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

Zervarnicins (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-ILA contains five a-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 Zr-v-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)-enantiorner, indicating a high stereoselectivity of the peptide synthetase for this substrate. When the culture was supplemented with [N-15](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 protemogenic residues of Zrv. The partial catabolism of exogenous [N-15](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 (c) 2004 European Peptide Society and John Wiley & Sons, Ltd.