Fig. 4 in Polyketide-derived macrobrevins from marine macroalga-associated Bacillus amyloliquefaciens as promising antibacterial agents against pathogens causing nosocomial infections

Fig. 4. Proposed biosynthesis of 21- membered macrocyclic lactones classified as macrobrevin analogues (1–4) in B. amyloliquefaciens through successive decarboxylative Claisen condensation between acetyl-S-KS domain and malonate-SACP units. Claisen condensation was activated by acyl carrier protein (ACP), ketoreductase (KR), ketosynthase (KS), thioesterase (TE), dehydratase (DH), methyl transferase (MT), acyl transferase (AT), enoyl reductase (ER) and S-adenosyl-methionine (SAM). The elongation process comprised of 16 modules with KS, KR and ACP domains. The initial step includes the decarboxylative Claisen condensation between 2-methylbutanethioic-S-KS and malonate-S-ACP. The final step of macrobrevin formation could occur through the cyclization of linear chain of 21-membered carbon framework by TE. Consequently, alterations of 21-membered carbon framework classified as macrobrevin scaffold could result in the formation of macrobrevin analogues 1–4.