The nucleophilic OH group of Ser 195 attacks the carbonyl carbon of the scissile bond (Michaelis complex), forming the tetrahedral transition state. Here, the C=O bond becomes a single bond, leaving a negative charge on the O atom (an oxyanion) while the fourth valence of the carbon atom is occupied by a bond with the serine O-gamma.
The proton donated from the OH group of Ser 195 to His 57 is then donated to the N atom of the scissile bond, cleaving the C-N peptide bond (or the C-O ester bond) to produce the amine and the acyl-enzyme intermediate. The amine is that part of the substrate which follows the scissile bond in the sequence; the acyl-enzyme intermediate is the remaining fragment covalently bound to Ser 195.
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