## 1 Introduction  
Mannan is a component of hemicellulose, which is second only to xylan and is widespread in wood, tubers, beans and plant seeds [1, 2]. According to the way that the sugar residues are connected, Mannan is composed of four subfamilies: linear mannan, glucomannan, galactomannan and galactoglucomannan. These glycans are made up of \(\beta\)-1,4 linked mannose or \(\alpha\)-1,6-linked mannose residues [3]. Because most mannans are highly branched heterogeneous polysaccharides with many different substituents, the biodegradation of mannans require a complex enzyme system in which many components degrade mannans by synergistic action. The enzyme system of mannan biodegradation involves many kinds of hydrolases which hydrolyze the main chain and side chains of mannan, such as exo-\(\beta\)-mannosidase (EC 3.2.1.25), endo-\(\beta\)-mannanase (EC 3.2.1.78), \(\alpha\)-galactosidase (EC 3.2.1.22), \(\beta\)-glucosidase (EC 3.2.1.21) and acetylmann esterases (EC 3.1.1.6) [1, 4]. Mannanas can be divided into several glycosyl hydrolase families (GH families) and their primary structures are quite different, whereas possess high similarity in their spatial arrangements which contain (\(\beta\)/\(\alpha\))8-barrel folding structures and are classified as GH-A clan [5]. They usually present modular structures which are composed of catalytic domain(s), carbohydrate binding module(s) (CBM), and additional functional domain(s) [6]. The hydrolysis of mannan-based polysaccharides involves two crucial enzymes, namely, \(\beta\)-mannanase (endo-\(\beta\)-mannose) and \(\beta\)-mannosidase (exo-\(\beta\)-mannose). \(\beta\)-mannanase cleaves glucosidic bonds inside mannan to produce smaller manno-oligosaccharides, and then, \(\beta\)-mannosidase hydrolyzes these smaller manno-oligosaccharides from their non-reducing end into fermentable mannose sugars. \(\beta\)-mannanase catalyzes the degradation of mannan, glucomannan, and galactomannan by incising the \(\beta\)-1,4-manoside bonds in the main chain in a random way, thereby degrades the abundant heterologous mannans to manno-oligosaccharides [7; 8]. \(\beta\)-mannanase has shown potential application values in many industrial fields, for instance, pulp bleaching, oligosaccharidesorducing, food quality improving [9; 10; 11; 12], oil drilling, feed additives and detergents [4; 13]. Up to now, a variety of \(\beta\)-mannanases and their corresponding genes have been isolated, purified and identified from microbial strains, for example, _Bacillus subtilis_ and other _Bacillus_[14], _Aspergillus niger_[15], _Caldocellum saccharolyticum_[16], _Caldibacillus cellulovorans_[17], _Thermoanoateobacterium_ spp. [18], and _Penicillium purpurogenum_[19]. \(\beta\)-mannanases of the GH26 family are found to originate primarily from bacteria, for example, _Bacillus subtilis_[20], _Clostridium cellulovorans_[21], _Paenibacillus polymyna_[22], _Pseudomonas fluorescens_, _Cellulomonas fini_[23].  
It is widely known that _Enterobacter aerogenes_ is a producing strain for 2,3-butanediol and 1,3-propanediol [24], and it is also regarded as a very useful strain for isobutanol production [25], ethanol production [26] and hydrogen production [27; 28]. In our previous work, we isolated a \(\beta\)-mannanase producing strain from forest soil. After physiological, biochemical and 16S rDNA identification, the strain belongs to _Enterobacter aerogenes_, named as _Enterobacter aerogenes_ B19. From this strain, we have cloned and characterized a CH1 \(\beta\)-mannanase with molecular weight of 82.5 kDa [29]. In this work, another new GH26 \(\beta\)-mannanase from _Enterobacter aerogenes_ B19 was investigated. We have successfully cloned and expressed Man26E gene in _Escherichia coli_, determined biochemical characteristics and substrate specificity of the enzyme. Three-dimensional structure of the \(\beta\)-mannanase was homologously modeled and its active domain was analyzed. The \(\beta\)-mannanase from _Enterobacter aerogenes_ B19 may have special potential applications in different industries.