Biology Reference
In-Depth Information
Chapter 1
Introduction to the Topic
B. Rowe Byers
1.1 The Value, Danger, and Capture of Iron by Bacteria
Astronomers tell us that all stars are furnaces fueled by fusion of hydrogen into
helium. When their fuel is spent, the central cores of stars collapse, reaching
100 million degrees Kelvin. Under these conditions nuclei fuse to form heavier ele-
ments, such as carbon, oxygen, and eventually iron. Some of these dying stars then
explode, scattering the heavy elements through space; subsequently, the remnants
of the explosion re-condense to form new stars surrounded by planets composed of
the heavier elements. Utilizing the electromagnetic force of nature (that is largely
responsible for associations of atoms and molecules) and the lucky coincidence of
the orbit of Earth and the mass of the sun, carbon bound with itself to yield our bio-
logical system. Other heavier elements were incorporated as integral members of the
system; their capacities to catalyze chemical reactions and to bond to specific atoms
were used to build a coherent entity of life. Iron became critical for metabolism.
Iron is valuable because it has two stable valence states that impart a wide
range to the metal's reactivity. However, iron is hazardous. In a biological cell,
free iron can catalyze the formation of hydroxyl radicals and peroxide, both of
which can destroy biological molecules. All cells that utilize iron (and almost all
do) must carefully handle internal iron to prevent escape of the metal. Moreover,
acquisition of iron can be difficult. In an oxidized environment near pH 7 the oxi-
dized ferric form precipitates from solution, a troublesome property requiring an
efficient iron-capture mechanism. In microorganisms the most studied iron-gather-
ing systems are those based on the production of low molecular mass ferric-chelat-
ing molecules called siderophores that bind and deliver iron to the cells. Like
microorganisms, human systems work within the constraints of iron's solubility
and reactivity and iron is present in almost a closed, protected circuit. This normal
B. R. Byers ( * )
University of Mississippi Medical Center, Jackson, MS, USA
e-mail: rowebyers@comcast.net
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