Interaction of class <scp>III</scp> cellobiose dehydrogenase with lytic polysaccharide monooxygenase

The genome of Fusarium solani, a well-known plant pathogen, encodes various lytic polysaccharide monooxygenases (LPMOs) involved in plant biomass degradation in combination with cellobiose dehydrogenase (CDH). To investigate the auxiliary role of the recently expressed and characterized class III CDH from F. solani (FsCDH), this enzyme was tested in combination with the well-characterized AA9C from Neurospora crassa (NcAA9C). Steady-state and stopped-flow methods as well as electrochemical measurements demonstrate how FsCDH efficiently transfers electrons to NcAA9C, with a rapid, observed heme reoxidation rate constant of 129 s^-1. In comparison to ascomycete class II CDHs, the H₂O₂ production by FsCDH is insufficient to promote LPMO activity. However, a cyclic cascade between NcAA9C and FsCDH was found. NcAA9C reaction products showed a high catalytic efficiency as FsCDH substrates, with K_M values close to its natural substrate cellobiose. This reaction was further investigated by a real time measurement, where FsCDH and NcAA9C were incubated with phosphoric acid-swollen cellulose and the reaction was sustained over a long period without the addition of an external reductant. The new class III CDH is similar to other CDH classes, except its very low reactivity with molecular oxygen, pointing towards a different function in Ascomycota than class II CDH. These findings contribute to the better understanding of oxidative cellulose degradation by fungi and thus, to potential biotechnological applications for the sustainable use of biomass.