Hydroxyl radicals production via quinone redox cycling by the ligninolytic bacteria Streptomyces cyaneus and its effectiveness to degrade kraft lignin

Abstract

Microbial degradation of lignocellulose is a complex process mainly carried out by filamentous fungi and bacteria. Among prokaryotes, the genus Streptomyces stands out, with laccases playing a key role in its lignocellulolytic enzyme system. However, bacterial laccases have a lower redox potential than fungal laccases, suggesting that their action on lignin is indirect, via high redox potential intermediates. Prominent examples of intermediates are hydroxyl radicals. In Basidiomycota fungi, the production of these radicals occurs through quinone redox cycling, involving a Fenton reaction. This study demonstrates, for the first time, extracellular hydroxyl radical production via quinone redox cycling in the bacterium Streptomyces cyaneus CECT 3335, with laccase playing an essential role. The process begins with the extracellular oxidation of quinones to semiquinones, catalyzed by laccase. In the presence of Fe³⁺, semiquinones produce hydroxyl radicals via a Fenton reaction. The cycle is restored through quinone reduction by mycelium-associated reductase activity. H₂O₂ production, Fe³⁺ reduction, and hydroxyl radical generation were confirmed in S. cyaneus. The key role of laccase was verified using a mutant strain lacking laccase activity, in which hydroxyl radical production was absent. The oxidative potential of this mechanism in S. cyaneus was evidenced by the degradation of non-phenolic lignin-related compounds homoveratric acid and veratraldehyde and by the ability to depolymerize kraft lignin. This novel finding of quinone redox cycling in bacteria has important implications for Streptomyces’ role in lignin degradation, as well as potential biotechnological applications, including lignin biotransformation and bioremediation of organic pollutants.