**1. Introduction**

Type 2 diabetes (T2D), a disease associated with insulin resistance and poor blood glucose control, is a major public concern due to the potential severe complications and associated morbidity and its increasing rate of occurrence in many developed and developing countries. The major characteristic of this metabolic disease is chronic hyperglycemia, with severe complications leading to the long-term damage of various organs, including the eyes, kidneys, heart, blood vessels and nerves [1]. According to projections from the International Diabetes Federation, it has been estimated that in 2019 there were 463 million people with diabetes globally, and this is expected to reach 700 million by 2045, which will cost US\$850 billion per year for diabetes healthcare [2,3]. Although several drugs have been used clinically to control T2D, such as biguanides [4], sulfonylureas [5], sodiumglucose co-transporter-2 (SGLT2) inhibitors [6], and thiazolidinediones [7], all of these drugs have some serious side-effects, especially resulting in gastrointestinal disorders [8,9] which affect both the drugs' efficacy and the patient's life. "Natural" medicine based on the concept of "food as medicine" has been proposed as an alternative strategy in the managing of metabolic diseases (such as T2D), due, among other things, to their safety [10]. Growing evidence has confirmed that certain bioactive nutrients in these foods, including polysaccharides [11,12], can help mitigate metabolic abnormalities [13].

Polysaccharides, one of the most important biomacromolecules for life, are polymers found in natural sustainable resources. From 1970, since the first discovery of the bioactive properties of lentinan, there has been a drive to research the biological functions of

**Citation:** Wan, Y.; Xu, X.; Gilbert, R.G.; Sullivan, M.A. A Review on the Structure and Anti-Diabetic (Type 2) Functions of *β*-Glucans. *Foods* **2022**, *11*, 57. https://doi.org/10.3390/ foods11010057

Academic Editor: Diego A. Moreno

Received: 29 November 2021 Accepted: 24 December 2021 Published: 27 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

polysaccharides [14,15]. *β*-glucans are a type of naturally-derived polysaccharide which can be widely found in bacteria, alga, fungi, cereals, and higher plants. They are generally biopolymers with *β*-glucopyranosyl units, which normally contain a *β*-(1,3)-linked and/or *β*-(1,4)-linked backbone and may be branched with *β*-(1,6)-linked glucose.

*β*-glucans have been used in the food industry and in clinical practice because of their significant biological functions. For example, *β*-glucans with high molecular weights have been reported to enhance the binding capacity to the receptors involved in immune responses (e.g., dectin-1: a natural killer (NK)-cell-receptor-like C-type lectin involving in innate immune responses) and therefore improve their immunomodulatory activities, which can help to control some chronic diseases such as diabetes [16]. However, many studies have reported that degrading *β*-glucans to yield lower molecular weights can increase their anti-diabetic effects *in vitro*, as well as their antioxidant and antibacterial activities [17,18]. These differences in reported results may arise from differences in the detailed molecular structures of the substrates [19]. Therefore, this paper reviews structural and conformational features of naturally derived *β*-glucans, summarizing the potential underlying mechanisms of their anti-diabetic functions, for a better understanding of the structure–function relationships of *β*-glucans.

#### **2. Structural Features of** *β***-Glucans**

It is commonly asserted that the functionalities of *β*-glucans are highly dependent on their molecular structure. The structural characteristics of *β*-glucans, including molecular weight distributions, glycosidic linkage patterns and branching degrees, vary with different sources and extraction methods. There are three main glycosidic linkage patterns identified in *β*-glucans (Figure 1): *β*-1,3-linked, *β*-1,4-linked and *β*-1,6-linked patterns. Normally, *β*-1,3-linked and *β*-1,4-linked patterns appear in the backbone of *β*-glucan, and *β*-1,6- linkages represent the branch points in the backbone.

**Figure 1.** Chemical structures of *β*-glucans. The glucose monomers are shown following the symbol nomenclature for glycans. (**A**) The *β*-1,3-linked backbone of *β*-glucan with different branching degree of *β*-1,6-linked glucose. (**B**) The *β*-1,3-1,4-linked backbone of *β*-glucan, DP3: cellotriosyl, DP4: cellotetraosyl).

Branched or linear *β*-1,3-glucans (Figure 1A) are commonly isolated from fungi (e.g., mushroom [20]) and bacteria (e.g., yeast [20]). The occurrence of *β*-1,3-linkages together with *β*-1,4-linked glycosidic bonds (Figure 1B) are observed in the *β*-glucans from cereal grains (e.g., oat [21]). As shown in Figure 1B, there are two types of oligosaccharide subunits; one is three *β*-1,4-linked glucose monomers, termed cellotriosyl (DP3), and the other type is *β*-1,4-linked glucose monomers called cellotetraosyl (DP4). The molar ratio of DP3 and DP4 in *β*-glucans is specific for different cereals; this can be used as a tool to trace the origin of a given *β*-glucan structure [21].

