Functional variation among LPMOs revealed by the inhibitory effects of cyanide and buffer ions

Enzymes known as lytic polysaccharide monooxygenases (LPMOs) are mono‐copper polysaccharide‐degrading peroxygenases that engage in several on‐ and off‐pathway redox reactions involving O 2 and H 2 O 2 . Herein, we show that the known metalloenzyme inhibitor cyanide inhibits reductive activation of LPMOs by binding to the LPMO‐Cu(II) state and that the degree of inhibition depends on the concentrations of the polysaccharide substrate, the reductant and H 2 O 2 . Importantly, this analysis revealed differences between fungal Nc AA9C and bacterial Sm AA10A, which have different secondary copper coordination spheres. These differences were also highlighted by the observation that phosphate, a commonly used buffer ion, strongly inhibits Nc AA9C while not affecting reactions with Sm AA10A. The results provide insight into LPMO inhibition and catalysis and highlight pitfalls in the analysis thereof.