Recombinant Neosartorya fumigata Protostadienol synthase A (pdsA), partial

Protostadienol synthase; part of the gene cluster that mediates the biosynthesis of helvolic acid, an antibacterial nortriterpenoid. Protostadienol synthase helA cyclizes (3S)-oxidosqualene to (17Z)-protosta-17(20),24-dien-3-beta-ol (protostadienol)(PubMed:19415934, PubMed:19216560, PubMed:19951700, PubMed:29158519). The synthesis of protostadienol is followed by several steps of monooxygenation, dehydrogenation, and acyl transfer to yield the final helvolic acid. Following the cyclization to the tetracyclic protostadienol by helA, cytochrome P450 monooxygenases helB1-mediated and helB2-mediated oxidation at C-4 and C-16, acyltransferase helD2-dependent acetylation of 16-OH, oxidation of C-21 by cytochrome P450 monooxygenase helB4, and short chain dehydrogenase helC-dependent oxidative decarboxylation yield the fusidane skeleton. This intermediate is further modified in three additional steps mediated by the cytochrome P450 monooxygenase helB3, the acyltransferase helD1, and the 3-ketosteroid 1-dehydrogenase helE to give helvolic acid. Compared with the late stages in the biosynthesis of helvolic acid, enzymes involved in the early stage modifications act in a relatively strict order. The hydroxylation of C-16 by helB1 and subsequent acetylation by helD2 should occur before the helB3-mediated oxidation of C-21. C-4 demethylation in fusidane-type antibiotics proceeds in an unusual manner though it is also achieved by oxidative decarboxylation. The methyl group at C-4 beta position is oxidized by helB1 and subsequently removed by the short chain dehydrogenase helC.