The extraction methods of *β*-glucans vary from sample to sample. There are commonly four types of isolation method used in extracting *β*-glucans, such as water extraction [22–24], alkaline extraction [25,26], acidic extraction [27] and enzymatic extraction [28]. Some suggested structures of naturally derived *β*-glucans are shown in Table 1. These structures

are generally inferred from a combination of results collected by both chemical tests and instrumental analysis. Interestingly, some similar repeating structural units exist in several *β*-glucans from different sources. For example, *β*-glucans from *Dictyophora indusiate*, *Hericium erinaceus*, *Grifola frondosa*, *Schizophyllan* and brown algae have the same repeating unit, viz., three *β*-1,3-Glc*p* backbone residues with a branch comprising one *β*-1,6-linked glucose residue. The branches of *β*-glucans, connected to the backbone via *β*-1,6 glycosidic bonds, play a major role in the solubility of the *β*-glucan. For example, curdlan, a linear *β*-glucan (i.e., without side chains) is insoluble in water [29], while the *β*-glucans with branched glucose residues, such as lentinan and Schizophyllan, are water-soluble [30,31]. However, these *β*-glucans exhibit some hydrophobicity due to hydrophobic carbon rings, resulting in limited water-solubility. Thus, *β*-glucans adopt different chain conformations to achieve stability. It is essential to consider their chain conformations in different solvents for the application of *β*-glucans in the food industry and medicine.

**Table 1.** Sources and deduced chemical structures of several *β*-glucans.


<sup>a</sup> The uppercase letters within this column represent the repeating units shown in Figure 1 (A. *β*-1,3-1,6 glucan; B. *β*-1,3-1,4 glucan). The lowercase letters indicate the molar ratio of each part in the repeating units.

#### **3. Conformational Features of** *β***-Glucans**

Several chain conformations of *β*-glucans are found in different solutions (Figure 2), from a disordered conformation (e.g., random coil) to an ordered conformation (e.g., helix conformation). These more organized conformations can easily form a stable network (e.g., a triple helix), and the stabilization of this network arises from its inter- and intra-molecular hydrogen bonds. However, the dense triple helix conformation formed by the interaction of intramolecular polyhydroxy groups may result in its insolubility in aqueous solution [54].

**Figure 2.** Various chain conformations of polysaccharides in different solvents.

There are many parameters that can affect conformational features of polysaccharides, including the molar mass per unit of contour length (*M*L), the contour length of chains (*L*), the persistence length (*q*) and the chain diameter (*d*). The contour length of a polymer chain refers to its length at maximum physically possible extension, and the persistence length reflects the bending stiffness of a chain. Several reports indicate that *β*-glucans that

exhibit strong biological functions show a triple-helix conformation, such as lentinan [55], curdlan [56] and yeast *β*-glucan [57]. Based on both theoretical and experimental results, the *M*<sup>L</sup> and *q* values of a polysaccharide with a rigid triple-helix conformation usually range from 2000 to 2800 nm−<sup>1</sup> and from 100 to 250 nm, respectively [20]. For example, lentinan, the first reported *β*-glucan with antitumor activities, exists as a triple-helix conformation in aqueous solution, with a reported *M*<sup>L</sup> value of 2160 nm−<sup>1</sup> and *q* value of 110 nm [58]. Schizophyllan, a widely studied *β*-glucan from *Schizophyllum*, has a reported *M*<sup>L</sup> value of 2150 nm−<sup>1</sup> and *q* value of 200 nm [59]. However, the triple-helix conformation of these *β*-glucans can be transferred to other conformations under special conditions, such as high temperature [60], high pH solvents [61] and strong polar solvents [62]. With disrupted conformations, their bioactivities and solubilities are also changed. Therefore, the structural and conformational features of *β*-glucan need to be well characterized for a correct understanding of their functionalities.

#### **4. Characterization Methods for** *β***-Glucan Structure and Conformation Analysis**

The structural determination of polysaccharides is more complicated than for other biopolymers due to the diverse monosaccharide compositions and glycosidic linkage patterns. In addition to classical chemical characterization methods, many newly developed technologies for the characterization of polysaccharides have been, or could be, employed to help understand the structure-function relationship of bioactive polysaccharides. For example, enzymatic arrays [63] and matrix-assisted laser desorption ionization mass spectrometry [64] have been used to sequence polysaccharides, and ion mobility-mass spectrometry has been developed to analyze carbohydrate anomers [65]. Advanced microscopy techniques, such as atomic force microscopy [66] and confocal laser scanning microscopy [67], provide a new level of microstructure analysis. Recently, low-temperature scanning tunneling microscopy has been successfully applied to observe single glycans [68]. However, the exploration of polysaccharides has been much slower than that of polynucleotides and proteins because of limitations on the structural theories, the complexity of their structures and a poor understanding of the underlying mechanisms of their bioactivities.

