Fig. 1. The first step of the One Asp (top) and Two Asp (bottom) catalytic mechanisms on the carbonyl carbon of the substrate scissile bond, forming a tetrahedral intermediate. In the One Asp mechanism, the same aspartate residue (AspB) deprotonates the hydrolytic water molecule and protonates the peptide oxygen – one carboxylate oxygen acts as an acid and the other as a base. In the Two Asp mechanism, which is most widely accepted, one of the two active site aspartates (AspB) protonates the oxyanion generated by the water attack, while the other (AspA) deprotonates the nucleophilic water. Conventionally, AspB is the residue that is considered to be protonated – the two chains are equivalent.