Lysosomes and Peroxisomes

Lysosomes

Lysosomes are roughly spherical bodies enclosed by a single membrane. They are manufactured by the Golgi apparatus (pathway 2 in the figure).

Link to a discussion of how proteins synthesized in the endoplasmic reticulum are sent to the appropriate destinations.
They contain over 3 dozen different kinds of hydrolytic enzymes including

The pH within the lysosome is about pH 5, substantially less than that of the cytosol (~pH 7.2). All the enzymes in the lysosome work best at an acid pH. This reduces the risk of their digesting their own cell if they should escape from the lysosome.

At one time, it was thought that lysosomes were responsible for killing cells scheduled to be removed from a tissue; for example, the resorption of its tail as the tadpole metamorphoses into a frog. This is incorrect. These examples of programmed cell death (PCD) or apoptosis take place by an entirely different mechanism. Link to a discussion of apoptosis.

Materials within the cell scheduled for digestion are first deposited within lysosomes. These may be:

Lysosomal Storage Diseases

Lysosomal storage diseases are caused by the accumulation of macromolecules (proteins, polysaccharides, lipids) in the lysosomes because of a genetic failure to manufacture an enzyme needed for their breakdown. Neurons of the central nervous system are particularly susceptible to damage.

Most of these diseases are caused by the inheritance of two defective alleles of the gene encoding one of the hydrolytic enzymes.

Examples:

However, one lysosomal storage disease, I-cell disease ("inclusion-cell disease"), is caused by a failure to "tag" (by phosphorylation) all the hydrolytic enzymes that are supposed to be transported from the Golgi apparatus to the lysosomes. Lacking the mannose 6-phosphate (M6P) tag, they are secreted from the cell instead.

Discussion of how proteins are targeted to lysosomes.

The result: all the macromolecules incorporated in lysosomes remain undegraded forming "inclusion bodies" in the cell.

Secretory Lysosomes

In some cells, lysosomes have a secretory function — releasing their contents by exocytosis.

Peroxisomes

Peroxisomes are about the size of lysosomes (0.5–1.5 µm) and like them are enclosed by a single membrane. They also resemble lysosomes in being filled with enzymes.

However, peroxisomes bud off from the endoplasmic reticulum, not the Golgi apparatus (that is the source of lysosomes).

The enzymes and other proteins destined for peroxisomes are synthesized in the cytosol. Each contains a peroxisomal targeting signal (PTS) that binds to a receptor molecule that takes the protein into the peroxisome and then returns for another load.

Two peroxisomal targeting signals have been identified:

Each has its own receptor to take it to the peroxisome.

Some of the functions of the peroxisomes in the human liver:

Peroxisomes are also present in plant cells where they participate is such functions as

Peroxisome Disorders

A variety of rare inherited disorders of peroxisome function occur in humans.

Peroxisomes are also called microbodies.

Link to schematic showing lysosomes and peroxisomes in a typical animal cell.
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24 April 2011