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类化合物的化学结构复杂，在水中的溶解度低，活性官能团数量少，存在四个脂环环和两个季碳原子，因此其缺氧代谢具有挑战性。因此，胆固醇的有氧代谢依赖于加氧酶催化的反应，需要分子氧作为共底物。脱氮杆菌通过A环的氧化和侧链末端C-25的氧非依赖羟化来完全无氧代谢胆固醇。对催化这些初始反应的酶进行了提纯，并对其进行了详细的鉴定。最耐人寻味的酶是C-25侧链羟基酶，属于含钼辅因子的DMSO还原酶家族。这种独特的酶的结构和功能将被详细研究。脱氮链球菌的基因组序列揭示了一些编码类似含钼羟基酶的基因，这些基因被预测在胆固醇的厌氧分解代谢中催化迄今尚未确定的氧非依赖性羟基化反应。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的方法进行研究。

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.