細菌中的燃料電池
4G光元：大氣和有機物之間的氮交換對地球上的生命至關重要，因為氮是蛋白質和DNA等基本分子的主要成分。
++ **細菌中的燃料電池
氮迴圈（左）和厭氧氨氧化細菌如何促進亞硝酸鹽轉化為氮氣（右）。
僅在20世紀90年代發現的這種交換的一個主要途徑是在某些細菌中發現的厭氧氨氧化途徑。它透過肼進行，肼是人類用作火箭燃料的高反應性物質。馬克斯普朗克醫學研究所的研究人員與來自荷蘭馬克斯普朗克生物物理研究所和Radboud大學的科學家合作，現在描述了該過程中最後一步的酶結構：將肼轉化為氮氣並收穫以這種方式釋放的能量。剛剛發表在“科學進步”雜誌上的這些結果顯示了一個前所未有的血紅素基團網路，用於處理化學轉化過程中釋放的大量電子。
生物地球化學氮迴圈
氮氣（N2）形式的氮氣佔我們大氣的80%左右，但作為一種元素，氮氣僅在地殼中少量存在。然而，所有生物都需要氮，因為它是大部分必需分子的一部分。然而，它們不能直接使用大氣氮並且需要不同的化學形式。許多細菌透過產生更多反應形式的氮來進行這種轉化並有助於生化氮迴圈（影象）。
Anammox細菌 - 透過中間的捷徑
在20世紀90年代，科學家發現了一種叫做厭氧氨氧化（anammox）的細菌過程。“我們現在相信這個過程是每年從海洋中去除氮的30%到70%的原因，”海德堡醫學研究MPI組織負責人Thomas Barends解釋道。“由於這種特性，厭氧氨氧化細菌被用於世界各地的可持續廢水處理，”Radboud大學的Cornelia Welte補充道。在此過程中，細菌將亞硝酸鹽和氨轉化為二氮（N2）和水，同時為細胞產生能量。分子肼在中間步驟中產生。肼是火箭燃料的常見成分，但由於其高毒性，細菌作為代謝燃料的使用相當奇特 - 並且在生物體中令人驚訝。韋爾特：“到目前為止，肼只在厭氧氨氧化物中發現，而在其他細菌中卻沒有。”直到最近，關於這些細菌如何利用肼轉化過程中釋放的能量知之甚少。
此前，該研究小組及其合作者已經描述了酶肼合成酶和羥胺氧化還原酶的結構。研究人員現在透過描述肼脫氫酶的晶體結構進一步解開厭氧謎題，肼脫氫酶是將毒性肼轉化為無害的二氮氣體的酶。“肼的使用以及肼脫氫酶的結構都非常獨特，因此詳細揭示生物過程非常重要，”Welte解釋說。
從有毒火箭燃料到無害氮 - 肼脫氫酶（HDH）複合物
“人們可以將HDH複合物與具有僅適合某些型別插頭的電源插座的燃料電池進行比較，”Thomas Barends說道，他描述了HDH的結構和機制。“燃料”肼透過外部通道進入蛋白質複合物。然後該酶透過192個血紅素基團的前所未有的大網路催化肼轉化為氮氣。然後電子被帶到細菌的其他部分，就像電流傳遞給電氣消費者一樣。然後這些消費者產生細胞的能量。
縮小差距
“我們正在努力尋找能夠吸收儲存在血紅素網路中的電子的蛋白質，”該研究的第一作者，Barends小組的博士後Mohd Akram說。從他們觀察到的結構中，他們預計只有小蛋白質可以進入複合物，在內部的空心空間中吸收電子，並再次離開。選擇哪些蛋白質可以接近電子可以幫助確保電子被帶到正確的位置以用於電池中的能量產生。
肼：也叫聯氨。有機化合物，化學式NH2NH2。無色油狀液體，在空氣中發煙，具有氨的氣味，劇毒。是一種強還原劑，可將鹼溶液中的金屬離子還原成單質，可用於鏡面鍍銀、塑膠和玻璃上鍍金屬膜。並可用作火箭燃料、顯像劑、抗氧劑、製藥等。
羥胺：有機化合物，化學式NH2OH。白色薄片狀、針狀晶體或無色油狀液體，有腐蝕性。可用作還原劑或用於有機合成。也叫胲。
Fuel cells in bacteria
by
Max Planck Society
The nitrogen cycle （left） and how anammox bacteria facilitate the conversion of nitrite to nitrogen gas （right）。 Credit： MPI for Medical Research
The exchange of nitrogen between the atmosphere and organic matter is crucial for life on Earth because nitrogen is a major component of essential molecules such as proteins and DNA。 One major route for this exchange， discovered only in the 1990s， is the anammox pathway found in certain bacteria。 It proceeds via hydrazine， a highly reactive substance used by humans as a rocket fuel。 Researchers at the Max Planck Institute for Medical Research， in cooperation with scientists from the Max Planck Institute for Biophysics and Radboud University in the Netherlands， now describe the structure of the enzyme performing the last step in this process： turning hydrazine into nitrogen gas and harvesting the energy set free in this way。 The results， which were just published in Science Advances， show an unprecedented network of heme groups for handling the large number of electrons released during the chemical conversion。
The biogeochemical nitrogen cycle
Nitrogen， in the form of
nitrogen gas
（N2）， makes up about 80 percent of our atmosphere， but as an element nitrogen occurs only in small quantities in the Earth‘s crust。 However， all living organisms require nitrogen， because it is part of most of their essential molecules。 However， they cannot use atmospheric nitrogen directly and require it in a different chemical form。 A number of
