Effect of Glycosylation on the Reorganization at the Active Site of Human Carbonic Anhydrase IX

A classical molecular dynamics (MD) simulation study is reported on the catalytic domain of human carbonic anhydrase IX in water, with and without glycosylation, employing two force fields, CHARMM36M and ff19SB. The equilibrium structure and stability of the simulated systems in water are found to be largely preserved upon glycosylation. The fluctuations of the distal glycan chain appear to be highly correlated with two catalytically important reorganization processes at the active site. For extended glycan conformations pointing away from the enzyme surface, the active site water network becomes more disordered compared to its unglycosylated counterpart. Glycosylation also sustains the proton shuttle mediated by His-64 sidechain with rapidly interconverting in and out conformations. But the relative stability and rates of transition between these conformations depend critically on the force field used. In the presence of a few rare, back-folded conformations of the glycan chain, a new transient hydrogen-bond network connects the glycan chain to proton paths leading up to the His-64 mediated shuttle, revealing a possible role of glycan fluctuation on the catalysis.