Biochemical and Economical Aspects of Levan Synthesis by Zymomonas Mobilis

Structure-function relationships of glucansucrase and fructansucrase enzymes from lactic acid bacteria

Lactic acid bacteria (LAB) employ sucrase-type enzymes to convert sucrose into homopolysaccharides consisting of either glucosyl units (glucans) or fructosyl units (fructans). The enzymes involved are labeled glucansucrases (GS) and fructansucrases (FS), respectively. The available molecular, biochemical, and structural information on sucrase genes and enzymes from various LAB and their fructan and alpha-glucan products is reviewed. The GS and FS enzymes are both glycoside hydrolase enzymes that act on the same substrate (sucrose) and catalyze (retaining) transglycosylation reactions that result in polysaccharide formation, but they possess completely different protein structures. GS enzymes (family GH70) are large multidomain proteins that occur exclusively in LAB. Their catalytic domain displays clear secondary-structure similarity with alpha-amylase enzymes (family GH13), with a predicted permuted (beta/alpha)(8) barrel structure for which detailed structural and mechanistic information is available. Emphasis now is on identification of residues and regions important for GS enzyme activity and product specificity (synthesis of alpha-glucans differing in glycosidic linkage type, degree and type of branching, glucan molecular mass, and solubility). FS enzymes (family GH68) occur in both gram-negative and gram-positive bacteria and synthesize beta-fructan polymers with either beta-(2-->6) (inulin) or beta-(2-->1) (levan) glycosidic bonds. Recently, the first high-resolution three-dimensional structures have become available for FS (levansucrase) proteins, revealing a rare five-bladed beta-propeller structure with a deep, negatively charged central pocket. Although these structures have provided detailed mechanistic insights, the structural features in FS enzymes dictating the synthesis of either beta-(2-->6) or beta-(2-->1) linkages, degree and type of branching, and fructan molecular mass remain to be identified.

Metabolization of beta-(2,6)-linked fructose-oligosaccharides by different bifidobacteria

Low-molecular-mass beta-(2,6)-linked fructose-oligosaccharides (beta-(2,6)-FOS) were examined as a new carbohydrate source for growth of bifidobacteria. beta-(2,6)-FOS were prepared from microbial high-molecular-mass levan by acid hydrolysis and refined by cation-exchange chromatography. (13)C-NMR spectroscopy confirmed the presence of predominantly beta-(2,6)-fructosyl linkages in the oligosaccharides. More than 80% beta-(2,6)-FOS was recovered after in vitro incubation with amylolytic and proteolytic enzymes, implying resistance to degradation in the upper intestinal tract. Bifidobacterium adolescentis, B. longum, B. breve, and B. pseudocatenulatum were studied in vitro for their ability to metabolize beta-(2,6)-FOS. Growth, decrease in pH, formation of short- chain fatty acids (lactate, acetate, formate) and degradation of beta-(2,6)-FOS were markedly different among species. B. adolescentis showed the best growth, produced the highest amounts of organic acids and metabolized both short- and long-chain beta-(2, 6)-FOS.

Isolation and characterization of mutants from levan-producing Zymomonas mobilis

Levan is a fructose polymer with potential importance as a fructose source and as a thickening agent in food technology. By N-methyl N'-nitro N-nitroso guanidine (NTG) mutagenesis and selection, two mutants, ZML1 and ZML2, of Zymomonas mobilis B4286 overproducing levan (21.6 and 20.0 g/l at 24 h) were isolated. Despite showing similar rates of sucrose hydrolysis (2.38 g/l/h) in fermentation, these mutants exhibited higher rates of levan production (1.72 and 1.68 g/l/h, respectively) than their parent strain (0.69 g/l/h). In contrast, these mutants showed lower rates of ethanol production (1.06 and 1.55 g/l/h respectively) than the parent strain (3.75 g/l/h). The maximum levansucrase activity in the mutants was higher (30.6 and 36 U/mg, respectively, at 24 h) than in their parent strain (12.6 U/mg). Therefore, the increased rates of levan production by the mutants are attributed to their higher levansucrase activity.