A crucial and enigmatic step in the complex biosynthesis of aflatoxin B1 is the oxidative rearrangement of versicolorin A to demethylsterigmatocystin. This step is thought to proceed by an oxidation–reduction–oxidation sequence, in which the NADPH-dependent oxidoreductase AflM catalyzes the enclosed reduction step. AflM from Aspergillus parasiticus, after heterologous production in E. coli and puriflcation, however, catalyzed the reduction of the hydroquinoid form of the starting compound versicolorin A (25% conversion) to a so far unknown product of aflatoxin biosynthesis. The asymmetric reduction of emodin hydroquinone to (R)-3,8,9,10-tetrahydroxy-6-methyl-3,4-dihydroanthracen-1(2H)-one (up to 82% for AflM) has also been observed in previous studies using MdpC from Aspergillus nidulans (mono-dictyphenone biosynthetic gene cluster). The first (non-enzymatic) reduction of emodin to emodin hydroquinone, for example with sodium dithionite, is obligatory for the enzymatic reduction by AflM or MdpC. These results imply an unprecedented role of AflM in the complex enzymatic network of aflatoxin biosynthesis.

NAD(P)H‐dependent oxidoreductases from the short‐chain dehydrogenases/reductases (SDRs) family possess high functional diversity. Three SDRs, namely, tetrahydroxy‐ and trihydroxynaphthalene reductases (T4HNR, T3HNR) involved in the dihydroxynaphthalene‐melanin biosynthesis of the phytopathogenic fungus Magnaporthe grisea, and glucose dehydrogenase (GDH) from Bacillus subtilis, were characterized regarding their substrate range and functional behavior. T4HNR and T3HNR share activities towards the stereoselective reduction of 2‐tetralone derivatives and 2,3‐dihydro‐1,4‐naphthoquinones and show distinct but different stereochemical outcome in the case of epoxy‐1,4‐napthoquinones as substrates. GDH shares the activity towards 2,3‐dihydro‐1,4‐naphthoquinones, however, with low stereocontrol. Moreover, GDH reduces 2‐hydroxy‐2,3‐dihydro‐1,4‐naphthoquinone into trans‐4‐hydroxyscytalone with a high diastereomeric excess (96 %), whereas T4HNR gave the cis diastereomer (diastereomeric excess>99 %). Thus, SDRs provide a much higher functional and stereochemical diversity than previously thought, already exemplified by many transformations of three members of this enzyme family.

17β-Hydroxysteroid dehydrogenase (17β-HSDcl) from the filamentous fungus Curvularia lunata (teleomorph Cochliobolus lunatus) catalyzes NADP(H)-dependent oxidoreductions of androgens and estrogens. Despite detailed biochemical and structural characterization of 17β-HSDcl, its physiological function remains unknown. On the basis of amino acid sequence alignment, phylogenetic studies, and the recent identification of the physiological substrates of the homologous MdpC from Aspergillus nidulans and AflM from Aspergillus parasiticus, we propose an anthrahydroquinone as the physiological substrate of 17β-HSDcl. This is also supported by our analysis of a secondary metabolite biosynthetic gene cluster in C. lunata m118, containing 17β-HSDcl and ten other genes, including a polyketide synthase probably involved in emodin formation. Chemoenzymatic reduction of emodin by 17β-HSDcl in the presence of sodium dithionite verified this hypothesis. On the basis of these results, the involvement of a 17β-HSDcl in the biosynthesis of other anthrahydroquinone-derived natural products is proposed; hence, 17β-HSDcl should be more appropriately referred to as a polyhydroxyanthracene reductase (PHAR).

Phloroglucinol reductases (PGRs) are involved in anaerobic degradation in bacteria, in which they catalyze the dearomatization of phloroglucinol into dihydrophloroglucinol. We identified three PGRs, from different bacterial species, that are members of the family of NAD(P)H-dependent short-chain dehydrogenases/reductases (SDRs). In addition to catalyzing the reduction of the physiological substrate, the three enzymes exhibit activity towards 2,4,6-trihydroxybenzaldehyde, 2,4,6-trihydroxyacetophenone, and methyl 2,4,6-trihydroxybenzoate. Structural elucidation of PGRcl and comparison to known SDRs revealed a high degree of conservation. Several amino acid positions were identified as being conserved within the PGR subfamily and might be involved in substrate differentiation. The results enable the enzymatic dearomatization of monoaromatic phenol derivatives and provide insight into the functional diversity that may be found in families of enzymes displaying a high degree of structural homology.

Phloroglucinol reductases (PGRs) are involved in anaerobic degradation in bacteria, in which they catalyze the dearomatization of phloroglucinol into dihydrophloroglucinol. We identified three PGRs, from different bacterial species, that are members of the family of NAD(P)H‐dependent short‐chain dehydrogenases/reductases (SDRs). In addition to catalyzing the reduction of the physiological substrate, the three enzymes exhibit activity towards 2,4,6‐trihydroxybenzaldehyde, 2,4,6‐trihydroxyacetophenone, and methyl 2,4,6‐trihydroxybenzoate. Structural elucidation of PGRcl and comparison to known SDRs revealed a high degree of conservation. Several amino acid positions were identified as being conserved within the PGR subfamily and might be involved in substrate differentiation. The results enable the enzymatic dearomatization of monoaromatic phenol derivatives and provide insight into the functional diversity that may be found in families of enzymes displaying a high degree of structural homology.