β-Oxidation cycle reactions, which are key stages in the metabolism of fatty acids in eucaryotic cells and in processes with a significant role in the degradation of acids used by microbes as a carbon source, have also found application in biotransformations. One of the major advantages of biotransformations based on the β-oxidation cycle is the possibility to transform a substrate in a series of reactions catalyzed by a number of enzymes. It allows the use of sterols as a substrate base in the production of natural steroid compounds and their analogues. This route also leads to biologically active compounds of therapeutic significance. Transformations of natural substrates via β-oxidation are the core part of the synthetic routes of natural flavors used as food additives. Stereoselectivity of the enzymes catalyzing the stages of dehydrogenation and addition of a water molecule to the double bond also finds application in the synthesis of chiral biologically active compounds, including medicines. Recent advances in genetic, metabolic engineering, methods for the enhancement of bioprocess productivity and the selectivity of target reactions are also described.

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Metabolism of steroids in healthy and unhealthy human organs is the subject of extensive clinical and biomedical studies. For this kind of investigations, it is essential that the reference samples of new derivatives of natural, physiologically active steroids (especially those difficult to achieve in the chemical synthesis) become available. This study demonstrated for the first time transformation of 7-oxo-DHEA-a natural metabolite of DHEA, using Syncephalastrum racemosum cells. The single-pulse fermentation of substrate produced two new hydroxy metabolites: 1β,3β-dihydroxy-androst-5-en-7,17-dione and 3β,12β-dihydroxy-androst-5-en-7,17-dione, along with the earlier reported 3β,9α-dihydroxy-androst-5-en-7,17-dione and 3β,17β-dihydroxy-androst-5-en-7-one. Simultaneously, the same metabolites, together with small quantities of 7α- and 7β-hydroxy-DHEA, as well as the products of their reduction at the C-17 were obtained after transformation of DHEA under pulse-feeding of the substrate. The observed reactions suggested that this micro-organism contains enzymes exhibiting similar activity to those present in human cells. Thus, the resulting compounds can be considered as potential components of the eukaryotic, including human, metabolome.