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Multiheme Hydroxylamine Oxidoreductases Produce NO During Ammonia Oxidation in Methanotrophs

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Multiheme Hydroxylamine Oxidoreductases Produce NO During Ammonia Oxidation in Methanotrophs Multiheme hydroxylamine oxidoreductases produce NO during ammonia oxidation in methanotrophs Wouter Versantvoorta, Arjan Pola, Mike S. M. Jettena, Laura van Niftrika, Joachim Reimanna, Boran Kartalb,1, and Huub J. M. Op den Campa aDepartment of Microbiology, Institute for Water and Wetland Research, Faculty of Science, Radboud University, 6525 AJ Nijmegen, The Netherlands; and bMicrobial Physiology Group, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany Edited by Caroline S. Harwood, University of Washington, Seattle, WA, and approved August 6, 2020 (received for review June 2, 2020) Aerobic and nitrite-dependent methanotrophs make a living from of both carbon and nitrogen cycles and can act both as sources and oxidizing methane via methanol to carbon dioxide. In addition, sinks of climate active gases. Therefore, understanding their me- these microorganisms cometabolize ammonia due to its structural tabolism and how this is influenced by anthropogenic activities will similarities to methane. The first step in both of these processes is help us predict their effect on global warming and possibly utilize catalyzed by methane monooxygenase, which converts methane them to counteract emissions (6–10). or ammonia into methanol or hydroxylamine, respectively. Meth- Ammonia oxidation in the natural environment has been mainly anotrophs use methanol for energy conservation, whereas toxic attributed to a distinct group of microorganisms, termed ammonia hydroxylamine is a potent inhibitor that needs to be rapidly re- oxidizers (11, 12). However, in natural and engineered envi- moved. It is suggested that many methanotrophs encode a hy- ronments where methane and ammonia co-occur, methanotrophs can droxylamine oxidoreductase (mHAO) in their genome to remove also contribute significantly to ammonia oxidation (8, 9, 13–17).This hydroxylamine, although biochemical evidence for this is lacking. cross-reactivity is due to the structural similarity between methane and HAOs also play a crucial role in the metabolism of aerobic and ammonia, which allows both methanotrophs and ammonia oxidizers anaerobic ammonia oxidizers by converting hydroxylamine to to convert either substrate, although neither is capable of growing on nitric oxide (NO). Here, we purified an HAO from the thermophilic the alternative substrate (4, 9, 17). The first step in both methane and verrucomicrobial methanotroph Methylacidiphilum fumariolicum ammonia oxidation is the incorporation of oxygen by methane/ SolV and characterized its kinetic properties. This mHAO possesses ammonia monooxygenase (MMO, AMO), resulting in the for- MICROBIOLOGY the characteristic P460 chromophore and is active up to at least mation of methanol or hydroxylamine, respectively (4, 17–19). In 80 °C. It catalyzes the rapid oxidation of hydroxylamine to NO. In methane-oxidizing bacteria, methanol dehydrogenase (MDH) methanotrophs, mHAO efficiently removes hydroxylamine, which se- converts methanol to formaldehyde (or formate), which is fur- verely inhibits calcium-dependent, and as we show here, lanthanide- ther oxidized to CO2 (6, 20, 21). Whereas in ammonia-oxidizing dependent methanol dehydrogenases, which are more prevalent in microorganisms, hydroxylamine oxidoreductase (HAO) oxidizes the environment. Our results indicate that mHAO allows methano- hydroxylamine to nitric oxide (NO), which is subsequently con- trophs to thrive under high ammonia concentrations in natural and verted to nitrite (22–24). In octaheme HAO proteins catalysis engineered ecosystems, such as those observed in rice paddy fields, occurs at heme 4, which is cross-linked to a tyrosine residue from landfills, or volcanic mud pots, by preventing the accumulation of a neighboring subunit, covalently linking all three subunits of the inhibitory hydroxylamine. Under oxic conditions, methanotrophs HAO complex (22, 25–29). This cross-link forces the heme into a mainly oxidize ammonia to nitrite, whereas in hypoxic and anoxic environments reduction of both ammonia-derived nitrite and NO Significance could lead to nitrous oxide (N2O) production. hydroxylamine oxidoreductase | ammonia oxidation | methanotrophy | Methanotrophs oxidize methane to CO2, thereby mitigating the multiheme cytochrome | nitric oxide emission of this potent greenhouse gas. Understanding how these microorganisms are influenced by anthropogenic activities will help better predict their impact on global warming and utilize ethane (CH ) is a potent greenhouse gas with a global 4 them to reduce it. Ammonia-fertilizer input to the environment warming potential 34 times that of carbon dioxide (CO ) M 2 has greatly improved