Bacteria produce a wide array of metabolites to protect themselves from competing microbes. These antimicrobial compounds include
peptides with an S-[(Z)-2-aminovinyl]-d-
cysteine (AviCys) or S-[(Z)-2-aminovinyl]-(3S)-3-methyl-d-
cysteine (
AviMeCys) residue, which have been isolated from several different bacterial species. The
peptides are structurally diverse: some feature polycyclic backbones, such as the
lantibiotic epidermin, and others feature a mostly linear structure, such as
cypemycin. Each of the AviCys-containing
peptides characterized to date exhibit highly potent
biological activities, ranging from antimicrobial activity against methicillin-resistant Staphylococcus aureus (MRSA) to anticancer activity against mouse
leukemia cells. The AviCys-containing
peptides gallidermin and
mutacin 1140 have been suggested as possible treatments of
acne and of throat
infections, respectively. Unfortunately, their low production yield in fermentation (typically only 10-200 mg/L) remains a major hindrance to the widespread use and clinical testing of AviCys-containing
peptides for human
therapeutics. Although scientists have made great strides in the total chemical synthesis of polycyclic
peptides on solid support, an efficient method to form the AviCys ring has yet to be developed. In light of these difficulties, it may be possible to draw inspiration from the natural biosynthesis of AviCys-containing
peptides within the producer organisms. In this Account, we examine the characteristics of the
enzymes responsible for constructing AviCys to evaluate possibilities for generating high yields of bioactive AviCys- or
AviMeCys-containing
peptides for research and clinical use. The gene cluster for the biosynthesis of
epidermin has been studied in depth, leading to the proposal for a mechanism of AviCys formation. First, a
serine residue upstream of the C-terminus is enzymatically dehydrated to form a
dehydroalanine residue. Then, the C-terminal
cysteine residue is oxidatively decarboxylated to form an enethiolate, which subsequently cyclizes onto the
dehydroalanine to give the AviCys ring. Extensive research on EpiD, the
enzyme responsible for the oxidative decarboxylation reaction, has led to its purification and cocrystallization with a model substrate
peptide, yielding an X-ray crystal structure. An in vitro assay of the
enzyme with a library of synthetic heptapeptides has resulted in the discovery that EpiD has low absolute substrate specificity and can oxidatively decarboxylate a wide variety of C-terminal
cysteine-containing
peptides. Recently, the gene cluster for the biosynthesis of
cypemycin was also identified. Despite certain structural similarities between
cypemycin and the
lantibiotic peptides, analysis of the biosynthetic genes suggests that
cypemycin production is quite different from that of the
lantibiotics. In particular, the AviCys residue in
cypemycin is formed from two
cysteine residues instead of one
serine and one
cysteine, and the CypD
enzyme that catalyzes the oxidative decarboxylation of the C-terminal
cysteine shows little homology to EpiD. The knowledge accrued from studying EpiD and CypD could be used to develop a semisynthetic methodology to produce AviCys-containing
peptides. In particular, suitable precursor
peptides could be synthesized on solid support before being fed to either of these
enzymes in vitro to generate the C-terminal AviCys moiety. Exploring the potential of this methodology could lead to the efficient production of
epidermin,
cypemycin, and analogues thereof.