Fragments i i



Enzymes that degrade proteins are dangerous. They are usually kept in separate compartments and often made as inactive precursors.

Examples are trypsin (and its precursor trypsinogen) and pepsin (and its precursor pepsinogen). Plants that catch insects, fungi that trap nematodes and bacteria that live in rotting animal or plant tissue also secrete proteases. As with animals, these proteases are generally secreted as inactive precursors and only activated once outside the cells of the producer organism.

Lysosomes are membrane-bound organelles found in eukaryotic cells. They contain a variety of digestive enzymes, including proteases, and function in self-defense. When cells of the immune system have engulfed bacteria or virus particles, the vesicle containing the invader is merged with lysosomes and the infectious agent is, hopefully, digested (Fig. 8.33B). Bacteria do not always cooperate—for example, many pathogenic strains of Salmonella can survive the toxins and digestive enzymes inside lysosomes.

Proteases located in the cytoplasm itself must be very carefully controlled. Nonetheless, the cell needs some internal proteases to degrade damaged or mis-folded proteins. The proteases found inside bacterial cells tend to form rings, with the dangerous active site on the inside of the ring. Proteins slated for destruction are ferried to the protease ring and pushed into its center by accessory proteins. The number of lysosome Membrane bound organelle of eukaryotic cells that contains degradative enzymes mis-folded proteins and consequently the level of protein degradation increases greatly under certain conditions, in particular when cells are exposed to uncomfortably high temperatures that tend to disrupt protein structure. This induces the heat shock response described in more detail in Ch. 9.

Eukaryotes have more sophisticated structures, known as proteasomes. These are cylindrical, with the protease active sites inside. The top and bottom of the cylinder are covered by protein complexes that recognize and bind damaged or unwanted proteins. Proteins destined for degradation are recognized because they are tagged with ubiq-uitin. This is a small protein that is fixed to damaged or mis-folded proteins and also to certain proteins that are needed only for a brief period (Fig. 8.34). The ubiquitin tagged proteins are unfolded and then fed into the barrel of the proteasome where they are degraded into short peptides. The ubiquitin tags themselves are cleaved off and recycled.

proteasome Protein assembly found in eukaryotic cells that degrades proteins ubiquitin Small protein attached to other proteins as a signal that they should be degraded;used by eukaryotic cells, not bacteria

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