Intelligent Design, the best explanation of Origins

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Intelligent Design, the best explanation of Origins » Origin of life » Origin and emergence of Iron oxidation and uptake pathways of the first life

Origin and emergence of Iron oxidation and uptake pathways of the first life

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Origin and emergence of Iron oxidation and uptake pathways of the first life

Iron is essential to most life forms. To date, the only organisms that do not depend on iron belong to the Lactobacillus spp. This metal is an integral part of a number of proteins and enzymes. The two oxidation states of iron (Fe(II) and Fe(III)) make it suitable for numerous biochemical reactions. Iron is an important cofactor in several proteins required for a large range of metabolic processes like 

(i) the transport, storage and activation of molecular oxygen, 
(ii) the activation and decomposition of peroxides, 
(iii) the reduction of ribonucleotides and dinitrogen and 
(iv) the electron transfer via a variety of electron carriers with a wide range of redox potentials.

Two distinct molecular mechanisms have been characterized whereby environmental iron is solubilized and transported into the cytosol. Most prokaryotes produce siderophores, that have an extremely high affinity for iron. Siderophores form soluble ferric chelates, that are taken up by the cell via high-affinity receptors. In the case of mammalian cells, iron is acquired by a process involving transferrin that shows strong similarity with siderophores. The second mechanism for solubilizing iron, well-characterized in yeast, involves a reductase oriented toward the outside, that reduces Fe(III) into the more soluble Fe(II) 

Iron is a ubiquitous element in the universe. Ferrous iron (Fe(II)) was abundant in the primordial ocean until the oxygenation of the Earth's atmosphere led to its widespread oxidation and precipitation. This change of iron bioavailability likely put selective pressure on the evolution of life. This element is essential to most extant life forms and is an important cofactor in many redox-active proteins involved in a number of vital pathways. In addition, iron plays a central role in many environments as an energy source for some microorganisms. This review is focused on Fe(II) oxidation. The fact that the ability to oxidize Fe(II) is widely distributed in Bacteria and Archaea and in a number of quite different biotopes suggests that the dissimilatory Fe(II) oxidation is an ancient energy metabolism. Based on what is known today about Fe(II) oxidation pathways, we propose that they arose independently more than once in evolution and evolved convergently. The iron paleochemistry, the phylogeny, the physiology of the iron oxidizers, and the nature of the cofactors of the redox proteins involved in these pathways suggest a possible scenario for the timescale in which each type of Fe(II) oxidation pathways evolved. The nitrate dependent anoxic iron oxidizers are likely the most ancient iron oxidizers. We suggest that the phototrophic anoxic iron oxidizers arose in surface waters after the Archaea/Bacteria-split but before the Great Oxidation Event. The neutrophilic oxic iron oxidizers possibly appeared in microaerobic marine environments prior to the Great Oxidation Event while the acidophilic ones emerged likely after the advent of atmospheric O2. This article is part of a Special Issue entitled: The evolutionary aspects of bioenergetic systems. 1

In the Earth's crust, iron is the fourth most abundant element and the second metal after aluminium, with an abundance estimated to be ~5% while it is believed to be the main constituent of the Earth's core. Iron is not only important for human activities (the processing of iron for industrial purposes accounts for 95% of worldwide metal production), but is also a crucial element for living cells from all three domains because it is incorporated as a cofactor in many metalloproteins involved in vital metabolic pathways.

Iron is a transition metal; its chemical symbol is Fe from the Latin name, ferrum. The melting point of iron is 1536 °C, its boiling point is about 3000 °C. Iron can exist in various oxidation states (from −2 to +6), the principal forms that occur naturally however are either ferrous or ferric iron (Fe(II) or Fe(III), respectively). Fe(II) is more abundant in anoxic environments whereas, in an oxygen-containing environment, iron is readily oxidized from the Fe(II) to the Fe(III) state. Fe(III) ions have a very low solubility (about 10−17 M), making iron less bioavailable at circumneutral pH than at acidic pH. Iron is known to react with oxygen (O2) in water or air moisture to form various insoluble iron oxide compounds described commonly as rust; there are sixteen known iron oxides and oxyhydroxides. Iron not only complexes with oxygen ligands but also with a lot of different compounds such as carbonate and sulfur by abiotic or biotic reactions. 


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