Chemolithotrophic bacteria electron transport

Chemolithotrophic organisms often grow slowly, making study of their metabolism difficult.310 Nevertheless, these bacteria usually use electron transport chains similar to those of mitochondria. ATP is formed by oxidative phosphorylation, the amount formed per electron pair depending upon the number of proton-pumping sites in the chain. This, in turn, depends upon the electrode potentials of the reactions involved. For example, H2, when oxidized by 02, leads to passage of electrons through the entire electron transport chain with synthesis of 3 molecules of ATP per electron pair. On the other hand, oxidation by 02 of nitrite, for which E° (pH 7) = +0.42 V, can make use only of the site III part of the chain. Not only is the yield of ATP less than in the oxidation of H2 but also there is another problem. Whereas reduced pyridine... 

HAO catalyzes the four-electron oxidation of hydroxylamine to nitrite. " It is present in autotrophic nitrifying bacteria, like Nitrosomonas, which are obligate chemolithotrophs that use the oxidation of ammonia as their sole energy source. For each cycle of hydroxylamine oxidation, two electrons are returned for the initial step of ammonia oxidation and the other two are either transferred to the terminal oxidase via the components of the respiratory chain, or used to generate NADH by reverse electron transport. 

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