25. Self-Sustaining Chemical        Systems: Living Cells   Previous PageNext Page
       Lysosomes and Peroxisomes

Lysosomes are small cell vesicles that contain enzymes for degrading proteins, nucleic acids, and polysaccharides. The lysosomes segregate these dangerous enzymes from the body of the cell, thereby permitting them to play a digestive role without damaging their host. A white blood cell, scavenging for foreign bacteria, will absorb an intruder and destroy it with the hydrolytic enzymes in its lysosomes. Upon the death of a cell in a multicelled organism, lysosomes rupture and digest the cell contents. They have been called "suicide vesicles" and compared with the cyanide capsules familiar from spy novels, but this may unfairly neglect the digestive and degradative functions that they carry out during the life of the cell.

Peroxisomes are more of a mystery. They contain the enzyme catalase, which is possibly one of the earliest heme proteins and a precursor, or at least a predecessor, of cytochromes and globins. Catalase is one of the largest single-chain enzymes, containing more than 500 amino acids in one polypeptide chain, and a heme group. Its only known function is to destroy hydrogen peroxide, either with or without the release of oxygen:

In the non-oxygen-releasing reaction above, HR represents any oxidizable organic compound.

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