Comparison of three seemingly similar lytic polysaccharide monooxygenases from Neurospora crassa suggests different roles in plant biomass degradation

Abstract

Many fungi produce multiple lytic polysaccharide monooxygenases (LPMOs) with seemingly similar functions, but the biological reason for this multiplicity remains unknown. To address this question, here we carried out comparative structural and functional characterizations of three cellulose-active C4-oxidizing family AA9 LPMOs from the fungus Neurospora crassa , Nc LPMO9A (NCU02240), Nc LPMO9C (NCU02916), and Nc LPMO9D (NCU01050). We solved the three-dimensional structure of copper-bound Nc LPMO9A at 1.6-Å resolution and found that Nc LPMO9A and Nc LPMO9C, containing a CBM1 carbohydrate-binding module, bind cellulose more strongly and were less susceptible to inactivation than Nc LPMO9D, which lacks a CBM. All three LPMOs were active on tamarind xyloglucan and konjac glucomannan, generating similar products but clearly differing in activity levels. Importantly, in some cases, the addition of phosphoric acid–swollen cellulose (PASC) had a major effect on activity: Nc LPMO9A was active on xyloglucan only in the presence of PASC, and PASC enhanced Nc LPMO9D activity on glucomannan. Interestingly, the three enzymes also exhibited large differences in their interactions with enzymatic electron donors, which could reflect that they are optimized to act with different reducing partners. All three enzymes efficiently used H 2 O 2 as a cosubstrate, yielding product profiles identical to those obtained in O 2 -driven reactions with PASC, xyloglucan, or glucomannan. Our results indicate that seemingly similar LPMOs act preferentially on different types of copolymeric substructures in the plant cell wall, possibly because these LPMOs are functionally adapted to distinct niches differing in the types of available reductants.