α-aminoadipyl-cysteinyl-valine synthetases in β-lactam producing organisms -: From Abraham's discoveries to novel concepts of non-ribosomal peptide synthesis

The tripeptide delta-(L-alpha -aminoadipyl)-L-cysteinyl-D-valine (ACV) was discovered by ARNSTEIN and MORRIS in Penicillium chrysogenum and ABRAHAM and coworkers in Acremonium chryogenum. Other analogous tripeptides and tetrapeptides were later reported in these and other beta -lactam producing fungi and actinomycetes. The ACV tripeptide is synthesized by a large non-ribosomal peptide synthetase named ACV synthetase encoded by the 11 kb pcbAB gene. This gene has been cloned from the DNA of four different filamentous fungi and two actinomycetes. Detailed analysis of the multifunctional ACV synthetases reveals that they consist of three repeated modules (initially named domains) involved in activation of the corresponding amino acids L-alpha -aminoadipic acid, L-cysteine and L-valine. Each module consists of functional domains for amino acid activation (A), condensation (C) and thiolation (T). In addition the last module of the ACV synthetase contains an epimerization domain (E) involved in conversion of the L-valine to its D-isomer when the tripeptide is still enzyme linked. There are seven epimerization motifs conserved in the third module of all ACV synthetases. In addition, there is an integrated thioesterase domain in the C-terminal region of the ACV synthetases that appears to be involved in the selective release of the tripeptide with the correct LLD configuration. The structure of the ACV synthetase is similar to that of other modular non-ribosomal peptide synthetases of bacterial and fungal origin. This molecular knowledge opens the way for engineering novel tripeptide synthetases that may result in new bioactive compounds.