A typical bacterial cholesterol degradation pathway

A typical bacterial cholesterol degradation pathway is presented in Fig. 1. Generally, the pathway is supposed to start with the oxidation or dehydrogenation of cholesterol (1) to 5-cholesten-3-one (59; Fig. 2), followed by isomerization to 4-cholesten-3-one (2). Under aerobic conditions, this transformation is catalyzed by oxygen-dependent bifunctional cholesterol oxidases/isomerases or 3β-hydroxysteroid dehydrogenases/isomerases [1,2,3,4], but under anaerobic conditions anoxic bifunctional cholesterol dehydrogenase/isomerase enzymes take care of the conversion [5,6]. Under aerobic conditions, the degradation of the eight-carbon aliphatic side chain of cholesterol is initiated with the hydroxylation of the C26 or C27 atom by the cytochrome P450 monooxygenase Cyp125 [7,8] or Cyp142 [9], followed by oxidation of the hydroxyl group to a carboxylate by the same enzyme. The resulting C26- or C27-carboxylate intermediate is subsequently activated as its coenzyme A (CoA) derivative by an ATP-dependent steroid-CoA ligase [10,11]. The release of the side chain has been elucidated biochemically to proceed through three cycles of a process similar to the β-oxidation of fatty acids, yielding the nineteen-carbon steroid core intermediate, e.g. 4-androstene-3,17-dione (AD; 8), by releasing successively propionic acid, acetic acid, and another propionic SCH 58261 [12,13]. Under anaerobic conditions, bacteria use a similar route to degrade the side chain [14]. However, the degradation is initiated by hydroxylation of 4-cholesten-3-one (2) at C25, instead of at C26 or C27, to yield 25-hydroxy-4-cholesten-3-one (16), by an oxygen-independent hydroxylase using a water molecule as the oxygen donor [5,6], and subsequent isomerization to form 27-hydroxy-4-cholesten-3-one (3) [14]. The degradation of the steroid nucleus is primed with the introduction of the 1(2)-double bond into the steroid ring system (see below). The 1(2)-unsaturated intermediate then follows either the 9,10-seco pathway for aerobic degradation (magenta arrows in Fig. 1) or the 2,3-seco pathway for anaerobic degradation (blue arrows in Fig. 1). More detailed information on microbial cholesterol degradation can be found elsewhere [1,2,3]. Besides degrading cholesterol, the cholesterol degradation pathway also offers a route to obtain useful starting materials for the production of steroid drugs and hormones. Indeed, steroids are among the most marketed pharmaceuticals with about 300 approved steroid drugs [15] and a worldwide market of approximately 10 billion USD per year [16]. On the other hand, for some pathogenic microorganisms, the steroid catabolic pathway is intimately involved in pathogenicity and virulence. This is in particular the case for pathogenic bacteria such as Mycobacterium tuberculosis and Rhodococcus equi, which depend on cholesterol for survival inside macrophages [17,18]. Finally, the pathway is important for clearance of steroid hormones released into the aquatic environment by human activity, where they may affect the physiology of aquatic organisms [19,20]. Thus, microbial steroid degradation and conversion is of interest to multiple fields.