Detrminacion De La Actividad De La Formianto Deshidrogenas Y Nitrato Reductasa En Escherichia Coli

Páginas: 13 (3043 palabras) Publicado: 27 de octubre de 2012
Gerhard Gottschalk
Bacterial Metabolism
Second Edition
w York Berlin Heidelberg Tokyo
With 204 Figures

Chapter 9
Chemolithotrophic and Phototrophic Metabolism
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Chemolithotrophic and phototrophic bacteria have in common the ability to grow in mineral media, deriving their cell carbon from C02. The reducing power required for C-02 reduction is obtained from inorganiccompounds and energy is provided either by light-dependent reactions or by oxidation of inorganic compounds with oxygen or nitrate (aerobic chemolithotrophs). The anaerobic chemolithotrophs (methanogenic, ace- togenic and sulfidogenic bacteria) have been discusscd in Chapter 8,
I, Chemolithotrophic Metabolism
A, Physiological groups of aerobic chemolithotrophs
As already mentionedchemolithotrophs gain energy by oxidation of an inorganic compound. Depending on the nature of the inorganic compound oxidized, six groups of aerobic chemolithotrophs are recognized; they are summarized in Table 9.1. The reactions earned out and their free energy changes are also given.
Hydrogen-oxidizing bacteria have in common that they use molecular hydrogen as energy source. In other physiologicalproperties and morphologically, however, they are very diverse. Since they all are facuitative chemolithotrophs, taxonomists have preferred to classify them with their chemoheterotrophic relatives. The hydrogen-oxidizing bacteria include Pseudomonas saccharophila, P. faciiis, A¡ca! ¡genes eutrophus, Nocardia

aulotrophica, the nitrogen fixing Xanthobacter autoirophicus and Paracoc- cusdenitrificans. The latter is able to use nitrate instead of oxygen as electron acceptor.
Carbon monoxide-oxidizing bacteria are also found in various genera. Examples are: Pseudomonas carboxydovorans, Atcaligenes carboxydus, and the thermophile Bacillus schlegelii. All CO-oxidizing bacteria are likewise H2 oxidizers {not vice versa!); they are faculative chemolitho- trophs.
Sulfur oxidizers are thethiobacilli, Thiomicrospira pelophiia, a marine, spiral organism, and Sulfolobus, a thermophilic archaebacterium of irregular cell form. In addition, filamentous gliding organisms, such as Beggiatou and Thiothrix arc able to oxidize sulfide to elemental sulfur and subsequently to sulfate. Most sulfur bacteria are obligate chemolitho- trophs. Some of them, e.g., Thiobacillus intermedius, can grow asaerobic heterotrophs. T. denitrificans can utilize nitrate instead of oxygen as electron acceptor.
The Fe'1+-oxidizflr Thiobacillus ferrooxidans is able to use reduced sulfur compounds and ferrous ions alternatively as electron donors. The iron bacterium. Gallionella probably also gains energy by oxidation of Fe2+ to Fe3 +. The free energy change of Fe2+ oxidation at low pH values is targe enough tobe coupled to ATP synthesis; it is rather small at neutral pH and iron bacteria cannot grow at pH values above about 4.
The oxidation of ammonia to nitrite is carried out by Nitrosomonas, Nitrosospira, Nitrosovibrio, and Nitrosococcus species; they all are obligate chemolithotrophs and so are the nitrite oxidizers Nitrobacter, Nitro- spina, and Nitrococcus, with the exception of some Nitrobacterstrains. Because nitrite is very toxic to most organisms,, the processes of nitrite production and nitrite oxidation are remarkable.
B. Energy production and generation of reducing power in H2 and CO oxidizers
The ratio in which H2, 02, and C02 are consumed by a growing cuJture of hydrogen-oxidizing bacteria is about the following:
4H2 + 202 4H20
2H2 + C02 —-*■ <CH20> + H20 6H2 +202 + C02 -—> <CH20) + 5H20
Thus, the oxidation of 4H2 to water yields enough ATP to allow the synthesis to cell material ((CH20)) from C02 and H2.
ATP synthesis proceeds by a chemiosmotic mechanism as in aerobic respiration. Cytochromes, ubiquinone, and menaquinone have been found in membrane fractions of hydrogen-oxidizing bacteria. Differences between hydrogen-oxidizing species have...
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