In the first step the serine side-chain (CH2-OH) reacts with the carbonyl group of the substrate peptide amino acid, forming a tetrahedral transition state. The H atom of the OH group is hydrogen bonded to the imidazole ring of the histidine, which acts as a base (proton acceptor). The transition state reforms the carbonyl double bond and breaks the nitrogen-carbon bond, thus breaking the peptide bond. The amino portion accepts the hydrogen from the His and one part of the peptide chain breaks free. This is the acylation phase and can be seen in the acylation reactions picture.