Lignocellulose is the most abundant biomass on earth. Agricultural, forest, and agroindustrial activities generate tons of lignocellulosic wastes annually, which present readily procurable, economically affordable, and renewable feedstock for various lignocelluloses based applications. Lignocelluloses are the focus of present decade researchers globally, in an attempt to develop technologies based on natural biomass for reducing dependence on expensive and exhaustible substrates. Lignocellulolytic enzymes, that is, cellulases, hemicellulases, and lignolytic enzymes, play very important role in the processing of lignocelluloses which is prerequisite for their utilization in various processes. These enzymes are obtained from microorganisms distributed in both prokaryotic and eukaryotic domains including bacteria, fungi, and actinomycetes. Actinomycetes are an attractive microbial group for production of lignocellulose degrading enzymes. Various studies have evaluated the lignocellulose degrading ability of actinomycetes, which can be potentially implemented in the production of different value added products. This paper is an overview of the diversity of cellulolytic, hemicellulolytic, and lignolytic actinomycetes along with brief discussion of their hydrolytic enzyme systems involved in biomass modification.
Actinomycetes, a separate taxonomic group within domain bacteria, are members of the order Actinomycetales [
Lignocellulolytic enzymes, one of the potent enzymes produced by actinomycetes, can be exploited widely in various lignocelluloses based industries [
Lignocellulolytic enzymes can be obtained from diverse types of microorganisms including bacteria and fungi [
Lignocellulose is comprised of three main components, that is, cellulose, hemicellulose, and lignin [
Chemical structure of lignocellulose.
Cellulose polymer
Hemicellulose monomers
Constituents of lignin
Celluloses, hemicelluloses, and lignin are packed closely in a crisscross network and glued with the help of a variety of noncovalent and covalent linkages. Lignin acts as a cementing agent and provides structural support. It also provides impermeable barrier to the enzymes, making whole structure robust and resistant [
Cellulose is used for many purposes such as in manufacturing of paper and textile fabric, in production of biofuel (from fermentable sugar glucose), as inert packing and insulating material, and in food and drugs as base and stabilizer [
Cellulolytic enzymes are a group of glycosyl hydrolases classified into different families depending on their sequence homologies. The mechanisms of action and substrate specificities vary among different cellulases, but they are generally divided into exoglucanases (EC 3.2.1.74), endoglucanases (EC 3.2.1.4), cellobiohydrolases (EC 3.2.1.91), and
Scheme of cellulose hydrolysis.
Microbial cellulase systems are either complexed or noncomplexed [
Among cellulase producing actinomycetes,
The expression of cellulolytic genes in
The extracellular cellulases are secreted by actinomycetes using either one or both of the common bacterial systems for secretion of extracellular proteins, that is,
Hemicellulases are generally synthesised along with cellulases [
Most of the hemicellulases belong to glycosyl hydrolases families; however, some enzymes involved in hemicellulose hydrolysis belong to glycosyltransferases (EC 2.4.1.x) [
Lignin degradation is mediated by a complex of enzymes containing three principal enzymes laccases (EC 1.10.3.2), manganese peroxidases (MnP, EC 1.11.1.13), and lignin peroxidases (LiP, EC 1.11.1.14) [
Laccases or Laccase-like multicopper oxidases containing (LMCO) four copper atoms are classified in types 1, 2, and 3 [
The genes from several lignocellulolytic actinomycetes have been successfully cloned to show heterologous expression in different microbes. GH1 and GH3 enzymes of
Cellulolytic potential of actinomycetes has been explored since inspection of other microorganisms for cellulase production. Various research studies support high cellulose degradation potential of microbes from actinomycetales. Table
Cellulase producing actinomycetes.
