Kinetic study of a β-mannanase from the Bacillus licheniformis HDYM-04 and its decolorization ability of twenty-two structurally different dyes

β-Mannanase (endo-1,4-β-D-mannanase, EC 3.2.1.78) is a hydrolase that catalyzes the random hydrolysis of β-1,4-mannosidic linkages in the main chain of β-1,4-D-mannan and releases linear/branched oligosaccharides of various lengths, and it could be classed to the glycosyl hydrolase (GH) families 5 and 26 based on amino acid sequence similarities (Van Zyl et al. 2010; Cantarel et al. 2009). β-mannanases have been characterized from a wide range of organisms, including invertebrate, plants, filamentous fungi, yeasts and bacteria. There has been growing interest over the years in the industrial potential of β-mannanase degrading enzymes, especially microbial β-mannanase (Wang et al. 2010a, b; Chauhan et al. 2012). Microbial β-mannanases are the primary endo-type enzymes responsible for degradation of mannan polysaccharides (Liepman et al. 2007; Scheller and Ulvskov 2010). The microbial β-mannanases have been increasingly exploited for bioconversion of biomass materials and various potential industrial applications, such as bleaching of softwood pulps, scouring and desizing, food and feed additive, and oil and textile industries due to the various advantages it can act in a wide range of pH and temperature because of which they play important roles in basic research (Dhawan and Kaur 2007; Zhou et al. 2012). So far, various microbial β-mannanases from Streptomyces sp. (Takahash et al. 1984), Bacillus subtilis (Jiang et al. 2006), Aplysia kurodai (Zahura et al. 2011), Bacillus licheniformis (Songsiriritthigul et al. 2010) and Trichoderma harzianum (Ferreira and Ferreira 2004) have been purified and characterized.

Synthetic dyes are classified as anthraquinone, azo, heterocyclic, triphenylmethane (TPM) dyes, and extensively used in several industries including textile, cosmetic, paper, printing, leather-dyeing, pharmaceutical and food industries (Chauhan et al. 2014a, b), but they have caused a serious environmental pollution. Moreover, the exiting dyes usually come from synthetic origin and contribute to more complicate molecular structures making them difficult to biodegrade which most of them are toxic, mutagenic and carcinogenic (Brown and De Vito 1993). The process of dye decolorization based on enzyme is an efficient method and is attracting increasing interest (Erkurt et al. 2007). By means of enzymatic catalyzed oxidative reactions, β-mannanase can detoxify phenolic contaminants (Asgher et al. 2008). At present, a lot of studies have focused on microbial enzymes. Certain fungal laccases combined with synthetic or natural mediators have been reported to proved to be suitable tools for textile effluent and dye removal treatments (Kaushik and Malik 2009). Although the β-mannanases from Bacillus have been already well characterized, there is still absence of information on the enzyme’s kinetic properties and factors that influence stability and the use of β-mannanases is still restricted due to high-production costs and low yields (Zhang et al. 2000; Zakaria et al. 1998). Surprisingly, no studies have been implemented to estimate β-mannanases from Bacillus licheniformis that may take part in the decolorize and biodegrade dyes.

In our previous studies, a β-mannanase from B.licheniformis HDYM-04 was purified (Ge et al. 2016). However, the stability of chemical reagents and application performances of β-mannanase have not been studied. The aim of this study was to carry out preliminary investigation of biocatalytic kinetic properties, stability of organic solvents, including inhibitors, detergents, chelating agents and oxidizing agents and decolorization of multifarious dyes of β-mannanase from B.licheniformis HDYM-04. Investigation of the application performance of β-mannanase would enhance the potential usability in industrial processing.

In conclusion, this study reported the some characterization of a β-mannanase produced by HDYM-04. The property of enzyme to maintain a high production at a short time was interesting from the point of view of industry. To the best of our knowledge, this was the first report about the production of β-mannanase from HDYM-04 at 37 °C. Similarly, Paenibacillus sp. DZ23 and B.subtilis NM-39 produced enzyme at 37 °C with glucomannan and locust bean gum as the substrate (Chandra et al. 2011; Mendoza et al. 1994). Other B.subtilis strains separated so far from producing maximum enzyme at below 50 °C (Zhou et al. 2012). Some B.subtilis strains can produced enzyme at up to 45 °C (Khanongnuch et al. 1998). The purified β-mannanase produced by HDYM-04 shows higher affinity toward glucomannan substrate (K _m and V _max were 2.69 mg/mL and 251.41 U/mg, respectively) than that of other mannans like guar gum. But, the result was higher than that β-mannanase of Paenibacillus sp. DZ3 (K _m 1.05 mg/mL) to amorophophallus konjac (Mendoza et al. 1994), and β-mannanase of B.nealsonii PN-11 (K _m 11.59 mg/mL) to guar gum (Chauhan et al. 2014a, b). Kinetic studies revealed that the enzyme had more affinity toward natural glucomannan, and hence it was applicable in the food industry for the production of oligosaccharides. In the present study, the β-mannanase retained > 80 % activity in dimethyl sulfoxide, acetone, chloroform, benzene, hexane. This could be due to well-known fact that hydrophilic solvents are usually superior to hydrophobic solvents for the better enzyme activity, as the earlier have a greater tendency to bind water tightly, which is essential for catalytic activity. Similar results were observed with mannanase from B. subtilis G1 which showed 11–53 % reduction in enzyme activity by the addition of organic solvents (Vu et al. 2012). In the presence of solvents; citric acid, ethylene diamine teracetic acid and potassium iodide, it retained > 80 % residual activity. The strong inhibitory effect of cetyl trimethyl ammonium bromide, a potent cation surfactant, could be due to the destruction of the conformation of mannanase.

