Cyanide metabolism of Pseudomonas pseudoalcaligenes CECT5344: role of siderophores

Cyanide is one of the most potent and toxic chemicals produced by industry. The jewellery industry of Córdoba (Spain) generates a wastewater (residue) that contains free cyanide, as well as large amounts of cyano–metal complexes. Cyanide is highly toxic to living systems because it forms very stable complexes with transition metals that are essential for protein function. In spite of its extreme toxicity, some organisms have acquired mechanisms to avoid cyanide poisoning. The biological assimilation of cyanide needs the concurrence of three separate processes: (i) a cyanide-insensitive respiratory chain, (ii) a system for iron acquisition (siderophores) and (iii) a cyanide assimilation pathway. Siderophores are low-molecular-mass compounds (600–1500 Da) that scavenge iron (Fe³⁺) ions (usually with extremely high affinity) from the environment under iron-limiting conditions. There are two main classes of siderophores: catechol and hydroxamate types. The catechol-type siderophores chelate ferric ion via a hydroxy group, whereas the hydroxamate-type siderophores bind iron via a carbonyl group with the adjacent nitrogen. In the presence of cyanide, bacterial proliferation requires this specific metal uptake system because siderophores are able to break down cyano–metal complexes. Pseudomonas pseudoalcaligenes CECT5344 is able to use free cyanide or cyano–metal complexes as nitrogen source. A proteomic approach was used for the isolation and identification, in this strain, of a protein that was induced in the presence of cyanide, namely CN0, that is involved in siderophore biosynthesis in response to cyanide. An overview of bacterial cyanide degradation pathways and the involvement of siderophores in this process are presented.