Glu60 acts through a water molecule to donate a proton to N10 of CH2THF allowing the formation of an imminium ion at the N5 position. This later permits the donation of a methyl group to dCMP.
The sulphur atom of Cys148 undergoes nucleophilic attack on the cytosine ring with the aid of Asp179 which donates a proton to the ring. Asp179 subsequently accepts the proton back, initiating attack on the carbon of the imminium ion by C5 of cytosine forming a bridging methyl group between the C5 of cytosine and N5 of THF. Cytosine loses a proton from the C5 position to reform the double bond and at the same time accepts a proton from Asp179. Loss of this proton to Asp179 causes a double bond to be formed at the new bridging methyl with concomitant lysis of the C-THF bond.
The pi system is then attacked by water at the new carbon which causes the lysis of the bond to cystine. A loss of a proton from the water results in formation of the product .