The white rot basidiomycete Ceriporiopsis subvermispora delignifies wood selectively and has potential biotechnological applications. Its ability to remove lignin before the substrate porosity has increased enough to admit enzymes suggests that small diffusible oxidants contribute to delignification. A key question is whether these unidentified oxidants attack lignin via single electron transfer (SET), in which case they are expected to cleave its propyl sidechains between Cα and Cβ and to oxidize the threo-diastereomer of its predominating β-O-4-linked structures more extensively than the corresponding erythro-diastereomer. We used two-dimensional solution-state NMR techniques to look for changes in partially biodegraded lignin extracted from spruce wood after white rot by C. subvermispora. The results showed that (a) benzoic acid residues indicative of Cα–Cβ cleavage were the major identifiable truncated structures in the lignin after decay, and (b) depletion of β-O-4-linked units was markedly diastereoselective with a threo preference. The less selective delignifier Phanerochaete chrysosporium also exhibited this diastereoselectivity on spruce, and a P. chrysosporium lignin peroxidase operating in conjunction with the P. chrysosporium metabolite veratryl alcohol did likewise when cleaving synthetic lignin in vitro. However, C. subvermispora was significantly more diastereoselective than P. chrysosporium or lignin peroxidase/veratryl alcohol. Our results show that the ligninolytic oxidants of C. subvermispora are collectively more diastereoselective than currently known fungal ligninolytic oxidants, and suggest that SET oxidation is one of the chemical mechanisms involved.
Yelle, Daniel J.; Kapich, Alexander N.; Houtman, Carl J.; Lu, Fachuang; Timokhin, Vitaliy I.; Fort Jr., Raymond C.; Ralph, John; Hammel, Kenneth E. 2014. A Highly Diastereoselective Oxidant Contributes to Ligninolysis by the White Rot Basidiomycete Ceriporiopsis subvermispora . Appl. Environ. Microbiol., Volume 80, Number 24, 2014; pp. 7536-7544.