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Studies on the structure of exopolysaccharides produced by lactic acid bacteria

机译：乳酸菌产生的胞外多糖的结构研究

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Microbial exopolysaccharides (EPSs) are widely applied in food industry as thickening, gelling and stabilizing agents (1). Lactic acid bacterial, which carry the GRAS (Generally Recognized As Safe) status, are good sources of such food grade EPSs. There is a great interest in understanding the relation between structure and physical properties, is order to enable the tailoring of the EPSs according to required properties. The EPSs produced by lactic acid bacteria can be homopolysaccharides or heteropolysaccharides. The homopolysaccharides can be α-D-glucans, β-D-glucans or fructans, showing strain type specific differences with regard to degree of branching and position of the glycosidic linkages (for a review see 1). The heteropolysaccharides consist of repeating units that are built up from three to seven monosaccharides. The average number of repeating units per molecule can be larger than 1000. The molecular masses of these polysaccharides are in the order of 4-6 * 10~3 kDa. For a large number of Lactobacillus species the structure of the repeating unit has been established (for a review see 2). EPSs from lactobacilli are mostly built up from combinations of D-Glc, D-Gal and/or L-Rha, but also D-GlcNAc, D-GalNAc and GlcA may occur. Noncarbohydrate substituents like phosphate, acetate and glycerol groups are found. An interesting, common feature of these heteropolysaccharides is the presence of a side chain. Apparently, side chains are relevant for the physical properties. For further insight into the structure/function relations it is necessary to study the conformation of the polymer in solution as well as its dynamic behaviour. A complicating factor in evaluating the potency of EPSs as food additives stems from the interaction with the other food constituents. The three-dimensional structure of a polysaccharide is dependent on the time averaged ring conformation of the constituting monosaccharide residues and the relative orientations of adjacent monosaccharides. The conformations around the glycosidic linkages are major characteristics to be determined. In addition, intramolecular hydrogen bonds contribute to the conformation. In many instances NMR spectroscopy does not provide sufficient information to define the conformational equilibrium unambiguously. Computational methods like MM calculations and MD simulations can furnish more data to describe the conformation in solution. Parameters like average molecular mass, end-to-end distance, radius of gyration and persistence length are important for defining the spatial structure of EPSs. Despite the information available today, it is not yet sufficient to allow the prediction of properties on the basis of structures.
机译：微生物胞外多糖（EPS）作为增稠剂，胶凝剂和稳定剂在食品工业中得到广泛应用（1）。具有GRAS（公认安全）状态的乳酸菌是此类食品级EPS的良好来源。理解结构和物理特性之间的关系引起了极大的兴趣，以使EPS能够根据所需的特性进行定制。乳酸菌产生的EPS可以是均多糖或杂多糖。均多糖可以是α-D-葡聚糖，β-D-葡聚糖或果聚糖，它们在糖苷键的支化程度和位置方面显示出菌株类型的特异性差异（综述见1）。杂多糖由三个至七个单糖组成的重复单元组成。每个分子的平均重复单元数可以大于1000。这些多糖的分子量约为4-6 * 10〜3 kDa。对于大量的乳杆菌属物种，已经确定了重复单元的结构（有关综述，请参见2）。乳杆菌的EPS大多由D-Glc，D-Gal和/或L-Rha组合而成，但也可能会出现D-GlcNAc，D-GalNAc和GlcA。发现了非碳水化合物取代基，例如磷酸根，乙酸根和甘油基。这些杂多糖的一个有趣的共同特征是侧链的存在。显然，侧链与物理性质有关。为了进一步了解结构/功能关系，有必要研究聚合物在溶液中的构象及其动态行为。评估EPS作为食品添加剂的效力的一个复杂因素来自与其他食品成分的相互作用。多糖的三维结构取决于构成的单糖残基的时间平均环构象和相邻单糖的相对方向。糖苷键周围的构象是要确定的主要特征。另外，分子内氢键有助于构象。在许多情况下，NMR光谱无法提供足够的信息来明确定义构象平衡。 MM计算和MD仿真等计算方法可以提供更多数据来描述溶液中的构象。平均分子量，端到端距离，回转半径和持续长度等参数对于定义EPS的空间结构很重要。尽管有当今可用的信息，但尚不足以允许根据结构预测属性。

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