The rotamer of the second sphere histidine in AA9 Lytic polysaccharide monooxygenase is pH-dependent

Biophys J. 2024 Apr 2:S0006-3495(24)00246-7. doi: 10.1016/j.bpj.2024.04.002. Online ahead of print.ABSTRACTLytic polysaccharides monooxygenases (LPMOs) catalyze a reaction that is crucial for the biological decomposition of various biopolymers and for the industrial conversion of plant biomass. Despite the importance of LPMOs, the exact molecular-level nature of the reaction mechanism is still debated today. Here, we investigated the pH dependent conformation of a second sphere histidine (His) that we call the "stacking histidine", which is conserved in fungal AA9 LPMOs, and is speculated to assist catalysis in several of the LPMO reaction pathways. Using constant pH and accelerated molecular dynamics simulations, we monitored the dynamics of the stacking His in different protonation states, for both the resting Cu(II) and active Cu(I) forms of two fungal LPMOs. Consistent with experimental crystallographic and neutron diffraction data, our calculations suggest that the side chain of the protonated and positively charged form (HIP) is rotated out of the active site towards the solvent. Importantly, only one of the possible neutral states of histidine (HIE state) is observed in the stacking orientation at neutral pH or when bound to cellulose. Our data predict that, in solution, the stacking His may act as a stabilizer (via H-bonding) of the Cu(II)-superoxo complex after the LPMO-Cu(I) has reacted with O2 in solution, which in fine leads to H2O2 formation. Also, our data indic...
Source: Biophysical Journal - Category: Physics Authors: Source Type: research