bacteria
perform such conversions and contribute to the biochemical nitrogen cycle （image） by producing more reactive forms of nitrogen。
Anammox Bacteria – a shortcut through the middle
In the 1990s， scientists discovered a bacterial process called anaerobic ammonium oxidation （anammox）。 “We now believe this process is responsible for 30 to 70 percent of the yearly nitrogen removal from the oceans，” explains Thomas Barends，
group leader
at the MPI for Medical Research in Heidelberg。 “Due to this characteristic，
anammox bacteria
are used in sustainable wastewater treatment all over the world，” Cornelia Welte of Radboud University adds。 During this process， bacteria convert nitrites and ammonia into dinitrogen （N2） and water， while generating energy for the cell。 The molecule
hydrazine
is produced in an intermediate step。 Hydrazine is a common component of
rocket fuel
，but its use by bacteria as a metabolic fuel is rather exotic-and surprising in living organisms because of its high toxicity。 Welte： “So far， hydrazine has only been found in anammox and not in other bacteria。” Until recently， little was known about how these bacteria harness the energy released during the hydrazine conversion。
Previously the research group and their collaborators have described the structures of the enzymes hydrazine synthase and hydroxylamine oxidoreductase。 The researchers now further unravel the anammox puzzle by describing the crystal structure of hydrazine dehydrogenase， the enzyme involved in the conversion of toxic hydrazine to harmless
dinitrogen
gas。 “Both the use of hydrazine as well as the structure of hydrazine dehydrogenase are quite unique， making it important to uncover the biological process in detail，” Welte explains。
From toxic rocket fuel to harmless nitrogen – the hydrazine dehydrogenase （HDH） complex
“One could compare the HDH complex to a fuel cell with electrical outlets that only fit certain types of plugs，” says Thomas Barends， describing the structure and mechanism of HDH。 The ’fuel‘ hydrazine enters the protein complex through a channel on the outside。 The enzyme then catalyzes the conversion of hydrazine into
nitrogen
gas through an unprecedentedly large network of 192 heme groups。 Then the electrons are carried to other parts of the bacterium， like the transfer of current to electrical consumers。 These consumers then generate the cell’s energy。
Closing the gap
“We are now working on finding the protein that takes up the electrons stored in the heme network，” says Mohd Akram， postdoc in the Barends group and first author of the paper。 From the structure they observed they expect that only small proteins can enter the complex， take up the electrons in a hollow space inside， and leave again。 Selecting which proteins can access the electrons may help ensure the electrons are brought to the right place to be used for energy generation in the cell