The catalytic glutamic acid residue in UPOs is well known to coordinate to histidine or arginine stabilising the negative charge formed at the acid-base catalyst. MD simulations proved that H88 can establish persistent H-bond interactions with both E158 and E158D (Fig. ). We therefore further investigated interactions of the positions E158 and H88 by generating the variants H88A and H88A/E158D. Variant H88A led to no product formation while variant H88A/E158D was able to convert NBD with a 4.5-fold decreased TON relative to the wildtype. MD simulations indicated that in the absence of H88 (H88A) the α-7-helix becomes even more flexible and disordered, whereby the side chain of position E158 is barely preorganised to activate H2O2. On the other hand, the shorter side chain of E158D in H88A/E158D is still well-positioned to generate compound 1 (Fig. ).