Fungal biosynthesis of non-ribosomal peptide antibiotics and α, α-dialkylated amino acid constituents

Zervamicins (Zrv) IIA and IIB are membrane modifying peptide antibiotics of fungal origin, characterized by a sequence of 15 amino acid residues. The primary structure of Zrv‐IIA contains five α‐aminoisobutyric acid residues at positions 4, 7, 9, 12 and 14 of the linear peptide. The sequence of Zrv‐IIB is similar, but contains a D‐isovaline at position 4. When the free amino acid Aib was added to the peptone‐glucose culture medium, the fungus Emericellopsis salmosynnemata produced Zrv‐IIA as the major secondary metabolite, whereas addition of DL‐Iva to the culture led to a high production of Zrv‐IIB. This observation is rationalized by a lack of selectivity of the non‐ribosomal peptide synthetase with respect to the thiolester activated amino acid substrates during step 12 of peptide synthesis. Analysis of the configuration of the Iva residue of Zrv‐IIB showed a high enantiomeric purity of the D‐enantiomer, indicating a high stereoselectivity of the peptide synthetase for this substrate. When the culture was supplemented with [15N]DL‐Iva, the nitrogen isotope was not only found at the D‐Iva residue, but surprisingly also at the Aib residues as well as at the proteinogenic residues of Zrv. The partial catabolism of exogenous [15N]DL‐Iva is explained by the assumption of a decarboxylation‐dependent transamination reaction, catalysed by 2,2‐dimethylglycine decarboxylase. The same enzyme might also be involved in the reversed carboxylation reactions of acetone and 2‐butanone, during the anabolic biosynthesis of Aib and Iva, respectively. Zrv might possibly act as a thermodynamic sink to shift these equilibrium reactions towards the reversed side. Copyright © 2004 European Peptide Society and John Wiley & Sons, Ltd.