Structural features of microbial exopolysaccharides in relation to their antioxidant activity
Graphical abstract
Introduction
Microbial resources are bountiful in nature and are enriched with valuable biological properties. Till now microorganisms are exceptional sources for various metabolites endowed with novel functions and essential medicinal properties [1,2]. Although microbes survive in diverse habitats such as high salinity, temperature, pressure, pH, they extend their endurance in such environments by fortifying themselves with molecules/mechanisms that can combat the stressful environment. Such a defence mechanism, leading to adaptability and extended survival of microbes are carried out by various metabolites such as osmolytes, enzymes, peptides, polysaccharides [2,3]. Microbial metabolites are very much preferred over other products for several applications like healthcare, agriculture and nutrition for their valuable therapeutic efficiency and availability [4]. Of these several metabolites microbial polysaccharides are secreted both in extracellular and intracellular region of cells. Polysaccharides constitute several monosaccharide units linked by glycosidic bonds which blend into various structural compositions. These are ubiquitous carbohydrate macromolecules that can be either linear or branched. In the natural environment, exopolysaccharides (EPS) protects the cell during adverse conditions, desiccation and against invading predators; it also promotes nutrient uptake [[5], [6], [7]].
In general, EPS can either be homopolysaccharide in which all the monosaccharides are identical or heteropolysaccharide in which more than one monosaccharide is combined to form a polysaccharide. More often these are attached to proteins, lipids or non-carbohydrate metabolites such as pyruvate, acetate, phosphate, and succinate as their additional structural component. It is well known that EPS with a straight chain of monosaccharides is a linear polysaccharide, whereas a chain that has arms and turns is referred to as branched polysaccharide [3,[7], [8], [9]]. The overall properties of any microbial EPS depend on its molecular weight, chemical composition and architecture, all of which contribute in determining its final structure and biological activity. The significance of microbial EPS has become prominent due to their novel physiological, functional and biological properties. Notable among these polymers are dextran, xanthan, alginate, levan, succinoglycan, welan and hyaluronan which are derived from bacterial sources. Some fungal derived EPS are pullulan, scleroglucan, chitosan, lentinan, schizophyllan, and pleuran [10]. Reports have shown that structural features and molecular characteristics of microbial EPS have been exploited successfully for various industrial and medicinal applications [8,9]. Despite traditional benefits, the neoteric research on microbial EPS have been exploited for several new biological properties such as drug delivery, drug targeting, cell carriers, tissue engineering, vaccine preparation, diagnostics tool making, wound healing [11], anti-proliferation [12], antitumor [13], immunomodulation and antiviral activities [14,15].
Besides the advancement of medicine and food technology, most of the emerging diseases and metabolic disorders have led to the exigency of reliable and natural therapeutic drugs with minimal side effects. Among various disease causative sources, the oxidative stress in cells plays a very crucial role in the occurrence of various complications such as mutations, DNA damage, and protein structure alteration through the generation of free radicals. Moreover, reports have demonstrated that pathology of several diseases like coronary artery disease, atherosclerosis, diabetes and several types of cancers evolve due to oxidative stress and extensive oxidative damage in the cells [[16], [17], [18]]. Induction of oxidative stress by free radical accumulation causes malfunctioning of the system and certain chronic abnormalities in the central nervous system leading to neurodegenerative disorders and alteration in the immune system of the human body [19]. Owing to the fact that antioxidants are essential factors in resolving several oxidative stresses, they are often used as protective agents, therapeutic agents, functional foods and additives in the preservation of human nutritional products. So the exploration of metabolites from microbes, plants and marine invertebrates have been augmented by the researchers to combat functional abnormalities and dreadful diseases [20,21]. The use of synthetic antioxidants in food products is prone to cause several adverse effects on human health despite its numerous benefits [22]. In the concern of safety to humans, the preference of antioxidants derived from natural resources perhaps can minimize harmful effects for further beneficial applications. There have also been reports on microbes that produce EPS with desirable structural features that assist in vitro free radical scavenging abilities. In order to utilize such effective antioxidant characteristics from microbial sources, few explorations have disclosed diverse EPS molecules with potential antioxidant properties obtained from different environments. Moreover, in recent years several reports have been established on the in vitro antioxidant activities of microbial EPS [[23], [24], [25], [26]]. Even though there has been great progress in the EPS-related applications over the last decade, an elaborated structure-activity relationship in this emerging field of research is lacking. This article attempts to review the recent exploration of microbial EPS especially for antioxidant properties and correlate their structural features with their powerful antioxidant mechanism.
Section snippets
Antioxidants: the game-changing molecules against free radical play
In nature, free radicals consisting of any unpaired electron undergo reactions which capture electrons from other sources for their neutralization. More often these free radicals are generated as byproducts of cellular metabolism, inflammation, stress and ageing process. Moreover, they are harmful molecules which have the deleterious sequel to the cellular membrane and components through their direct interaction. This series of reactions leads to a reduction of cell function and sometimes
Slippage of synthetic antioxidants and exigency of natural antioxidants
Antioxidants are considered as a necessary factor in food industries for preservation, maintenance of flavour, shelf life and consistency of nutrients in food products. Since modern society has developed, the production of processed and readymade food products has dominantly increased all over the world. Consequently, the manufacturing of food products is associated with safety and the long-term quality which demands ultra-modern preservation methods, thus requiring suitable antioxidant
Microbial EPS as antioxidants -the rationale behind the utilization
As research interests have turned towards marine microbes [46], EPS have proved to be a reliable molecule for various therapeutic and biomedical applications [4,47]. Since nature has an abundance of microbial resources, the exploitation of EPS could be noteworthy for future demands, inclusive of pharmaceutical industries [11,21]. Fig. 1 denotes statistics about the number of publications per year in PubMed from 2009 until 2019 April on microbial EPS. This information unveils the need for more
Mode of scavenging action
The free radical scavenging action is one of the remarkable roles of antioxidants. Stable radicals such as DPPH and ABTS are reduced either by receiving electron or hydrogen from affordable antioxidants. Toxic hydroxyl radicals and highly reactive ROS are scavenged either directly or through the prevention of further generation. Additionally, superoxide anion is a precursor for a few free radicals which often could generate hydroxyl radical and hydrogen peroxide that subsequently causes
Conclusion and future perspective
Oxidative stress being a crucial factor in the evolving diseases that embark severe impact on human health is rapidly increasing due to unhealthy lifestyles. Antioxidants serve as key molecules that are involved in the degradation of oxidants, exert protection and assault free radical generation. Since the necessity of favorable and stable natural antioxidants is exceeding globally, it is essential to explore more natural antioxidants from various resources. In accordance with fact, EPSs are a
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors acknowledge the host institution for providing the necessary support.
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