules and prescription drugs, particularly in liver and kidney peroxisomes. For example, 50% of ingested ethyl alcohol is degraded to acetic aldehyde in liver and kidney peroxisomes. Liver and kidney peroxisomes show a higher variation in their enzyme complement than do other peroxisomes. Their homogeneous matrix contains D- and L-amino oxidases, catalase, and hydroxyacid oxidase. In some species, but not humans, a crystalline nucleoid is present that is composed of urate oxidase.
Peroxisomes contain enzymes involved in lipid metabolism. Thus, the -oxidation of long-chain fatty acids (18 carbons and longer) is preferentially accomplished by peroxisomal enzymes that differ from their mitochondrial counterparts. Certain reactions leading to the formation of bile acids and cholesterol also have been localized in highly purified peroxisomal fractions.
Peroxisomal enzymes are synthesized on free cytosolic polyribosomes, with a small sequence of amino acids located near the carboxyl terminus that functions as an import signal. Proteins with this signal are recognized by receptors located in the membrane of peroxisomes and internalized into the organelle. The peroxisome grows in size and is divided into two smaller peroxisomes, by a mechanism not completely understood.
A large number of disorders arise from defective peroxisomal proteins, because this organelle is involved in several metabolic pathways. Probably the most common peroxisomal disorder is X-chromosome-linked adrenoleukodystrophy, caused by a defective integral membrane protein that participates in transporting very long-chain fatty acids into the peroxisome for -oxidation. Accumulation of these fatty acids in body fluids destroys the myelin sheaths in nerve tissue, causing severe neurological symptoms. Deficiency in peroxisomal enzymes causes the fatal Zellweger syndrome, with severe muscular impairment, liver and kidney lesions, and disorganization of the central and peripheral nervous systems. Electron microscopy reveals empty peroxisomes in liver and kidney cells of these patients.
Secretory Vesicles, or Granules
Secretory vesicles are found in those cells that store a product until its release is signaled by a metabolic, hormonal, or neural message (regulated secretion). These vesicles are surrounded by a membrane and contain a concentrated form of the secretory product (Figure 2–29). The contents of some secretory vesicles may be up to 200 times more concentrated than those in the cisternae of the RER. Secretory vesicles containing digestive enzymes are referred to as zymogen granules.
Electron micrograph of a pancreatic acinar cell from the rat. Numerous mature secretory granules (S) are seen in association with condensing vacuoles (C) and the Golgi complex (G). x18,900.