Angayarkanni Jayaraman*¹, Thandeeswaran Murugesan¹, Nisshanthini Durairaj², Karunya Jairaman¹ and Muthusamy Palaniswamy^3
1Department of Microbial Biotechnology, Biotechnology, Bharathiar University, Coimbatore – 641 046 . ² Molecular Diagnostics Lab, Bhat Bio- Tech India Pvt Ltd, Bengaluru -561229. ³Department of Microbiology, Karpagam Academy of Higher Education, Coimbatore – 641 021
e-mail: angaibiotech@buc.edu.in
*For correspondence

Abstract

         Cyanide is highly toxic for most living organisms because it forms very stable complexes with transition metals (ie. Iron), that is essential for protein function, as in cytochrome oxidase, haemoproteins as well as other metal–containing oxidases or oxygenases. The removals of cyanide by the physical and chemical methods are more expensive and thus alternative process like biodegradation technologies are under focus. The microorganisms utilize potassium and sodium cyanide as a sole source of carbon and nitrogen for the degradation process. Cyanide degrading bacteria are noted to produce pteridines, a cofactor for the activation of cyanide monooxygenase which is needed for cyanide degradation. Peridines being a potential therapeutic agent, the production of pteridines by these organisms are needed to be explored in future.

Keywords: Biodegradation, Cyanide degradation, Cyanide Monooxygenase, Pteridine.
         

Introduction

         In the wake of Technology development and Industrial revolution, rapid industrialization has resulted in amassment of waste in the form of both solid and liquid. While addressing the global challenge in sustainable development, the waste degradation must be given priority over the production process. The product would fulfill certain needs of human kind but the waste accumulation deprives the whole community of healthy environment by piling up the pollutants. If improperly managed, this waste can pose dangerous health and environmental consequences. In this context, biodegradation is found to be the best approach to retract the adverse impact and reduce the pollution effect. Biodegradation is the nature’s way of cleaning up the environment by breaking down the complex toxic matter to simple nontoxic matter for the utilization of the biota.
       

Cyanide waste

          Cyanide is an ancient molecule that might be involved in the prebiotic synthesis of different nitrogenous compounds, including amino acids and nitrogenous bases. Cyanides include a type of chemicals that present the cyano (−C≡N) group and they can be found in nature in many different forms owing to the chemical properties of this group. Cyanide is generated as a natural compound by some bacteria, algae, fungi, higher plants and even by insects, either as a biomolecule for guarding mechanism or as repulsive molecule. Plants are the main source of cyanide in the biosphere because they cogenerate cyanide with ethylene (Peiser et al., 1984) in addition to generating cyanoglycosides and cyanolipids. Moreover, cyanide has also been shown to be produced as part of active iron-cyanide complexes of catalytic proteins (Reissmann et al., 2003). Even though natural processes generate cyanide, the human activity is the major contributors which tip the balance in nature creating environmental havoc.

Toxicity of cyanide waste

          Cyanide can enter the human system either by inhalation or ingestion or adsorption. The fatal doses for human adults have been prescribed as 1-3 mg/kg body weight if ingested, 100-300 mg/L if inhaled, and 100 mg/kg body weight if adsorbed (Huiatt, 1984). Cyanide released from industries worldwide has been estimated to exceed 14 million kg per year which is an alarming quantity (Naveen et al., 2011). A short-term exposure of cyanide, causes rapid breathing, tremors and other neurological effects, and long-term exposure to cyanide causes weight loss, thyroid effects, nerve damage, and even death. Skin contact with cyanide-containing liquids may produce irritation and sores (Dash et al., 2009). Cyanide is also known as a major inhibitor of the enzyme cytochrome oxidase as well as haemoproteins and other metal–containing oxidases or oxygenases (Knowles, 1976). As cyanide is a metabolic inhibitor of terminal cytochromes of electron transport chains (Dumestre et al., 1997; Yanase et al., 2000), cyanide pollution causes great damage to ecosystems.       

Microbial degradation of cyanide waste

          In India, Central Pollution Control Board has set a minimal national standard limit for cyanide in wastewater as 0.2 mg/L. In the current scenario wastewater treatments for cyanide removal physical and chemical methods are employed which are often expensive and involve the use of additional hazardous reagents (chlorine and sodium hypochlorite) for alkaline chlorination, ozonization, wet-air oxidation and sulfur-based technologies (Watanabe et al., 1998, Patil and Paknikar, 2000). Further each of these technologies has its own cost and disposal considerations (Saarela and Kuokkanen, 2004). Thus cyanide treatment hollered out for an alternative treatment process capable of achieving high degradation efficiency at low costs. Biodegradation technologies are scrupulously appealing for cyanide wastes with added organic supplements for microbial growth which results in production of eco-friendly products like CO₂, formate, formamide and methane (Dubey and Holmes, 1995; Raybuck, 1992). It was earlier contemplated that cyanide was the pioneer organic compound on earth, from which the chemical building blocks of life evolved (Oparin, 1957; Rawls, 1997). Many microorganisms can use potassium or sodium cyanide as a sole source of carbon and nitrogen. Despite the toxicity of cyanide towards living organisms, biodegradation of cyanide bank upon the easy adaptation and enrichment of indigenous microorganisms which can utilize cyanide as substrate (Dash et al., 2009).

Propitious omen of cyanide degradation by oxidation

          The oxidative pathway of cyanide conversion involves oxygenolytic conversion to carbon dioxide and ammonia. There are two types of oxidative pathway involving three different enzymes. The first oxidative pathway involves cyanide monooxygenase and cyanase. The second oxidative pathway utilizes cyanide dioxygenase to form ammonia and carbon dioxide directly (Ebbs, 2004). Between the two pathways the first pathway involves a positive product named pteridine which has several biological implications both in prokaryotes and eukaryotes.  

          Cyanide monooxygenase in the first oxidative pathway (Raybuck, 1992; Ebbs, 2004) converts cyanide to cyanate. The cyanate is then catalyzed by cyanase resulting in the conversion of cyanate to ammonia and carbon dioxide. Cyanases have been identified in numerous bacteria, fungi, plants and animals. Cyanide monooxygenase (CNO) is located in the cytosolic fraction of cells induced with cyanide and requires both reduced pyridine nucleotide (NADH) and a source of reduced pterin as a cofactor (Kunz et al., 1992; Fernandez et al., 2004). Cyanide monooxygenase is a pterin-dependent hydroxylase which means this enzyme requires pterin as a cofactor (Cabuk et al., 2006). It is usually observed that cyanide-grown cells contain elevated levels of both cyanide mono- oxygenase and formate dehydrogenase (Kunz et al., 1992). It was hypothesised that the cofactors production also increase with increased production of metabolic enzymes and it was proposed that cyanide degrading bacteria produces pteridines in large amounts (Nisshanthini et al., 2015). 

          Owing to the therapeutical importance of pteridines, the production of pteridines by cyanide degrading bacteria using cyanide waste as substrate is a typical process of wealth from waste which needs to be explored in detail.  

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