crop yields but increased emissions of over a 100-y time span (1). Microbial anaerobic degradation of greenhouse gasses like N O as well. In methane-rich environ- biomass yields organic acids and hydrogen that fuel methano- 2 ments, methanotrophs play a prominent role in ammonia oxida- genesis. Methanogenic archaea are responsible for the produc- − tion. Here, we purified a protein (mHAO) from Methylacidiphilum tion of the majority of biogenic methane (about 583 Tg·y 1) (2), fumariolicum, capable of rapid oxidation of hydroxylamine to which is subsequently released to the environment. Emission of NO. We propose that mHAO enables methanotrophs to cope this methane to the atmosphere is partly mitigated by aerobic with high ammonia concentrations, leading to reduced methane and anaerobic methanotrophs (3). These microorganisms use emissions. However, this activity simultaneously contributes to methane as an energy source by oxidizing it to CO2 and act as a ammonia loss and nitrite production, and potentially leads to methane sink, thereby reducing the contribution of methane on N2O emissions. global warming. On the other hand, aerobic and nitrite-dependent methanotrophs can be the source of another, even more potent Author contributions: W.V., A.P., J.R., B.K., and H.J.M.O.d.C. designed research; W.V. and greenhouse gas, nitrous oxide (N2O), which has a global warming A.P. performed research; W.V., A.P., M.S.M.J., L.v.N., J.R., B.K., and H.J.M.O.d.C. analyzed data; and W.V. and B.K. wrote the paper. potential 296 times that of CO2 over a 100-y time span (1, 4). According to the Fifth Assessment Report of the Intergovern- The authors declare no competing interest. mental Panel on Climate Change, the total radiative forcing of CH4 This article is a PNAS Direct Submission. and N2O increased by 2% and 6%, respectively, since the previous Published under the PNAS license. 1 report (1). This increase makes N2O the third largest contributor To whom correspondence may be addressed. Email: [email protected]. to radiative forcing. Furthermore, since the chlorofluorocarbons This article contains supporting information online at https://www.pnas.org/lookup/suppl/ were phased out of use, N2O became the primary ozone-depleting doi:10.1073/pnas.2011299117/-/DCSupplemental. greenhouse gas (5). Consequently, methanotrophs sit in the nexus www.pnas.org/cgi/doi/10.1073/pnas.2011299117 PNAS Latest Articles | 1of5 Downloaded by guest on September 23, 2021 highly ruffled conformation, giving rise to the characteristic ab- sorbance peak at 460 nm (P460) in the reduced enzyme (26, 27). The active site of HAOs strongly favors oxidative reactions and the cross-linking tyrosine is highly conserved within members of the HAO family (28). Hydroxylamine is a highly toxic compound and has been shown to inhibit calcium-dependent MDHs (30), which necessitates the rapid turnover of hydroxylamine in methane-oxidizing bacteria. Many methanotrophs encode a HAO-like protein (mHAO) ho- mologous to that of bacterial ammonia oxidizers. These HAO-like proteins have been postulated to perform hydroxylamine oxida- tion in methanotrophs, but biochemical evidence for this is lacking (4). Up-regulation of hao transcription in response to ammonia has been shown for several methanotrophic bacteria that oxidize ammonia faster compared to the ones not encoding hao genes (31–34). Most cultured methanotrophs oxidize ammonium to ni- trite (17, 33–35), whereas a few methanotrophs have been shown to produce N2O from ammonia oxidation without apparent nitrite production (36). In addition, N2O production has been observed for many aerobic methanotrophs under nitrite-reducing condi- Fig. 1. UV-vis absorbance spectrum of Methylacidiphilum fumariolicum tions, either as a product of reactive nitrogen detoxification or SolV HAO in the as-isolated (solid) and dithionite reduced state (dashed). respiration under O2 limitation (31, 34, 36–38). N2O production Inset shows the reduced minus oxidized difference spectrum. The P460 from ammonia and hydroxylamine is thought to occur via the chromophore characteristic for HAOs is clearly present in both the reduced initial formation of nitrite, which until recently was the presumed and reduced minus oxidized difference spectrum. end product of hydroxylamine oxidation by all HAOs. In this hy- pothesis, nitrite reductases nirS and nirK would produce NO, oxidation stain. The UV-vis spectrum of purified mHAO displayed which would subsequently be reduced to N ObyanorBC complex 2 a prominent maximum at 468 nm in the reduced form. In addition, (29, 31–34). However, the close amino acid sequence similarity of characteristic heme c features, namely a maximum in the Soret the mHAO proteins to those encoded by ammonia-oxidizing region at 408 nm in the oxidized form (Fig. 1, solid) and maxima at bacteria suggests that the end product of mHAO could also be NO
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