THE ENETHIOLATE ANION REACTION-PRODUCTS OF EPID - P-KAPPA(ALPHA) VALUE OF THE ENETHIOL SIDE-CHAIN IS LOWER THAN THAT OF THE THIOL SIDE-CHAIN OF PEPTIDES

One of the steps involved in the biosynthesis of the lantibiotic epide rmin is the oxidative decarboxylation reaction of peptides catalyzed b y the flavoenzyme EpiD. EpiD catalyzes the formation of a (Z)-enethiol derivative from the C-terminal cysteine residue of the precursor pept ide of epidermin and related peptides. The UV-visible spectra of the r eaction products of EpiD are pH-dependent, indicating that the enethio l side chain is converted to an enethiolate anion. The pK(alpha), valu e of the enethiol group was determined to be 6.0 and is substantially lower than the pK(alpha) value of the thiol side chain of cysteine res idues. The increased acid strength of the enethiol side chain compared with that of the thiol group is attributed to the resonance stabiliza tion of the negative charge of the anion.