Characterization of the Aminocarboxycyclopropane-Forming Enzyme CmaC
The biosynthesis of the coronamic acid fragment of the pseudomonal phytotoxin coronatine involves construction of the cyclopropane ring from a γ-chloro-l-allo-Ile intermediate while covalently tethered as a phosphopantetheinyl thioester to the carrier protein CmaD. The cyclopropane-forming catalyst is CmaC, catalyzing an intramolecular displacement of the γ-Cl group by the α carbon. CmaC can be isolated as a Zn2+ protein with about 10-fold higher activity over the apo form. CmaC will not cyclize free γ-chloro amino acids or their S-N-acetylcysteamine (NAC) thioester derivatives but will recognize some other carrier protein scaffolds. Turnover numbers of 5 min-1 are observed for Zn−CmaC, acting on γ-chloro-l-aminobutyryl-S-CmaD, generating 1-aminocyclopropane-1-carbonyl (ACC)-S-CmaD. Products were detected either while still tethered to the phosphopantetheinyl prosthetic arm by mass spectrometry or after thioesterase-mediated release and derivatization of the free amino acid. In D2O, CmaC catalyzed exchange of one deuterium into the aminobutyryl moiety of the γ-Cl-aminoacyl-S-CmaD, whereas the product ACC-S-CmaD lacked the deuterium, consistent with a competition for a γ-Cl-aminobutyryl α-carbanion between reprotonation and cyclization. CmaC-mediated cyclization yielded solely ACC, resulting from C−C bond formation and no azetidine carboxylate from an alternate N−C cyclization. CmaC could cyclize γ,γ-dichloroaminobutyryl to the Cl-ACC product but did not cyclize δ- or ε-chloroaminoacyl-S-CmaD substrates.
© 2007 American Chemical Society
“Characterization of the aminocarboxycyclopropane-forming enzyme CmaC,” Kelly, W.L.; Boyne, M.T., II; Yeh, E.; Vosburg, D.A.; Galonic, D.P.; Kelleher, N.L.; Walsh, C.T.; Biochemistry 2007, 46, 359-368. DOI: 10.1021/bi061930j