Insights into an unusual Auxiliary Activity 9 family member lacking the histidine brace motif of lytic polysaccharide monooxygenases

J Biol Chem. 2019 Nov 8;294(45):17117-17130. doi: 10.1074/jbc.RA119.009223. Epub 2019 Aug 30.


Lytic polysaccharide monooxygenases (LPMOs) are redox-enzymes involved in biomass degradation. All characterized LPMOs possess an active site of two highly conserved histidine residues coordinating a copper ion (the histidine brace), which are essential for LPMO activity. However, some protein sequences that belong to the AA9 LPMO family display a natural N-terminal His to Arg substitution (Arg-AA9). These are found almost entirely in the phylogenetic fungal class Agaricomycetes, associated with wood decay, but no function has been demonstrated for any Arg-AA9. Through bioinformatics, transcriptomic, and proteomic analyses we present data, which suggest that Arg-AA9 proteins could have a hitherto unidentified role in fungal degradation of lignocellulosic biomass in conjunction with other secreted fungal enzymes. We present the first structure of an Arg-AA9, LsAA9B, a naturally occurring protein from Lentinus similis The LsAA9B structure reveals gross changes in the region equivalent to the canonical LPMO copper-binding site, whereas features implicated in carbohydrate binding in AA9 LPMOs have been maintained. We obtained a structure of LsAA9B with xylotetraose bound on the surface of the protein although with a considerably different binding mode compared with other AA9 complex structures. In addition, we have found indications of protein phosphorylation near the N-terminal Arg and the carbohydrate-binding site, for which the potential function is currently unknown. Our results are strong evidence that Arg-AA9s function markedly different from canonical AA9 LPMO, but nonetheless, may play a role in fungal conversion of lignocellulosic biomass.

Keywords: His-brace; N-terminal Arg-AA9; biomass degradation; copper monooxygenase; crystal structure; glycobiology; phosphorylation; polysaccharide; xylooligosaccharide.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Motifs
  • Amino Acid Sequence
  • Amino Acid Substitution
  • Binding Sites
  • Histidine*
  • Ligands
  • Mixed Function Oxygenases / chemistry*
  • Mixed Function Oxygenases / genetics
  • Mixed Function Oxygenases / metabolism*
  • Models, Molecular
  • Phosphorylation
  • Phylogeny
  • Polysaccharides / metabolism*


  • Ligands
  • Polysaccharides
  • Histidine
  • Mixed Function Oxygenases

Associated data

  • PDB/4D7U
  • PDB/4EIR
  • PDB/5ACH
  • PDB/4QI8
  • PDB/3EJA
  • PDB/3ZUD
  • PDB/4EIS
  • PDB/2VTC
  • PDB/6RS6
  • PDB/6RS7
  • PDB/6RS8
  • PDB/6RS9
  • PDB/5NLS
  • PDB/5NLO