Hydrogen cyanide or hydrocyanic acid (HCN) or cyanide (-CN moiety) is known to all of us as a toxic chemical. It is generally found in automobile exhaust, cigarette smoke; it is produced during incomplete burning of fire and has been important component of chemical weapons. It is also one of the byproducts from various industries like textile, dyeing, electroplating, chemical fertilizers, plastics etc., and as alike other industrial byproducts, is capable of polluting large number of ecosystems and their components directly. On the other hand, HCN is also produced naturally by biological systems of biosphere such as plants, bacteria, fungi, algae and animals. These cyanogenic biocomponents produce cyanide for different purposes. To name some cyanogenic plants-lima beans, velvet grass, corn, white clover, flax, seeds of apple, plum, cherry, peach; roots of turnip, cassava, young bamboo shoots etc. Some algal species of Chlorella and Platymonas and cyanobacterial species of Nodularia, Nostoc and Anacystis are known to be cyanogenic. Fungi like Tricholoma, Stemphylium, Polyporus, Clitocybe, Agaricus (Marasmius). Cyanogenic bacterial genera include Pseudomonas, Rhizobium, Xanthomonas, Erwinia, Arthrobacter, Bacillus, Acinetobacter and Aeromonas.
Why is HCN toxic?
HCN is toxic to bacteria, fungi, protozoa, plants, mammals and other animals. Typical symptoms of cyanotoxicity in bacteria are phenotypic changes, decreased motility and growth, mutations, altered respiration or death. The toxicity of HCN is attributed to its ability to covalently bind metal ions of metalloenzymes associated with cellular respiration; especially cytochrome oxidase. HCN forms strong bond with iron (Fe) of cytochrome oxidase, inhibiting electron transfer in respiratory chain and thus disrupting cellular respiration. Since cyanide binds iron metal, it may create iron deficiency for the cyanide producing bacterium or to the plant if bacterium is native to its rhizosphere region. To overcome such cyanide bound iron deficiency, these bacteria also synthesize iron sequestering siderophore compounds. Cellular organs of plants and animals with essential oxygen consumption are extremely sensitive to HCN and so are sensitive the aerobic and facultative anaerobic bacteria.
Surprisingly, it is not toxic to organisms producing it! HCN producing bacteria shield themselves of toxic effects of HCN by various mechanisms. Some bacteria can adapt to the presence of HCN and growth is sustained because of their cyanide tolerant respiratory systems. The cyanide tolerance is offered by branched respiratory chain, presence of alternate oxidase, multiple terminal oxidases and other mechanisms still unknown. Bacteria do possess cyanide detoxification mechanisms that include synthesis of enzymes which convert toxic cyanide into the product/s which is/are non-toxic to growing culture. Pseudomonas aeruginosa possesses a cyanide insensitive terminal oxidase which defends terminal electron acceptor i.e. oxygen from deactivation by cyanide. In this bacterium enzyme rhodanese is also expressed constitutively which detoxifies cyanide by converting it to thiocyanate (Cipollone et al, Appl.Environ.Microbiol. 2006; 73(2):390-398). It has also been found that growing bacteria are non-sensitive to HCN during lag and logarithmic phases; the mechanism is partially known.
Bacterial cyanide utilization and degradation:
For some bacterial genera, HCN is not only non-toxic but also a source of energy. They can grow on cyanide containing substrates, utilize it as carbon and nitrogen source for the synthesis of their amino acids and can also degrade HCN to non-toxic products. It is not always that HCN producers utilize it as carbon, nitrogen or energy source or necessarily degrade it. Cyanide production and degradation are two irrelevant biochemical processes of a concerned bacterium. In cyanogenic bacteria, amino acid glycine is main precursor of cyanide synthesis. Glycine is decarboxylated into HCN and carbon dioxide via formaldoxime as an intermediate; enzyme HCN synthase catalyzes this oxidation reaction.
