Summons Roger E, Bradley Alexander S, Jahnke Linda L, Waldbauer Jacob R
Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Avenue E34-246, Cambridge, MA 02139-4307, USA.
Philos Trans R Soc Lond B Biol Sci. 2006 Jun 29;361(1470):951-68. doi: 10.1098/rstb.2006.1837.
There is a close connection between modern-day biosynthesis of particular triterpenoid biomarkers and presence of molecular oxygen in the environment. Thus, the detection of steroid and triterpenoid hydrocarbons far back in Earth history has been used to infer the antiquity of oxygenic photosynthesis. This prompts the question: were these compounds produced similarly in the past? In this paper, we address this question with a review of the current state of knowledge surrounding the oxygen requirement for steroid biosynthesis and phylogenetic patterns in the distribution of steroid and triterpenoid biosynthetic pathways. The hopanoid and steroid biosynthetic pathways are very highly conserved within the bacterial and eukaryotic domains, respectively. Bacteriohopanepolyols are produced by a wide range of bacteria, and are methylated in significant abundance at the C2 position by oxygen-producing cyanobacteria. On the other hand, sterol biosynthesis is sparsely distributed in distantly related bacterial taxa and the pathways do not produce the wide range of products that characterize eukaryotes. In particular, evidence for sterol biosynthesis by cyanobacteria appears flawed. Our experiments show that cyanobacterial cultures are easily contaminated by sterol-producing rust fungi, which can be eliminated by treatment with cycloheximide affording sterol-free samples. Sterols are ubiquitous features of eukaryotic membranes, and it appears likely that the initial steps in sterol biosynthesis were present in their modern form in the last common ancestor of eukaryotes. Eleven molecules of O2 are required by four enzymes to produce one molecule of cholesterol. Thermodynamic arguments, optimization of function and parsimony all indicate that an ancestral anaerobic pathway is highly unlikely. The known geological record of molecular fossils, especially steranes and triterpanes, is notable for the limited number of structural motifs that have been observed. With a few exceptions, the carbon skeletons are the same as those found in the lipids of extant organisms and no demonstrably extinct structures have been reported. Furthermore, their patterns of occurrence over billion year time-scales correlate strongly with environments of deposition. Accordingly, biomarkers are excellent indicators of environmental conditions even though the taxonomic affinities of all biomarkers cannot be precisely specified. Biomarkers are ultimately tied to biochemicals with very specific functional properties, and interpretations of the biomarker record will benefit from increased understanding of the biological roles of geologically durable molecules.
特定三萜类生物标志物的现代生物合成与环境中分子氧的存在之间存在着密切联系。因此，在地球历史早期对类固醇和三萜类碳氢化合物的检测已被用于推断产氧光合作用的古老程度。这就引发了一个问题：这些化合物在过去的产生方式是否类似？在本文中，我们通过回顾围绕类固醇生物合成的氧气需求以及类固醇和三萜类生物合成途径分布的系统发育模式的当前知识状态来解决这个问题。类异戊二烯和类固醇生物合成途径分别在细菌域和真核域内高度保守。多种细菌可产生细菌藿烷多元醇，产氧蓝细菌会在C2位置大量甲基化。另一方面，甾醇生物合成在远缘相关的细菌分类群中分布稀少，且这些途径不会产生构成真核生物特征的广泛产物。特别是，蓝细菌进行甾醇生物合成的证据似乎存在缺陷。我们的实验表明，蓝细菌培养物很容易被产甾醇的锈菌污染，用环己酰亚胺处理可以消除这种污染，从而得到无甾醇的样品。甾醇是真核细胞膜的普遍特征，甾醇生物合成的初始步骤似乎很可能以现代形式存在于真核生物的最后共同祖先中。生成一分子胆固醇需要四种酶消耗十一分子的O₂。热力学观点、功能优化和简约性都表明，古老的厌氧途径极不可能。已知的分子化石地质记录，尤其是甾烷和三萜烷，其观察到的结构基序数量有限。除了少数例外，碳骨架与现存生物脂质中的碳骨架相同，尚未报道明显灭绝的结构。此外，它们在数十亿年时间尺度上的出现模式与沉积环境密切相关。因此，生物标志物是环境条件的极佳指标，尽管并非所有生物标志物的分类学亲缘关系都能精确确定。生物标志物最终与具有非常特定功能特性的生物化学物质相关联，对生物标志物记录的解释将受益于对地质耐久性分子生物学作用的更多了解。
