Anaerobic bacterial metabolism of cholesterol

胆固醇是自然界中含量最丰富的类固醇之一。这种普遍存在的三萜的缺氧代谢是具有挑战性的，因为其复杂的化学结构，在水中的低溶解度，活性官能团的数量少，存在四个脂环和两个季碳原子。因此，胆固醇的有氧代谢依赖于需要分子氧作为共底物的加氧酶催化反应。已显示，Sterolibacterium acetificans通过环A的氧化，随后通过侧链末端C-25的氧非依赖性羟基化，在没有氧的情况下完全代谢胆固醇。对催化这些初始反应的酶进行了纯化和一些详细的表征。最有趣的酶是C-25侧链羟化酶，属于含Mo-辅因子的DMSO还原酶家族。将详细研究这种独特酶的结构和功能。序列分析表明，S.嗜热链球菌揭示了许多编码推定的类似含钼羟化酶的基因，并且预测这些基因在胆固醇的厌氧催化中催化迄今为止未表征的氧非依赖性羟化反应。这些酶在缺氧胆固醇降解步骤中的作用将通过蛋白质组学和代谢组学方法进行研究。

Cholesterol is one of the most abundant steroids in the nature. The anoxic metabolism of this ubiquitous triterpene is challenging because of its complex chemical structure, low solubility in water, low number of active functional groups, the presence of four alicyclic rings and two quaternary carbon atoms. Consequently, the aerobic metabolism of cholesterol depends on oxygenase catalyzed reactions requiring molecular oxygen as co-substrate. Sterolibacterium denitrificans was shown to metabolize cholesterol completely without oxygen via the oxidation of ring A, followed by an oxygen independent hydroxylation of the terminal C-25 of the side chain. The enzymes catalyzing these initial reactions were purified and charcterized in some detail. The most intriguing enzyme is the C-25 side chain hydroxylase belonging to the Mo-cofactor containing family of DMSO reductases. The structure and function of this unique enzyme will be investigated in detail. The genome sequence of S. denitrificans revealed a number of genes that code for putative similar Mo-containing hydroxylases, and that are predicted to catalyze so far non-characterized oxygen-independent hydroxylation reaction in the anaerobic catabolism of cholesterol. The role of such enzymes in anoxic cholesterol degradation steps will be studied by proteomic and metabolomic approaches.