Microbial HCN degradation is enzyme catalyzed reaction. HCN is degraded differently under aerobic and anaerobic conditions and it also depends on cyanide concentration. Generally, cyanide is converted to ammonia and further to nitrate aerobically. The process is similar to biological nitrogen fixation in which atmospheric nitrogen is fixed to ammonia and then oxidized to nitrate to be assimilated by green plants; a unique reaction catalyzed by enzyme nitrogenase. Remarkably, HCN is also one of the substrates of nitrogenase. Under anaerobic environment, HCN is degraded to ammonium ion, nitrogen, formate, thiocyanate and/or carbon dioxide. As said at the start of this paragraph, bacterial cyanide degradation is a cascade of enzyme catalyzed metabolic reactions consisting of sequential oxidation, hydrolysis followed by substitution reactions. Degradation initiates by oxidation. HCN is oxidized to cyanate by cyanide monoxygenase; enzyme cyanase then converts cyanate to ammonia (derived from reduction of nitrogen in HCN) and bicarbonate/formate. Direct oxidation pathway consisting conversion of cyanide to ammonia and carbon dioxide directly by enzyme cyanide dioxygenase also operates sometimes during degradation (anaerobic). Ammonia is next oxidized to nitrate and nitrate is reduced to nitrogen by soil ammonia oxidizing and denitrifying bacterial community respectively. Fate of HCN is thus into the release of atmospheric nitrogen. Bicarbonate or formate formed in aerobic oxidation are either used for bacterial growth or hydrolyzed to formic acid by enzyme cyanidase. As stated in earlier paragraph, about Cipollone et al’s reference of Pseudomonas, cyanide detoxification into thiocyanate is catalyzed by enzyme rhodanese or sulfur transferase. This is achieved as cyanide has strong affinity of sulfur contained in the enzyme. In this reaction hydrogen of HCN is substituted by thio or sulfur group, transforming it into non-toxic thiocyanate. The bacterial genera which are prominent HCN degraders are: Arthrobacter, Alcaligenes, Pseudomonas, Methylococcus, Bacillus, Thiobacillus, Klebsiella, Corynebacterium, Actinomyces, Enterobacter, Flavobacterium, Burkholderia, Chromobacterium etc.
The reason for bacterial cyanide utilization and degradation lies in the simple structure of HCN: one carbon (C1) compound-easy to break and assimilate. Bacterial cyanide degradation is environment friendly, non- expensive and a complete procedure. In addition to these advantages, it also occurs under aerobic, facultative and anaerobic conditions at cyanide concentrations ranging from 2-200 ppm. On the other hand, chemical degradation of cyanide presents exactly contrary drawbacks and therefore it would be wise to commercialize cyanide biodegradation. Since HCN is naturally biodegradable simple compound, effective strategies for biodegradation of industrial effluents containing cyanide as pollutant can be formulated and scaled up for the implementation.
Detrimental but important:
Cyanogenic activity of bacteria living in rhizosphere of plants has been considered as one of the phytoinhibitory, growth suppressing mechanisms. Their phytoinhibitory properties include loss of seed germination, under developed root system, loss of seedling vigor and growth. Hence cyanogenesis is regarded as harmful and cyanogenic bacteria are (depending upon plant response) recognized as deleterious rhizobacteria (DRB). Association of DRB can be host specific or non-specific. Covalent binding of HCN to enzymes of photosynthetic electron transport, nitrate assimilation and carbohydrate metabolism is the principle cause of phytoinhibition. Details of biochemical mechanisms involved are still under-investigated. Cyanogenic DRB also interfere with root colonization and establishment of beneficial plant growth promoting rhizobacteria (PGPR). There are relentless interactions of DRB and PGPR for niche and nutrients in plant rhizosphere. Although, bacterial cyanogenesis is viewed as disadvantageous; it has been suggested that cyanogenic DRB could be offered potential application for biocontrol of weeds, soil phytopathogenic fungi and nematodes. They can be eco-friendly and superlative replacement for chemical weedicides, fungicides and nematicides.
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