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.