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.
Highlights
We provide an overview of recent advances in the identification and use of bacterial enzymes acting on lignin or lignin-derived products
dyedecolorizing peroxidases (DyP)-type peroxidases are generally active on monophenolic substrates, but several bacterial DyPs have shown significant activity towards the nonphenolic veratryl alcohol: BsDyP from Bacillus subtilis KCTC2023 (Min et al, 2015), PpDyP from Pseudomonas putida MET94 (Santos et al, 2014), SviDyP from Saccharomonospora viridis DSM 43017 (Yu et al, 2014) and TfuDyP from Thermobifida fusca (Van Bloois et al, 2010)
The product 5-carboxyvanillic acid is converted into vanillic acid in a process catalyzed by two decarboxylase enzymes (LigW and LigW2). These findings suggest that dioxygenases represents another tool used by bacteria to assist in lignin degradation
Summary
White-rot fungi produce several different kinds of heme-containing peroxidases to trigger lignin decomposition. DyP-type peroxidases are generally active on monophenolic substrates, but several bacterial DyPs have shown significant activity towards the nonphenolic veratryl alcohol: BsDyP from Bacillus subtilis KCTC2023 (Min et al, 2015), PpDyP from Pseudomonas putida MET94 (Santos et al, 2014), SviDyP from Saccharomonospora viridis DSM 43017 (Yu et al, 2014) and TfuDyP from Thermobifida fusca (Van Bloois et al, 2010). Both SviDyP and TfuDyP are class A DyPs and are secreted via the Tat-system. This indicates that in some way encapsulation which enhances DyP-mediated lignin degradation
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