The catalytic His447 is more stable with Glu202 being protonated
In our previous study, we found that the catalytic His438 of BChE is eventually distorted after the collapse of the key hydrogen bond network. It is thus interesting to know whether the same effects can be observed in AChE. Shown in Figure 3A is the RMSD traces of the catalytic His447 of AChE with Glu202 being protonated. Clearly, the His447 stays firmly at its crystal structure position throughout the entire simulation. However, from Figure 3A where Glu202 is deprotonated, the RMSD traces indicate that the catalytic His447 stays at its crystal structure position for only the initial 20 ns. Then, the His447 deviates significantly from its crystal structure position. Shown in Figure 3C is the His447 in the representative MD-simulated structure compared to that in the crystal structure. Similar to what is observed for the key hydrogen bond network, the simulated His447 is also almost identical to that in the crystal structure. The distances of the two hydrogen bonds in the catalytic triad are 2.4±0.8 Å and 1.8±0.1 Å, respectively, indicating a stable catalytic triad throughout the simulation. However, this catalytic triad is observed as disrupted in a simulation with Glu202 being deprotonated. As we mentioned above, due to the repulsion between the deprotonated Glu202 and the key water molecule, the key hydrogen bond network is eventually collapsed. From Figure 4D, we can see that the collapsed key hydrogen bond network produces an empty space for the catalytic His447. This empty space decreases the stability of His447 and promotes it to deviate largely from its crystal structure position, causing the disruption of the catalytic triad.
Therefore, the key hydrogen bond network plays an important role in stabilizing the catalytic triad through supporting the His447. Without the key hydrogen bond network, the His447 could deviate largely to occupy the space that originally belongs to the hydrogen bond network, and this large deviation of His447 apparently results in the distortion of the catalytic triad.