Anions and citrate inhibit L$s$AA9A, a lytic polysaccharide monooxygenase (LPMO)

Lytic polysaccharide monooxygenases (LPMOs) are oxidative enzymes thatbreak the glycosidic linkage in recalcitrant polysaccharides such as celluloseand chitin. The LPMO LsAA9A (AA9 family lytic polysaccharide monooxygenase A) from the basidiomycete fungus Lentinus similis is biochemically and structurally well characterized, with crystallographic complexeswith oligosaccharides having been obtained. Chloride ions from the crystallization solution are known to bind to the LsAA9A–substrate complex incrystals at the copper equatorial coordinating position, where activation ofthe co-substrate oxygen species is expected. An investigation of the effectof high concentration salts on LsAA9A activity showed that salts containing chloride and other halide anions, except for fluoride, had a clear inhibitory effect on the activity at concentrations > 100 mM, although chlorideions are known to increase the LPMO affinity for oligosaccharide binding.Surprisingly, LsAA9A crystals can be transferred for short times to considerably different chemical environments, allowing crystallographic analysisat reduced chloride concentrations. Unfortunately, these washing steps donot eliminate the chloride binding at the copper equatorial coordinatingposition. Furthermore, we observed that citrate buffer, also present, boundunder these changed chemical conditions at the copper active site. Thisinteraction completely blocks access to the oligosaccharide substrate and isadditionally supported here by citrate inhibition of LsAA9A activitiesagainst azurine cross-linked hydroxyethylcellulose (AZCL-HEC), tamarindxyloglucan, and cellopentaose. The conclusions from our study indicatethat citrate should be absolutely avoided in LPMO research, not onlybecause of possible abstraction of copper ions from the LPMO active sitebut also because it might directly compete with binding of LPMOs to theirtarget substrates.