Proteolytic dehydration

Figure 4 The biosynthesis of nisin A as a representative example of the posttranslational maturation process of lantibiotics. Following ribosomal synthesis, NisB dehydrates serine and threonine residues in the structural region of the prepeptide NisA. NisC subsequently catalyzes intramolecular addition of cysteine residues onto the dehydro amino acids in a stereo- and regioselective manner. Subsequent transport of the final product across the cell membrane by NisT and proteolytic cleavage of the leader sequence by NisP produces the mature lantibiotic. For the sequence of the leader peptide, see Figure 6. Adapted with permission from J. M. Willey W. A. van der Donk, Annu. Rev. Microbiol. 2007, 61, 477-501.

This extremely facile restoration process indicates that the environment of the scissile bond in the complex is quite hydrophobic in nature so the dehydration reaction, which is highly endergonic in aqueous environment, proceeds unusually rapidly. One can therefore conclude that the Michaelis intermediate with the intact scissile bond is indeed the thermodynamically stable form and not the kinetically controlled one. The subtilisin is being entrapped in the complex since it is excessively stable, hence its proteolytic activity is inhibited by SSI. 

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