Auxiliary activity 9 (AA9) lytic polysaccharide monooxygenases (LPMOs) are copperdependent
oxidoreductases that use O2 or H2O2 to perform oxidative cleavage of cellulose in the
presence of an electron donor. Combined with cellulases, they can assist in a more efficient cleavage
of cellulose. AA9 LPMOs have therefore attracted considerable attention in recent years for use in
biotechnological applications. Here, a native AA9 LPMO (nTaAA9A) from the thermophilic fungus
Thermoascus aurantiacus was purified and characterized. The enzyme was shown to be active and able
to cleave cellulose and xylan to produce C1- and C4-oxidized products. It was also found to retain
about 84.3, 63.7, and 35.3% of its activity after incubation for 30 min at 60, 70, and 80 C, respectively,
using quantitative activity determination. The structure was determined to 1.36 Å resolution and
compared with that of the recombinant enzyme expressed in Aspergillus oryzae. Structural differences
in the glycosylated Asn138 and in solvent-exposed loops were identified.