Note: a dashed line (without arrowheads) between two compound names is meant to imply that the two names are just different instantiations of the same compound -- i.e. one may be a specific name and the other a general name, or they may both represent the same compound in different stages of a polymerization-type pathway. This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: iron oxidation
|Superclasses:||Degradation/Utilization/Assimilation → Inorganic Nutrients Metabolism → Iron Metabolism|
Some taxa known to possess this pathway include : Acidithiobacillus ferrooxidans
Expected Taxonomic Range:
Oxidation of ferrous iron (Fe(II)) by microbes is an important component of the iron geochemical cycle. In many environments iron is oxidized spontaneously and rapidly in the presence of oxygen. However, at low pH the chemical oxidation of iron is greatly reduced.
Acidithiobacillus ferrooxidans is an obligate chemolithotroph that lives at extremely low pH, and is the main bacterium used in the industrial extraction of copper and uranium from ores, using the microbial leaching technique. The sole energy-producing process that it uses for growth and cell maintenance involves oxidation of reduced sulfur compounds and/or Fe(II) under acidic conditions, using molecular oxygen as oxidant.
Fe(II) is oxidized by iron-rusticyanin reductase (encoded by cyc2), which transfers tthe electrons to the blue copper protein rusticyanin [Blake94]. Cyc2 has a redox potential of 560 mV, the highest measured for a cytochrome c.
Rusticyanin (680 mV) functions as a split point in the pathway, as it can transfer the electrons to two different periplasmic diheme cytochromes c4: Cyc1 shuttles them to the inner-membrane located aa3-type cytochrome oxidase, while CycA1 shuttles the electrons to the similarly located cytochrome bc1. This phenomenon is known as electron bifurcation. In this manner the electrons that are derived from Fe(II) are directed towards one of two competing pathways.
The "downhill" pathway directs the electrons towards an aa3-type cytochrome oxidase (EC 220.127.116.11) that uses the electrons to reduce oxygen to water. This reaction is coupled to pumping of protons from the cytoplasm to the acidic periplasm, maintaining a strong proton motive force. The proteins that are involved in the downhill pathway are encoded by genes present on a single operon, and form a large iron-oxidizing/O2-reducing supercomplex that spans the outer membrane, the periplasm, and the inner membrane [Castelle08].
The "uphill" pathway shuttles the electrons toward a cytochrome bc1 (EC 18.104.22.168) that utilizes the proton motive force that exists between the acidic periplasm and the much more basic cytoplasm to drive the reduction of ubiquinone, forcing this pathway to operate in the reverse direction to that observed in most organisms. Ubiquinol transfers its electrons to an NADH-quinone oxidoreductase (EC 22.214.171.124, NADH dehydrogenase complex), which also utilizes the proton motive force to reduce NAD(P)+ to NAD(P)H), required for biosynthesis and carbon fixation.
Castelle08: Castelle C, Guiral M, Malarte G, Ledgham F, Leroy G, Brugna M, Giudici-Orticoni MT (2008). "A new iron-oxidizing/O2-reducing supercomplex spanning both inner and outer membranes, isolated from the extreme acidophile Acidithiobacillus ferrooxidans." J Biol Chem 283(38);25803-11. PMID: 18632666
AppiaAyme98: Appia-Ayme C, Bengrine A, Cavazza C, Giudici-Orticoni MT, Bruschi M, Chippaux M, Bonnefoy V (1998). "Characterization and expression of the co-transcribed cyc1 and cyc2 genes encoding the cytochrome c4 (c552) and a high-molecular-mass cytochrome c from Thiobacillus ferrooxidans ATCC 33020." FEMS Microbiol Lett 167(2);171-7. PMID: 9809418
Battchikova05: Battchikova N, Zhang P, Rudd S, Ogawa T, Aro EM (2005). "Identification of NdhL and Ssl1690 (NdhO) in NDH-1L and NDH-1M complexes of Synechocystis sp. PCC 6803." J Biol Chem 280(4);2587-95. PMID: 15548534
Braun98a: Braun M, Bungert S, Friedrich T (1998). "Characterization of the overproduced NADH dehydrogenase fragment of the NADH:ubiquinone oxidoreductase (complex I) from Escherichia coli." Biochemistry 37(7);1861-7. PMID: 9485311
Bruscella07: Bruscella P, Appia-Ayme C, Levican G, Ratouchniak J, Jedlicki E, Holmes DS, Bonnefoy V (2007). "Differential expression of two bc1 complexes in the strict acidophilic chemolithoautotrophic bacterium Acidithiobacillus ferrooxidans suggests a model for their respective roles in iron or sulfur oxidation." Microbiology 153(Pt 1);102-10. PMID: 17185539
Calhoun93: Calhoun MW, Gennis RB (1993). "Demonstration of separate genetic loci encoding distinct membrane-bound respiratory NADH dehydrogenases in Escherichia coli." J Bacteriol 1993;175(10);3013-9. PMID: 8387992
Cavazza96: Cavazza C, Giudici-Orticoni MT, Nitschke W, Appia C, Bonnefoy V, Bruschi M (1996). "Characterisation of a soluble cytochrome c4 isolated from Thiobacillus ferrooxidans." Eur J Biochem 242(2);308-14. PMID: 8973648
Elbehti99: Elbehti A, Nitschke W, Tron P, Michel C, Lemesle-Meunier D (1999). "Redox components of cytochrome bc-type enzymes in acidophilic prokaryotes. I. Characterization of the cytochrome bc1-type complex of the acidophilic ferrous ion-oxidizing bacterium Thiobacillus ferrooxidans." J Biol Chem 274(24);16760-5. PMID: 10358017
Euro09: Euro L, Belevich G, Bloch DA, Verkhovsky MI, Wikstrom M, Verkhovskaya M (2009). "The role of the invariant glutamate 95 in the catalytic site of Complex I from Escherichia coli." Biochim Biophys Acta 1787(1);68-73. PMID: 19061856
Finel94: Finel M, Majander A (1994). "Studies on the proton-translocating NADH:ubiquinone oxidoreductases of mitochondria and Escherichia coli using the inhibitor 1,10-phenanthroline." FEBS Lett 339(1-2);142-6. PMID: 8313963
GiudiciOrticoni00: Giudici-Orticoni MT, Leroy G, Nitschke W, Bruschi M (2000). "Characterization of a new dihemic c(4)-type cytochrome isolated from Thiobacillus ferrooxidans." Biochemistry 39(24);7205-11. PMID: 10852719
Hayashi89: Hayashi M, Miyoshi T, Takashina S, Unemoto T (1989). "Purification of NADH-ferricyanide dehydrogenase and NADH-quinone reductase from Escherichia coli membranes and their roles in the respiratory chain." Biochim Biophys Acta 977(1);62-9. PMID: 2679883
Hellwig00: Hellwig P, Scheide D, Bungert S, Mantele W, Friedrich T (2000). "FT-IR spectroscopic characterization of NADH:ubiquinone oxidoreductase (complex I) from Escherichia coli: oxidation of FeS cluster N2 is coupled with the protonation of an aspartate or glutamate side chain." Biochemistry 39(35);10884-91. PMID: 10978175
Levican02: Levican G, Bruscella P, Guacunano M, Inostroza C, Bonnefoy V, Holmes DS, Jedlicki E (2002). "Characterization of the petI and res operons of Acidithiobacillus ferrooxidans." J Bacteriol 184(5);1498-501. PMID: 11844787
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