balhimycina and the vancomycin producer Amycolatopsis orientalis, and support in vitro turnover of peptidyl carrier protein-bound peptide substrates into monocyclic cross-linked products. These results show that ferredoxins encoded in the antibiotic-producing strain can act in a degenerate manner

in supporting the catalytic functions of glycopeptide biosynthetic P450 enzymes from the same as well as heterologous gene clusters. Cytochrome P450 enzymes typically catalyze the hydroxylation of substrates, using molecular oxygen and reducing equivalents supplied by NAD(P)H. The class I bacterial cytochrome P450 hydroxylases require a flavin-dependent ferredoxin reductase (FdR), which is reduced by NAD(P)H, and a ferredoxin (Fd) iron–sulfur protein to mediate electron transfer to the P450 BGB324 heme (Munro et al., 2007). Less well studied is a small group of P450s that catalyze oxidative phenol coupling reactions on substrates containing phenolic EPZ5676 groups (Isin & Guengerich, 2007). Molecular oxygen is again required, but no oxygen atom is incorporated into the product of the enzymic reaction, although there is again a requirement for electrons, which must be shuttled from NAD(P)H to the heme during the catalytic cycle. Three bacterial class I cytochrome P450 enzymes called OxyA, OxyB and OxyC catalyze three

key cross-linking reactions in the biosynthesis of glycopeptide antibiotics of the vancomycin/balhimycin family (Fig. 1). X-ray crystal structures of OxyB and OxyC from the vancomycin producer Inositol monophosphatase 1 Amycolatopsis orientalis confirmed that each contains a typical P450 fold, with a conserved cysteine residue acting as a proximal axial ligand for the heme (Zerbe et al., 2002; Pylypenko et al.,

2003). The order of the cross-linking reactions in balhimycin biosynthesis has been defined through gene inactivation experiments in Amycolatopsis balhimycina (Süssmuth et al., 1999; Bischoff et al., 2001a, b; 2005). The first cross-link, introduced by OxyB, is an aryl-ether bridge (C-O-D ring) between the side chains of residues-4 and -6. The second cross-link (D-O-E ring) is introduced by OxyA, and the final biaryl link (the AB ring) is created by OxyC. In vitro experiments have shown that linear hexa- or heptapeptides attached as C-terminal thioesters to the pantetheinyl group of a peptide carrier protein (PCP) domain from the glycopeptide nonribosomal peptide synthetase (NRPS) are the preferred substrates for OxyB (Zerbe et al., 2004; Woithe et al., 2007). The first cross-link, therefore, is introduced while the peptide chain of the antibiotic is still attached to the NRPS assembly line. So far, in vitro assays with the two remaining cross-linking enzymes OxyA and OxyC have not been reported, and so the timing of these cross-linking steps remains undefined.