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Fig. 4 | AMB Express

Fig. 4

From: Genome-scale metabolic model analysis indicates low energy production efficiency in marine ammonia-oxidizing archaea

Fig. 4

Three proposed ammonia oxidation pathways in AOA. Pathway 1 was proposed by Walker et al. In this pathway, ammonia is oxidized to hydroxylamine by the membrane enzyme complex AMO/CuMMO (Walker et al. 2010). Subsequently, hydroxylamine is oxidized to nitrite in the periplasm by a heme-rich hydroxylamine oxidoreductase (CuHAO) complex. Four electrons from this oxidation are transferred to the quinone pool. Two electrons from the reduced quinone pool return to AMO (marked by red) and are required to initiate ammonia oxidation. The remaining two electrons enter the electron transport chain composed of pcy protein to generate the proton motive force (PMF) necessary for ATP synthesis and NADH synthesis. In Pathway 2 (Schleper and Nicol 2010; Stahl and de la Torre 2012), NO is speculated functioning as a redox shuttle to deliver electrons to the AMO (marked by green) since measurable amounts of NO are produced during ammonia oxidation. In pathway 3, iterative production and consumption of NO is involved in conversion of hydroxylamine to nitrite facilitated by a proposed novel copper enzyme capable of performing known P460 activity (CuP460) (Kozlowski et al. 2016). N2O was formed abiotically from NO by interaction with media components or with debris in killed cell. AMO/CuMMO, ammonia monooxygenase; CuHAO, hydroxylamine dehydrogenase; NIR, Cu-containing NO-forming nitrite reductase; pcy, plastocyanin; Q/QH2, quinone/quinol pool

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