Fig. 4. HSD-1 is involved in ⁴-dafachronic acid synthesis. (A) HSD-1 may function in the proposed biosynthetic pathway of ⁴- and ⁷-dafachronic acids [modified, with permission from Rottiers et al. (Rottiers et al., 2006)]. Predicted intermediates for biosynthesis of ⁷- and ⁴-dafachronic acids from cholesterol are indicated in the upper and lower branches of the pathway, respectively. The marked positions on the cholesterol structure (1-7) represent the standard numbering of the carbon atoms. 5-cholesten-3-one, in parentheses, is the presumed product, from cholesterol, after the first of the two-step 3β-HSD reactions. Shaded triangles and ovals indicate the sequential changes that HSD-1 is hypothesized to direct: triangles indicate 3β-dehydrogenase activity, ovals indicate isomerase activity. (B) The structures of ergosterol and β-sitosterol, the major fungal and plant sterols, respectively.