Investigating the P450-Mediated Cross-Linking of the Atypical GPA Complestatin Using Alternate Peptide Probes

The glycopeptide antibiotics (GPAs) are clinically relevant antibiotics defined by their highly cross-linked structure, which is required for their antimicrobial activity. While GPAs are typically characterized as type I-IV GPAs - heptapeptides that bind to the d-Ala-d-Ala terminus of Lipid II - an expansion of these subtypes has recently been seen for type V GPAs that possess altered structural characteristics and mechanisms of action. The major structural changes seen with this class are due to varied cross-linking occurring between the aromatic side chain residues, with these cross-links installed by cytochrome P450 (Oxy) enzymes that are recruited to the peptide substrate during nonribosomal peptide synthesis by a specialized recruitment domain, known as the X-domain. Given the importance of these cross-links for the structural rigidity and activity of GPAs, in vitro characterization of their activity remains a priority for the field. In this study, we synthesized a range of peptides, including an authentic complestatin precursor, to explore the cross-linking of these peptides by a selection of Oxy enzymes. Our results show the importance of stereochemistry in the C-terminal portion of the peptide substrate, revealing alternate cross-linking patterns and emphasizing the challenge of working with complete type V GPA peptides in vitro.