Most experimental evidence for the mechanism of RNase H catalysis comes from measurements performed on members of the H1 group, usually the "E. coli" homolog. According to measurements of this protein, one of the aspartate residues has an elevated pKa, while another has an abnormally low pKa. It is unclear whether any of the active-site residues participates in the reaction as a general base. In addition, it is possible that one of the substrate's oxygen atoms participates directly in the reaction as a base.