The remaining (from the acylation step) carbonyl part, still attached to the serine, must be removed and the active site is regenerated. The second step removes the carbonyl part of the peptide amino acid from the serine through a reaction with water (hydrolysis of the acylated serine residue). The -OH portion of the water molecule reacts with the carbonyl group forming a second tetrahedral intermediate, and the -H is bonded to the histidine. When the carbonyl bond reforms, the bond to the serine oxygen is broken, freeing the second part of the peptide chain. The hydrogen bonded to the histidine is lost to the serine oxygen and a hydrogen bond is reformed between the serine and histidine. This is the deacylation phase and can be seen in the deacylation reactions picture.