The regulation of bacterial nitrous oxide reduction

人类完全依赖我们呼吸的氧气来维持我们的生命过程。值得注意的是，对于许多种类的细菌来说，情况并非如此。面对环境中缺氧的情况，许多细菌物种能够转而使用硝酸盐而不是氧气来维持生命。这些维持生命的过程之一是反硝化，其中水溶性硝酸盐转化为气态产物，一氧化氮，氧化亚氮（N2O）和二氮。这种反硝化过程可以在农业土壤中广泛发生，在农业土壤中添加富氮肥料以刺激植物生长也可以刺激细菌生命。能够反硝化的土壤细菌需要保护自己免受一氧化氮的影响，一氧化氮是一种通过自身代谢产生的强效毒素。它们有一种叫做“一氧化氮还原酶”的酶，这种酶通过将一氧化氮转化为相对无害的一氧化二氮（N2O），使细菌中的一氧化氮水平保持在较低水平，而一氧化氮有时会被释放到大气中。这种细菌生存策略具有显著的环境后果，因为一氧化二氮是一种能破坏臭氧层的强效温室气体。在讨论温室气体排放时，公众对二氧化碳和甲烷所带来的问题有着敏锐的认识。然而，一氧化二氮的排放，也许是最著名的牙科麻醉“笑气”，也应该引起公众和政治的关注。1793年，英国化学家约瑟夫·普里斯特利（Joseph Priestley）首次发现了一氧化二氮，当时它的大气水平已经稳定了几千年。然而，在过去的100年中，大气中的N2O增加了20%，并且每年以0.25%的速度进一步增加。大多数评论家将这一增长与20世纪为提高农田生产力而大量使用化肥有关。虽然它在大气中的含量只是二氧化碳的一小部分，但它的全球变暖潜力是二氧化碳的300倍。因此，当以二氧化碳当量表示时，它约占全球温室气体排放总量的10%。由于今天产生的一氧化二氮在大气中的寿命约为150年，它将影响我们的曾曾孙所经历的气候，因此，现在就制定减少这些排放的策略是很重要的。反硝化细菌从硝酸盐中产生NO的途径是从分子水平上理解的，因为已知将硝酸盐转化为亚硝酸盐（硝酸盐还原酶）和将亚硝酸盐转化为一氧化氮（亚硝酸盐还原酶）的酶的结构。这些酶的活性依赖于钼、铁和铜等金属。将N2O分解成惰性N2的酶是一种含铜的酶，称为氧化亚氮还原酶。它是地球上主要的酶，负责破坏强效的一氧化二氮温室气体。如果没有它，大气中的一氧化二氮水平将比现在高得多。氧化亚氮还原酶含有12个铜原子，因此其在环境中的活性高度依赖于铜的生物利用度。它对pH值和氧气也很敏感，因此它在环境中的活性取决于许多不同的环境变量。由于施用氮肥，农业土壤是人为N2O的最大来源。自英国签署《京都议定书》以来，许多非生物源的一氧化二氮排放已经减少，但生物源的排放却不太容易管理。改善农业N2O排放预测和管理的努力将受益于更好地了解影响细菌N2O净产量的因素。这需要在受控的实验室环境中对模式生物进行基础研究。这个项目代表了这样一项研究，重点是铜调节N2O排放的机制。

Humans are entirely dependent on the oxygen we breathe to support our life processes. Significantly, this is not so for many species of bacteria. Faced with a shortage of oxygen in their environment many bacterial species are able to switch to using nitrate, rather than oxygen to support life. One of these life-supporting processes is denitrification, in which water-soluble nitrate is converted to gaseous products, nitric oxide, nitrous oxide (N2O) and dinitrogen. This denitrification process can take place extensively in agricultural soils where nitrogen rich fertilisers added to stimulate plant growth can also stimulate bacterial life. Soil bacteria that can denitrify need to protect themselves from the effects of NO, a potent toxin, produced through their own metabolism. They have an enzyme called 'nitric oxide reductase' that has evolved to keep NO levels low in the bacteria by converting it to the relatively benign nitrous oxide (N2O) which can sometimes be released into the atmosphere. This bacterial survival strategy has significant environmental consequences as N2O is a potent greenhouse gas which can damage the ozone layer.When discussing greenhouse gas emissions the general public are acutely aware of the problems posed by carbon dioxide and methane. However, emissions of N2O, perhaps best known as the dental anaesthetic "laughing gas", should also be a cause for public and political concern. N2O was first discovered by the British chemist Joseph Priestley in 1793 when its atmospheric levels had been steady for millennia. However, over the last 100 years N2O in the atmosphere has increased by 20% and this atmospheric loading is increasing further by 0.25% each year. Most commentators linking this increase to intensive use of fertiliser to increase farmland productivity in the 20th Century. Although its atmospheric levels are only a fraction of that of CO2 it has a 300-fold greater global warming potential. Thus when expressed in terms of CO2 equivalents it represents around 10% of total global emissions of greenhouse gases. Since it has an atmospheric lifetime of some 150 years the N2O produced today will influence the climate experienced by our great-great grandchildren thus it is important to devise strategies to mitigate these releases now.The pathways by which denitrifying bacteria produce NO from nitrate are understood from a molecular level with structures of enzymes that convert nitrate to nitrite (nitrate reductases) and nitrite to nitric oxide (nitrite reductases) being known. These enzymes depend on metals such as molybdenum, iron and copper for their activity. The enzyme that breaks down N2O to inert N2 is a copper-containing enzyme called nitrous oxide reductase. It is the major enzyme on the planet that is responsible for the destruction of the potent N2O greenhouse gas. Without it the atmospheric levels of N2O would be much greater that they currently are. The nitrous oxide reductase contains twelve atoms of copper and so its activity in the environment is highly dependent on the bioavailability of copper. It is also sensitive to pH and oxygen and so its activity in the environment is dependent on a number of different environmental variables. As a result of the application of nitrogenous fertilisers, agricultural soils are the largest source of anthropogenic N2O. Since the UK signed up to the Kyoto Protocol, many non-biological sources of N2O emissions have been reduced, but emissions from biological sources are less easy to manage. Efforts to improve the prediction and management of agricultural N2O emissions will benefit from a better understanding of the factors that influence the net production of N2O by bacteria. This requires fundamental studies on model organisms in controlled laboratory environments. This programme represents just such a study focused on the mechanism by which copper regulates N2O emission.