Mapping the protonation states of the histidine brace in an AA10 lytic polysaccharide monooxygenase using CW-EPR spectroscopy and DFT calculations

Abstract

The active site of the polysaccharide-degrading lytic polysaccharide monooxygenase (LPMO) enzyme features a single copper ion coordinated by a histidine brace. The primary coordination sphere of the copper contains several ligating atoms which are bonded to ionisable protons (e.g. OH₂, NH₂), the pK_as of which are unknown. Using a combination of CW-EPR X-band spectroscopy over a range of pH values and DFT calculations, we show that the active site of a chitin-active AA10 LPMO can exist in three different protonation states (pK_a1 = 8.7, pK_a2 ∼ 11.5), representing the ionisation of the coordinating groups. The middle pH species (fully formed at pH ∼ 10.5) is proposed to be Cu(II)(His)₂(OH)₂ (N₂O₂ coordination) with a decoordinated R–NH₃⁺ group at the amino terminus. This species also sees a rotation of the SOMO equatorial plane from the canonical histidine brace plane, whereby the nominal Cu d(x² − y²)-orbital has rotated some 45° along the His–Cu(II)–His axis, driven by the elongation and decoordination of the amino group. The highest pH species (>12) is proposed to exist as a Cu(II)–azanide, in which the NH₂ of the amino terminus has been deprotonated. The high pH means that this species is unlikely to be biologically relevant in the catalytic cycle of AA10 LPMOs.