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

Catalase is one of the most efficient and rapid enzymes known. One molecule of the enzyme can destroy ten million molecules of HO per second. The various kinases of the citric acid cycle have comparable turnover numbers (rates of reaction) of 1000 substrate molecules acted upon per second per enzyme molecule. Chymotrypsin has a turnover number of 300 molecules per second, and a succinate dehydrogenase molecule only dehydrogenates 20 succinic acid molecules per second. As a catalyst for the destruction of HO, catalase is ten million times as rapid as a simple heme group is, and ten billion times as fast as a ferric ion. It does a small job, but does it supremely well.

By this hypothesis, reactions of organic compounds with O first began, not as a means of extracting energy from organic molecules, but as a way of detoxifying the cell and eliminating the adverse effects of O. Harnessing of the energy released by combination with O came later. The theory is plausible. Peroxisomes also contain rudimentary metabolic cycles that reduce flavoproteins, and which may be the vestigial remains of bypassed respiratory mechanisms. The peroxisomes could be the remnants of a primitive respiratory system that was abandoned by the cell when it struck up a symbiotic relationship with the bacterial ancestors of mitochondria. By this theory, these metabolic relics are reduced today to doing the only thing that mitochondria cannot do better: eliminate peroxide.

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