Aerobic and nitrite-dependent methanotrophs make a living from oxidizing methane via methanol to carbon dioxide. In addition, these microorganisms cometabolize ammonia due to its structural similarities to methane. The first step in both of these processes is catalyzed by methane monooxygenase, which converts methane or ammonia into methanol or hydroxylamine, respectively. Methanotrophs use methanol for energy conservation, whereas toxic hydroxylamine is a potent inhibitor that needs to be rapidly removed. It is suggested that many methanotrophs encode a hydroxylamine oxidoreductase (mHAO) in their genome to remove hydroxylamine, although biochemical evidence for this is lacking. HAOs also play a crucial role in the metabolism of aerobic and anaerobic ammonia oxidizers by converting hydroxylamine to nitric oxide (NO). Here, we purified an HAO from the thermophilic verrucomicrobial methanotroph Methylacidiphilum fumariolicum SolV and characterized its kinetic properties. This mHAO possesses the characteristic P₄₆₀ chromophore and is active up to at least 80 °C. It catalyzes the rapid oxidation of hydroxylamine to NO. In methanotrophs, mHAO efficiently removes hydroxylamine, which severely inhibits calcium-dependent, and as we show here, lanthanide-dependent methanol dehydrogenases, which are more prevalent in the environment. Our results indicate that mHAO allows methanotrophs to thrive under high ammonia concentrations in natural and engineered ecosystems, such as those observed in rice paddy fields, landfills, or volcanic mud pots, by preventing the accumulation of inhibitory hydroxylamine. Under oxic conditions, methanotrophs mainly oxidize ammonia to nitrite, whereas in hypoxic and anoxic environments reduction of both ammonia-derived nitrite and NO could lead to nitrous oxide (N₂O) production.

需氧和依赖亚硝酸盐的甲烷氧化菌以通过甲醇将甲烷氧化成二氧化碳为生。此外，由于氨的结构与甲烷相似，这些微生物可以共代谢氨。这两个过程的第一步都是由甲烷单加氧酶催化，将甲烷或氨分别转化为甲醇或羟胺。甲烷氧化菌利用甲醇来节约能源，而有毒的羟胺是一种有效的抑制剂，需要迅速去除。有人认为，许多甲烷氧化菌在其基因组中编码羟胺氧化还原酶（mHAO）来去除羟胺，尽管缺乏这方面的生化证据。HAO 通过将羟胺转化为一氧化氮 (NO)，在需氧和厌氧氨氧化剂的代谢中也发挥着至关重要的作用。在这里，我们从嗜热疣状微生物甲烷氧化菌Mmethylacidiphilum fumariolicum SolV中纯化了 HAO ，并表征了其动力学特性。该 mHAO 具有特征性的 P ₄₆₀发色团，并且在至少 80 °C 的温度下仍具有活性。它催化羟胺快速氧化成NO。在甲烷氧化菌中，mHAO 能有效去除羟胺，羟胺会严重抑制钙依赖性甲醇脱氢酶，正如我们在此所示，镧系元素依赖性甲醇脱氢酶在环境中更为普遍。我们的结果表明，mHAO 通过防止抑制性羟胺的积累，使甲烷氧化菌能够在自然和工程生态系统（例如在稻田、垃圾填埋场或火山泥盆中观察到的生态系统）的高氨浓度下繁衍生息。在有氧条件下，甲烷氧化菌主要将氨氧化为亚硝酸盐，而在缺氧和缺氧环境中，氨源亚硝酸盐和NO的还原可能导致一氧化二氮（N ₂ O）的产生。