Actinomycete isolate | Cellulolytic enzyme | Observed results | Reference |
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Saccharification of rice straw | [ | |
Extracellular cellulases | Zone of hydrolysis in plate assay method | [ | |
Endoglucanases | Enzyme production | [ | |
Cellulases | Aid in composting | [ | |
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Carboxymethylcellulose (Cx) and Avicelase (C1) enzyme | Production of enzymes | [ |
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Endoglucanases/CMCase (carboxymethylcellulose) | CMCase production | [ |
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Scale-up of enzyme production | [ | |
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Enzyme production using fruit waste | [ | |
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Optimization studies for production of cellulases | [ | |
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Cellulases | Enzyme characterization |
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Cellulases | Optimization of enzyme production | [ |
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Exoglucanase, |
Catabolite repression studies |
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Avicelase | Gene expression studies |
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Extracellular cellulases | Cellulose decomposition | [ |
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Endoglucanase | Successful application in detergent and textile processing | [ |
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Extracellular cellulases | Zone of hydrolysis in plate assay method | [ |
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Extracellular cellulase, CMCase/endoglucanase | Zone of hydrolysis in plate assay method | [ |
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Endoglucanase | Optimization of CMCase production | [ |
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CMCase/endoglucanase | CMCase production | [ |
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CMCase/endoglucanase | Zone of hydrolysis in plate assay method |
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Optimization studies | ||
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Endoglucanase | Enzyme production from agroindustrial residues | [ |
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Avicelase, CMCase, and total cellulase | Saccharification of rice straw and ethanol production | [ |
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Extracellular cellulases | Degradation of cellulosic materials | [ |
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Cellulases | Enzyme production optimization | [ |
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Endoglucanase and exoglucanase | Enzyme production optimization | [ |
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Carboxymethylcellulose | Production and purification of enzyme | [ |
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Cellulase | Activity observed, | [ |
FPase (total cellulases) | saccharification of rice straw and ethanol production, |
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CMCase, FPase | CMC depolymerisation and filter paper disintegration | [ |
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Degradation of flax, sisal, and cotton fibres | ||
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FPase, endo- |
Enzyme production using cellulosic substrates |
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CMCase | Degradation of flax, sisal, and cotton fibres | [ |
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Study of substrate inhibition kinetics | [ | |
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Endoglucanase/CMCase | Enzyme production using cassava bagasse | [ |
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Isolation from silver fish and cellulase characterization | [ | |
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Extracellular cellulase | Zone of hydrolysis in plate assay method | [ |
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Saccharification of rice straw | [ | |
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Demonstration of activity | [ | |
Enzyme production | [ | ||
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Saccharification of rice straw |
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Saccharification of rice straw | [ | |
CMCase | Enzyme production, purification, and characterization | [ | |
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Extracellular cellulase | Production and partial purification of enzyme | [ |
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CMCase/endoglucanase | CMCase production | [ |
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CMCase/endoglucanase | Zone of hydrolysis in plate assay method | [ |
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Activities demonstrated | [ | |
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CMCase and |
Detection of activity | [ |
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Endoglucanase, |
Degradation of microcrystalline cellulose | [ |
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Extracellular cellulase | High efficiency and bioactivity observed during compositing | [ |
Diverse types of actinomycetes belonging to wide range of habitats and active in different environmental conditions are known to produce hemicellulolytic enzymes.
In a study by Ninawe et al. [
Lignolytic activity is exhibited by diverse range of actinomycetes, which play important role in biodegradation processes in the environment. Search is in progress for more actinomycetes with high lignolytic potential, using advanced techniques combined with conventional methods. A study by Fernandes et al. [
Owing to the abundance and renewability of lignocellulosic biomass, it is considered as most appropriate and economical feedstock for production of various industrially useful products. Lignocellulases enzymes are, therefore, critical in processes associated with bioconversion of lignocelluloses. Presently most of the commercially exploited lignocellulases rely on fungal or bacterial microorganisms. Actinomycetes are relatively less explored for their biomass hydrolysis potential. The studies can be elaborated in search of new actinomycetes producing lignocellulose degrading enzyme systems. Different feedstock shows variation in their chemical composition. The production of enzymes needs to be optimized for different biomass. The production of lignocellulases from all microbial sources is still quite expensive. Efforts can be made for reducing the cost of production of these enzymes using high potency actinomycete enzyme systems with broader range of tolerance and active at diverse environmental conditions. Genetic engineering techniques can be used to construct enzyme systems with desirable characteristics. Also the studies can be expanded gradually to scale up to the industrial levels for their subsequent adoption in commercial processes.
Actinomycetes are an important source of lignocellulose hydrolysing enzymes. They constitute considerable proportion of the soil or aquatic microflora responsible for biomass degradation in nature. The research studies on search of lignocellulose hydrolysing actinomycetes revealed the abundance and diversity of these microbes in different ecological niches. The genetic and protein studies on their hydrolytic enzymes lead to the elucidation of structural and mechanism details of enzymes and their relatedness with other known lignocellulose producers and their enzyme systems. Relatively scanty information is available on lignocellulolytic actinomycetes. The research studies, therefore, need to be elaborated in view of utilization of lignocellulolytic potential of actinomycetes applicable in different industrial sectors.
The authors declare that there is no conflict of interests regarding publication of this review article.