Mycobacterial Iron Acquisition Mechanisms - Iron Acquisition by the Genus Mycobacterium - page 41

these mineral-rich hydrothermal mounds served as early energy transforming mol-
ecules for carbon fixation by CO 2 reduction [ 2 , 3 ]. Current dependency of most
energy transducing metabolic sequences on iron containing cofactors argues for
pivotal use of the primordial versions of these cofactors in the expansion of these
reactions. Subsequent enclosure of the nascent biological system in a membrane
vesicle allowed development of a greater range of iron cofactors, including those
required for redox reactions, as well as the iron sensors of cellular redox status and
ferritin-like proteins for iron storage and protection from iron induced damage [ 4 ].
For example, the iron-sulfur complexes (with a later acquired protein component)
probably were the precursors of today's ferredoxins [ 1 ] and in some microorgan-
isms non-heme iron proteins can replace the coenzyme NADP [ 5 ]. The encircling
membrane also permitted control of the entry and exit of substances. As early
Earth is considered to have been anaerobic, iron would have been present in its
soluble ferrous form and transport of the metal into the vesicle could have used the
early prototypes of metal symporters or antiporters.
In an unusual comparison to human technology, Wachtershäuser [ 1 ] suggested
that cell envelopes were like space suits which enabled existence in uninhabitable
regions. The surface of the planet was abundant with radiant energy from the sun
and within their “space suits” cells could both venture to the surface and evolve to
fit the new environment, utilizing light as an energy source not available to subsur-
face cells [ 6 , 7 ]. However, most of the required iron-promoted reactions may have
been in place before the origin of light capturing pigments completed the route to
anoxygenic photosynthesis. Today, some photoferrotrophic bacteria still display a
photobiologic process in which ferrous iron is oxidized with concomitant reduc-
tion of CO 2 to cell material, implying that anoxygenic photosynthesis was present
before oxygenic photosynthesis appeared.
The present increase in the planet's temperature may be due in part to increase
in greenhouse gases from human activities. Similarly, about 2.4 billion years ago
the anoxygenic life-form underwent a change to oxygenic photosynthesis in which
water served as the electron donor and oxygen was evolved [ 8 - 10 ]. Iron components
remained as essential units in the controlled downhill flow of electrons from photo-
synthetic pigments and the repercussions of this change led to development of new
biosynthetic pathways, modification of electron carriers, and alternative methods of
carbon fixation. The demand for iron may have been amplified by these changes.
The rise in oxygen level linked to oxygenic photosynthesis is a good example
of how a life-form can cause a self-inflicted alteration the planet's atmosphere.
As oxidation progressed, reduced iron was changed from its water soluble ferrous
valence to the nearly water insoluble oxidized ferric ion with resultant precipitation
of the metal from solution. Under these oxidized circumstances life could retreat
to, or remain in anaerobic niches or regions of low oxygen tension (some did), find
substitutes for iron (very few did), or capture ferric iron and deliver it to the cells
(most did). Ferric chelating siderophores that extracted the metal from insoluble
complexes and delivered the precious commodity to bacteria were a remarkable
answer to the iron acquisition problem in oxidized environments. The term sidero-
phore (“iron bearer”) is an insightful trivial designation made by Charles Lankford