Microbial degradation of lignin: how a bulky recalcitrant polymer is efficiently recycled in nature and how we can take advantage of this.
Bottom Line: Lignin is the second most abundant constituent of the cell wall of vascular plants, where it protects cellulose towards hydrolytic attack by saprophytic and pathogenic microbes.Ligninolytic microbes have developed a unique strategy to handle lignin degradation based on unspecific one-electron oxidation of the benzenic rings in the different lignin substructures by extracellular haemperoxidases acting synergistically with peroxide-generating oxidases.These peroxidases poses two outstanding characteristics: (i) they have unusually high redox potential due to haem pocket architecture that enables oxidation of non-phenolic aromatic rings, and (ii) they are able to generate a protein oxidizer by electron transfer to the haem cofactor forming a catalytic tryptophanyl-free radical at the protein surface, where it can interact with the bulky lignin polymer.
Affiliation: Centro de Investigaciones Biológicas, CSIC, Ramiro de Maeztu 9, E-28040 Madrid, Spain. FJRuiz@cib.csic.esShow MeSH
Mentions: As mentioned above, all peroxidases require hydrogen peroxide, or other hydroperoxides, to activate the haem cofactor yielding the so‐called compound‐I in a common catalytic cycle (Dunford, 1999) (Fig. 3). Compound‐I contains a reactive Fe4+ oxo complex with a cation radical at the porphyrin ring, formed by two‐electron oxidation of the Fe3+‐containing haem of the resting enzyme. One‐electron oxidation of one substrate molecule yields compound‐II, where the porphyrin cation radical has been reduced. The remaining Fe4+=O in compound‐II oxidizes a second substrate molecule, and the enzyme returns to its ferric resting state to initiate a new catalytic cycle.
Affiliation: Centro de Investigaciones Biológicas, CSIC, Ramiro de Maeztu 9, E-28040 Madrid, Spain. FJRuiz@cib.csic.es