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Hemicellulose is a kind of abundant and renewable biomass resources that need to be developed and utilized. Degrading hemicellulose into sugars to produce xylitol and other chemicals is a key step in the utilization of biomass resources. Acetylxylan esterase is an important enzyme that degrades hemicellulose. It can hydrolyze the O acetyl groups at positions 2 and 3 on the xylose residues in acetylated xylan. Acetylxylan have a broad vision of application agriculture and food industries.
Figure 1. Protein structure of acetylxylan esterase.
According to the similarity of amino acid sequence, acetyl xylan esterases from different microorganisms belong to CE (carbohydrate esterase) family 1-7 and 12 families respectively. Acetylxylan esterase from fungi such as Aspergillus, Penicillium, Schizophyllum, Trichoderma, and acetylxylan esterase from bacteria belong to CE families 1 and 5. Acetylxylan esterase from anaerobic bacteria Neocallimastix patriciarum belongs to CE In families 2, 3 and 6, acetylxylan esterase derived from Streptom yces belong to cE family 4, and they have homology with chitin deacetylases of rhizobia and some yeasts.
Generally speaking, the molecular mass of acetylxylan esterase as a monomer enzyme is 2348kDa. Most acetylxylan esterase derived from fungi and some bacteria are typical monomer enzymes, but some thermophilic anaerobic/ The acetylxylan esterase derived from bacteria R1 exists in the form of oligomers. The isoelectric point, molecular mass, temperature and pH range of acetylxylan esterase from different microorganisms will vary.
Thermal stability is an important parameter to measure the industrial use of enzymes. The optimum reaction temperature of acetyl xylan esterase from different sources is mostly 45 to 60°C. Some acetylxylan esterases extracted from thermophilic bacteria and certain fungi have higher optimal reaction temperatures. The optimum reaction temperature of the acetylxylan esterase purified from Tengcongensis is up to 60°C, and the latter only loses about 10% of the enzyme activity at 65°C for 100 min, indicating good thermal stability. The optimum reaction temperature of acetylxylan esterase purified from Streptomyces lividans is as high as 70°C. Higher optimum reaction temperature and heat resistance will make the enzyme have great industrial use value, so more thermophilic acetylxylan esterase genes have yet to be discovered.
The optimum pH of acetylxylan esterase from fungi is mostly weakly acidic or neutral, while the optimum pH of acetyl xylan esterase from bacteria is more alkaline than that of fungus. Moriyoshi et al. expressed the acetyl xylan esterase gene cloned from the bacterium e subterraneus in E. coli, and the optimal pH of the acetyl xylan esterase produced by its expression was 8. 0.
In addition to temperature and pH, metal ions also have a greater impact on enzyme activity. In industrial applications, the activation or inhibition of certain metal ions on the enzyme is also an important parameter to measure the industrial use of the enzyme. Studies have shown that metal ions have different effects on acetylxylan esterases from different sources.
Industrial degradation of xylan often adopts acid or alkaline methods, but the acid and alkaline hydrolysis processes are often accompanied by side reactions, which produce more toxic substances, which have an inhibitory effect on the later stage of microbial fermentation, and the acid and alkaline waste liquid produced will be Cause greater environmental pressure, therefore, enzymatic degradation of xylan will become the main way of degradation of xylan in the future due to its mild and environmental advantages. Acetylxylan esterase as one of the members of the hemicellulase family has a good application prospect. In terms of engineering bacteria construction, the research of acetylxylan esterase is of great significance for the construction of hemicellulase engineering bacteria with high yield and excellent synergistic effect. In industrial applications, acetylxylan esterase can synergize with endoxylanase and B-xylosidase to completely degrade hemicellulose into xylose, and then into xylitol. In addition, some acetylxylan esterases can be used as the acetyl group of chitin, so chitosan can be prepared by enzymatic method, which has broad application prospects in medicine, textile, paper, cosmetics, etc.
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