Besides, we have found that the β-mannanase produced by HDYM-04 we used show remarkably high activity and found to be capable of decolorizing and degrading different structures of synthetic dyes. Though lots of papers reported the degradation and decolourisation of synthetic dyes by other enzymes, some studies demonstrating that laccases from Leptomitus lacteus could made different dyes decolorization (Svobodová et al. 2008). Such as, B.cereus could decolorize 85 % of an azo dye for 120 h of incubation (Kanagaraj et al. 2012) and a novel laccase from B.subtilis WD23 could decolorize 50-90 % of congo red and methyl orange, which suggested the potential application of spore laccase in dyestuff treatment (Wang et al. 2010a, b). Unfortunately, the majority of dyes are chemically stable and still resistant to microbiological attack. The differences in the decoloriaztion efficiencies may be attributed to the various chemical structures of different dyes. The most employed dyes belong to the azo and triaromatic class which accounts for the 80 % of all textile dye produced. The best decolorization over was obtained with reactive conge red methyl orange and titan yellow. β-mannanase may be modify azo dye structures by destroying their chromophoric assemblies, phenoxyl radicals are generated in the reaction course (Muralikrishna et al. 1995). Compared with the β-mannanase produced by HDYM-04, these enzyme decolorization efficiency were relatively low. The β-mannanases are oxidase that cataluze oxidation reactions and hydrolysis reactions in many phenolic and organic substrates coupling with reduction reactions that transfer molecular oxygen to water (Murugesan et al. 2007). Triaromatic methane dyes represent an especially recalcitrant class of compounds. The present study confirms the ability of methyl orange and aniline blue purified β-mannanase to decolorize amaranth, chromotrope 2R, amaranth and eosin with decolorization efficiency of more than 90 % in short time. This could be due to the presense of electron donating methy and methoxy groups on the triaromatic methane dyes. The results obtained in this study were in agreement with results reported previrously for Pleurotusostreatus laccase (Kumar et al. 2012) and P.variabile laccase (Forootanfar et al. 2011). They also demonstrated that different decolorization rates were attributed to the specific catalytic properties of the individual enzymes and to the structure of dyes. To our knowledge, this is the first description of a bacterial β-mannanase from HDYM-04 able to degrade different dyes. Furthermore, the decolorization of dyes by β-mannanase produced by HDYM-04 is simple and cheap. So, the broad substrates specificity of enzyme rendered its great potentials in industrial applications, such as degradation of dyes from acidic textile effluents and the purified β-mannanase produced by HDYM-04 could be successfully employed for the treatment of dyes bearing industrial wastewater as it had prominent capacity to degrade other different dyes. Some studies found that, the degradation of azo dyes could result in the production of compounds of increased toxicity. However, most studies failed to evaluate the toxicity of either the dyes and/or the reaction products (Gottlieb et al. 2003). So, to further investigate its effect on the toxicity of dye decolorization, the toxicity experiment will be carry out.

To sum up, the purified β-mannanase produced by HDYM-04 showed higher affinity toward glucomannan substrate (K _m and V _max were 2.69 mg/ml and 251.41 U/mg, respectively) than that of other mannans like guar gum. The enzyme obtained from this research possessed much higher stability in inhibitors, detergents, chelating agents, oxidizing agents and reducing agents. Furthermore, this enzyme could resist citric acid, ethylene diamine teraacetic acid and potassium iodide with more than 80 % maximum activity remained. Besides, this study represented the first attempt to decolorize the mixtures of dyes by purified β-mannanase from HDYM-04. Thus, the β-mannanase has been successfully identified and, from this study, it has good potential in applying to decolorize dyes in textile wastewaters, particularly for water recycling. Further studies should be attempted to evaluate their feasibility in industrial uses.