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海底热液区生命的第三种化学能来源

已有 3782 次阅读 2011-8-11 10:07 |系统分类:科研笔记

   构建生命体的碳氮主要是以还原态的形式存在,如氨基酸和碱基中的氨基,氨基酸、DNA链、脂肪链等中的饱和或不饱和甲基、亚甲基等。而在最开始原始地球上,碳氮的主要存在形式却不是Miller在1953年建议的CH4、NH3等还原态,彼时地球大气主要成分是CO2、CO、H2O、N2等,这样的大气成分显然是不利于生物分子的形成和最早生命形式的出现的。
   因此,在1977年美国Alvin号深潜器于海底2000多米处发现海底烟囱附近存在共生生态系统之后,人们自然会想到地球生命是否最早诞生在那里。因为那里有还原态的硫和甲烷,都可以作为海底热液区生物链最底端生命形式——化能自养细菌——的代谢能量之源。在最新一期Nature上Petersen等有报道了第三种,即氢 (Nature,2011,476:176-180)。http://www.nature.com/nature/journal/v476/n7359/full/nature10325.html?WT.ec_id=NATURE-20110811
 
 
补充:其实类似的能源物质不止于S、H2S、H2和CH4,海底一种嗜酸嗜热古菌Acidianus A1-3体内的二硫化碳水解酶能够将CS2转变成H2S和CO2,为自身的生命活动提供能量。详见
Evolution of a new enzyme for carbon disulphide conversion by an acidothermophilic archaeon Journal name: Nature Volume: 478, Pages:412–416 DOI: doi:10.1038/nature10464

Extremophilic organisms require specialized enzymes for their exotic metabolisms. Acid-loving thermophilic Archaea that live in the mudpots of volcanic solfataras obtain their energy from reduced sulphur compounds such as hydrogen sulphide (H2S) and carbon disulphide (CS2)1, 2. The oxidation of these compounds into sulphuric acid creates the extremely acidic environment that characterizes solfataras. The hyperthermophilic Acidianus strain A1-3, which was isolated from the fumarolic, ancient sauna building at the Solfatara volcano (Naples, Italy), was shown to rapidly convert CS2 into H2S and carbon dioxide (CO2), but nothing has been known about the modes of action and the evolution of the enzyme(s) involved. Here we describe the structure, the proposed mechanism and evolution of a CS2 hydrolase from Acidianus A1-3. The enzyme monomer displays a typical β-carbonic anhydrase fold and active site, yet CO2 is not one of its substrates. Owing to large carboxy- and amino-terminal arms, an unusual hexadecameric catenane oligomer has evolved. This structure results in the blocking of the entrance to the active site that is found in canonical β-carbonic anhydrases and the formation of a single 15-Å-long, highly hydrophobic tunnel that functions as a specificity filter. The tunnel determines the enzyme’s substrate specificity for CS2, which is hydrophobic. The transposon sequences that surround the gene encoding this CS2 hydrolase point to horizontal gene transfer as a mechanism for its acquisition during evolution. Our results show how the ancient β-carbonic anhydrase, which is central to global carbon metabolism, was transformed by divergent evolution into a crucial enzyme in CS2 metabolism.

 


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