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in Mammals Molecular Biology 4 Last update: 8/14/05 ------------------------------------------------------------------------------------
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Mitochondria
are found in all human cells except red blood cells. Therefore, in
all of these cells except RBCs, it is safe to say that oxygen is used in
electron transport in mitochondria in the step involving respiratory complex
IV where the enzyme cytochrome c oxidase combines two electrons
(taken from cytochrome c), two protons and an oxygen atom to produce water:
2 e- + 2 H+ +
1/2 O2 ----> H2O.
However,
in many cells there are other organelles called peroxisomes that
also use oxygen in the oxidation of food molecules such as long-chain fatty
acids with carbon counts from 16 to 30. They also employ oxygen to
deal with a breakdown product from some proteins and nucleic acids known
as urate. Enzymes that use oxygen are known as oxidases. Oxygen
use does not result in water formation directly. Instead the very
toxic hydrogen peroxide molecule (H2O2) is formed.
You may recall that hydrogen peroxide is a common component of household
disinfectants. What do we do with organelles filled with peroxide? Fortunately
another enzyme,
catalase, degrades the hydrogen peroxide into water
and oxygen. Peroxisomes gained their names based on the discovery
that hydrogen peroxide is degraded only in peroxisomes.
Figure 1: Animal cell organelles. (Ref.)
Never
heard of peroxisomes? Sometimes they are referred to as "microbodies".
In most introductory biology textbooks, they get little attention in comparison
to the other "big" organelles such as mitochondria, ribosomes, chloroplasts,
endoplasmic reticula and the nucleus, but they are very important for survival.
This importance has been shown by the discovery of several inherited diseases
(Inborn Errors
of Metabolism) that can lead to dire consequences. One was brought
to national attention through the 1993 film Lorenzo's
Oil.
To learn more about how oxygen is used in peroxisomes, visit page 2.
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