Roles of Small Laccases fromStreptomycesin Lignin Degradation

Abstract

Laccases (EC 1.10.3.2) are multicopper oxidases that can oxidize a range of substrates, including phenols, aromatic amines, and nonphenolic substrates. To investigate the involvement of the small Streptomyces laccases in lignin degradation, we generated acid-precipitable polymeric lignin obtained in the presence of wild-type Streptomyces coelicolor A3(2) (SCWT) and its laccase-less mutant (SCΔLAC) in the presence of Miscanthus x giganteus lignocellulose. The results showed that strain SCΔLAC was inefficient in degrading lignin compared to strain SCWT, thereby supporting the importance of laccase for lignin degradation by S. coelicolor A3(2). We also studied the lignin degradation activity of laccases from S. coelicolor A3(2), Streptomyces lividans TK24, Streptomyces viridosporus T7A, and Amycolatopsis sp. 75iv2 using both lignin model compounds and ethanosolv lignin. All four laccases degraded a phenolic model compound (LM-OH) but were able to oxidize a nonphenolic model compound only in the presence of redox mediators. Their activities are highest at pH 8.0 with a low krel/Kapp for LM-OH, suggesting that the enzymes’ natural substrates must be different in shape or chemical nature. Crystal structures of the laccases from S. viridosporus T7A (SVLAC) and Amycolatopsis sp. 75iv2 were determined both with and without bound substrate. This is the first report of a crystal structure for any laccase bound to a nonphenolic β-O-4 lignin model compound. An additional zinc metal binding site in SVLAC was also identified. The ability to oxidize and/or rearrange ethanosolv lignin provides further evidence of the utility of laccase activity for lignin degradation and/or modification.

Structure of Thermobifida fusca DyP-type peroxidase and activity towards Kraft lignin and lignin model compounds

“…This phenomenon has been observed for R. jostii RHA1 DyPB [9], and also for bacterial laccases from Streptomyces coelicolor and S. viridosporus T7A [24], and represents a significant challenge for the use of these enzymes in vitro for lignin depolymerisation, though this phenomenon is generally not observed in living systems, so there may be mechanisms to combat this phenomenon in vivo.…”

“…Dimeric -aryl ether lignin model compounds have been frequently used to study the action of fungal lignin-oxidising enzymes [23], and bacterial DyPs [9,10] and laccases [24]. TfuDyP was incubated with guaiacylglycerol--guaiacyl ether (1) at 1.6 mg/ml concentration in 100 mM sodium acetate buffer pH 6.0 in the presence of 1 mM hydrogen peroxide.…”

Applications of Microbial Enzymes in Food Industry

“…Peroxidase activities are reported in bacterial taxa, such as Firmicutes, Proteobacteria, Actinobacteria and Acidobacteria (155,156). Moreover, actinomycetes, which are soil bacteria, are able to grow like fungi and have similar ecological niche, and can produce peroxidases for lignin degradation (154,157). The first secreted extracellular lignin peroxidase was produced by Streptomyces viridosporus T7A (158).…”

Identification of an extracellular bacterial flavoenzyme that can prevent re-polymerisation of lignin fragments

“…This problem has been encountered using fungal lignin peroxidase [12][13][14] and laccase enzymes [15,16] and with bacterial Dyp peroxidase [17] and laccase [18] enzymes, and is also encountered in chemocatalytic valorisation of lignin, where a high molecular weight char is observed alongside depolymerised products [1]. Kirk & Farrell commented on this problem in their review of microbial lignin oxidation in 1987, noting that polymerisation of lignin is not prominent in vivo, therefore they suggested that phenoxy radical intermediates "are reduced back to the phenols by an undiscovered enzyme and/or mechanism that prevents polymerization" [19].…”