Bacterial enzymes involved in lignin degradation

Abstract

Lignin forms a large part of plant biomass. It is a highly heterogeneous polymer of 4-hydroxyphenylpropanoid units and is embedded within polysaccharide polymers forming lignocellulose. Lignin provides strength and rigidity to plants and is rather resilient towards degradation. To improve the (bio)processing of lignocellulosic feedstocks, more effective degradation methods of lignin are in demand. Nature has found ways to fully degrade lignin through the production of dedicated ligninolytic enzyme systems. While such enzymes have been well thoroughly studied for ligninolytic fungi, only in recent years biochemical studies on bacterial enzymes capable of lignin modification have intensified. This has revealed several types of enzymes available to bacteria that enable them to act on lignin. Two major classes of bacterial lignin-modifying enzymes are DyP-type peroxidases and laccases. Yet, recently also several other bacterial enzymes have been discovered that seem to play a role in lignin modifications. In the present review, we provide an overview of recent advances in the identification and use of bacterial enzymes acting on lignin or lignin-derived products.

Lignin Degradation Processes and the Purification of Valuable Products

“…The emerging crosslinked network and aromatic nature of lignin explain its recalcitrance towards degradation [11]. The most frequent linkages formed during lignification are β-O-4 bonds, which represent~50% of all linkages between monolignol building blocks in hardwood and softwood lignin ( Table 1) [3,12].…”

“…Lignin biodegradation clearly occurs in nature, otherwise all carbon would eventually become trapped in lignocellulosic biomass [11]. However, it has not yet been possible to replicate these enzymatic processes on an industrial scale.…”

Insights into the Mechanism of Lignocellulose Degradation by Versatile Peroxidases

“…Although diverse laccase genes have been identified from the above families, the frailty of laccase in acting independently on non-phenolic units of lignin prevents complete depolymerization of the polymer with exclusive application of this enzyme. Bacterial ligninolytic system encompasses dye-decolourizing peroxidases (DyPs) substantiated by the recognition of multitude of putative dyedecolourizing peroxidase encoding genes in the bacterial genome 3 . However, no homologs to fungal peroxidases have been perused till date.…”

“…However, no homologs to fungal peroxidases have been perused till date. Dye-decolourizing peroxidases with diverse potential from varying bacterial species still lags behind the fungal counterpart in efficiency and entirety of oxidation of lignin 3 . The genes of ligninolytic peroxidases -lignin peroxidase, manganese peroxidase and versatile peroxidase are exclusive to white rot group of basidiomycetes, confirming their role in preferential degradation of lignin 26 .…”

“…Streptomyces viridosporus, Rhodococcus jostii, Nocardia autotrophica of actinomycetes and Pseudomonas putida, Sphingobium, Burkolderia of proteobacteria could depolymerize lignin through their extracellular enzymes. Albeit, the fungal system of biodegradation of lignin overarches most efficient bacterial lignin degraders in terms of redox potential and in oxidation of highly recalcitrant compounds 3 . The fungal system possesses integrative complete machinery requisite for selective degradation of lignin.…”