Bédard C, Knowles R
Microbiol Rev. 1989 Mar;53(1):68-84. doi: 10.1128/mr.53.1.68-84.1989.
Ammonia oxidizers (family Nitrobacteraceae) and methanotrophs (family Methylococcaceae) oxidize CO and CH4 to CO2 and NH4+ to NO2-. However, the relative contributions of the two groups of organisms to the metabolism of CO, CH4, and NH4+ in various environments are not known. In the ammonia oxidizers, ammonia monooxygenase, the enzyme responsible for the conversion of NH4+ to NH2OH, also catalyzes the oxidation of CH4 to CH3OH. Ammonia monooxygenase also mediates the transformation of CH3OH to CO2 and cell carbon, but the pathway by which this is done is not known. At least one species of ammonia oxidizer, Nitrosococcus oceanus, exhibits a Km for CH4 oxidation similar to that of methanotrophs. However, the highest rate of CH4 oxidation recorded in an ammonia oxidizer is still five times lower than rates in methanotrophs, and ammonia oxidizers are apparently unable to grow on CH4. Methanotrophs oxidize NH4+ to NH2OH via methane monooxygenase and NH4+ to NH2OH via methane monooxygenase and NH2OH to NO2- via an NH2OH oxidase which may resemble the enzyme found in ammonia oxidizers. Maximum rates of NH4+ oxidation are considerably lower than in ammonia oxidizers, and the affinity for NH4+ is generally lower than in ammonia oxidizers. NH4+ does not apparently support growth in methanotrophs. Both ammonia monooxygenase and methane monooxygenase oxidize CO to CO2, but CO cannot support growth in either ammonia oxidizers or methanotrophs. These organisms have affinities for CO which are comparable to those for their growth substrates and often higher than those in carboxydobacteria. The methane monooxygenases of methanotrophs exist in two forms: a soluble form and a particulate form. The soluble form is well characterized and appears unrelated to the particulate. Ammonia monooxygenase and the particulate methane monooxygenase share a number of similarities. Both enzymes contain copper and are membrane bound. They oxidize a variety of inorganic and organic compounds, and their inhibitor profiles are similar. Inhibitors thought to be specific to ammonia oxidizers have been used in environmental studies of nitrification. However, almost all of the numerous compounds found to inhibit ammonia oxidizers also inhibit methanotrophs, and most of the inhibitors act upon the monooxygenases. Many probably exert their effect by chelating copper, which is essential to the proper functioning of some monooxygenases. The lack of inhibitors specific for one or the other of the two groups of bacteria hampers the determination of their relative roles in nature.
氨氧化菌（硝化杆菌科）和甲烷氧化菌（甲基球菌科）将一氧化碳和甲烷氧化为二氧化碳，将铵离子氧化为亚硝酸根离子。然而，在不同环境中，这两类生物体对一氧化碳、甲烷和铵离子代谢的相对贡献尚不清楚。在氨氧化菌中，负责将铵离子转化为羟胺的氨单加氧酶也催化甲烷氧化为甲醇。氨单加氧酶还介导甲醇转化为二氧化碳和细胞碳，但具体途径尚不清楚。至少有一种氨氧化菌，即海洋亚硝化球菌，其甲烷氧化的米氏常数与甲烷氧化菌类似。然而，氨氧化菌中记录到的最高甲烷氧化速率仍比甲烷氧化菌低五倍，而且氨氧化菌显然不能以甲烷为碳源生长。甲烷氧化菌通过甲烷单加氧酶将铵离子氧化为羟胺，再通过一种可能类似于氨氧化菌中发现的酶的羟胺氧化酶将羟胺氧化为亚硝酸根离子。铵离子氧化的最大速率远低于氨氧化菌，而且对铵离子的亲和力通常也低于氨氧化菌。铵离子显然不能支持甲烷氧化菌的生长。氨单加氧酶和甲烷单加氧酶都将一氧化碳氧化为二氧化碳，但一氧化碳不能支持氨氧化菌或甲烷氧化菌的生长。这些生物体对一氧化碳的亲和力与其生长底物的亲和力相当，且往往高于羧基细菌。甲烷氧化菌的甲烷单加氧酶有两种形式：可溶性形式和颗粒形式。可溶性形式的特征明确，似乎与颗粒形式无关。氨单加氧酶和颗粒甲烷单加氧酶有许多相似之处。两种酶都含铜且与膜结合。它们能氧化多种无机和有机化合物，且抑制剂谱相似。被认为是氨氧化菌特异性抑制剂的物质已用于硝化作用的环境研究。然而，几乎所有被发现抑制氨氧化菌的众多化合物也抑制甲烷氧化菌，而且大多数抑制剂作用于单加氧酶。许多抑制剂可能通过螯合铜发挥作用，而铜对某些单加氧酶的正常功能至关重要。缺乏针对这两类细菌中某一类的特异性抑制剂阻碍了确定它们在自然界中的相对作用。