The characterization of *β*-glucan structure therefore requires a combination of chemical and instrumental analyses (Figure 3). The structural information of *β*-glucans usually reported include its purity, molecular weight, monosaccharide composition, anomeric configuration, glycosidic linkage pattern and sequence of residues.

**Figure 3.** Chemical and instrumental methods used for *β*-glucan structure characterization.

To investigate detailed structure–function relationships of polysaccharides, the "purity" is one of the most important factors. Generally, measuring the purity of *β*-glucans includes obtaining several parameters, such as the sugar content and the molecular weight distribution. Colorimetric methods are commonly used as the first step to determine the purity of crude polysaccharides, including the measurement of sugar, uronic acid and protein contents. Then, the molecular weight distributions of polysaccharides can be analyzed using several instrumental methods, such as size exclusion chromatography (SEC). Sometimes, *β*-glucans coexists with other polysaccharides, such as arabinogalactans [69]. Therefore, it is essential to analyze its monosaccharide composition to identify the purity of a *β*-glucan. Hydrolysis with acids or enzymes is the first step to analyze the monosaccharide composition, after which the hydrolysate is characterized with various instruments. High-performance anion exchange chromatography (HPAEC) is considered one of the most effective instrumental analysis techniques to determine monosaccharide composition due to the high sensitivity and simple sample preparation [70]. A high-purity *β*-glucan should have a narrow molecular weight distribution and high glucose content.

Extraction of *β*-glucans from grains always results in some starch (an *α*-glucan) [54], and it is difficult to distinguish *β*-glucans from *α*-glucans through their sugar content or monosaccharide composition alone. However, a combination of glycosidase hydrolysis and instrumental analysis, such as Raman spectra, FT-IR and NMR, can easily identify the anomeric configuration of glucans. The sequence of residue and the branching degree of naturally derived *β*-glucans is highly dependent on its source, and can be characterized using NMR. For a comprehensive characterization of naturally derived *β*-glucans, it is necessary to use both chemical methods and instrumental analysis.

To identify the conformation of *β*-glucans, weight-average molecular weight (*Mw*), intrinsic viscosity ([*η*]), radius of gyration (*R*g) and hydrodynamic radius (*R*h) of *β*-glucan samples can be measured by static light scattering (SLS), dynamic light scattering (DLS) and viscometry, using the molecular-weight dependence of their solution properties. Several models, including the helical wormlike chains model and the Kratky-Porod model, can be used to deduce the four main parameters, *M*L, *L*, *q* and *d* based on the results of measurements.

Conformation analysis can be performed using X-ray diffraction (XRD), e.g., for measuring the triple-helix *β*-glucan [71]. Atomic force microscopy (AFM) is a powerful tool to observe chain conformations in solution, including rod-, sphere- and fiber-like shapes. AFM can provide information on the chain length, chain diameter and even the *M*<sup>L</sup> of a *β*-glucan sample [58]. In addition to these experimental techniques, molecular dynamics simulations have been used as a tool to explore the conformation of polymers, which can help illustrate the chain movements and conformations of polymers in different solutions [32], although the results always depend on the assumed model.

Although the characterization of *β*-glucans is complicated, an understanding of the structure/function relationships of these molecules is crucial if they are to advance further as a potential antidiabetic drug. Without this understanding the efficacy of a particular *β*-glucan is difficult to predict.

### **5. Amelioration of Type 2 Diabetes and Associated Mechanisms**

T2D is the most frequent metabolic disorder which involves insulin resistance, followed by deficient insulin secretion by impaired pancreatic islet *β*-cells [72]. The two main factors that typically account for T2D are genetic factors and environmental factors. A genome-wide association study has confirmed that there are more than 400 gene variants associated with T2D, with most of them involving islet function [73]. The environmental factors that increase the risk of developing T2D include obesity, alcohol intake and smoking. The predominant factor accounting for T2D is the consumption of unhealthy diets, including those with energy-dense refined food [72].

#### *5.1. Pharmacotherapy for T2D and Anti-Diabetic Mechanisms*

There is a lack of effective drugs to treat T2D due to the complexity of pathogenesis [72,74]. However, several drugs are used in controlling T2D, and these drugs can be classified into seven main types based on their structures and mechanisms, including biguanides, sulfonylureas, thiazolidinediones, glucagon-like peptide (GLP-1), dipeptidyl peptidase (DPP-4) inhibitors, sodium-glucose co-transporter-2 (SGLT2) inhibitors and enzyme inhibitors. The underlying mechanisms and potential side-effects of these drugs are summarized in Table 2. As well as the injection of insulin being essential to control type 1 diabetes, this is also adopted to control T2D under certain conditions, such as functional failure of pancreatic islet *β*-cells due to the long-term suffering from T2D [75]. However, it should be noticed that all of these anti-diabetic drugs are companied by some severe side-effects (Table 2), such as gastrointestinal disorders, which have prolonged impact on the patient.

**Table 2.** Drugs used in amelioration of T2D.

