like azide. The final native enzyme complex is a complex of 5 dimers. The crystallization of cyanase shows that cyanase crystals are triclinic with one homodecamer in the asymmetric unit. The active site of cyanase accommodates two anions, bicarbonate and cyanate. Three catalytic residues, Arg96, Glu99, and Ser122, form this pocket that stabilizes the anions with ionic interactions. The positively charged and polar amino acid residues allow the active site to bind anions. The active site sits between dimers, noted by the Arg96 in each monomer to be in close proximity. The structure of cyanase has been shown to be similar in its prokaryotic and eukaryotic forms. Enzyme Activity Cyanase is regulated both transcriptionally and in its enzymatic activity. Transcription of cyanase is elevated with the presence of extracellular cyanate, a toxic ion, but also down regulated by the presence of excess arginine, the catalytic amino acid. Cyanase requires the product of the reaction catalyzed by cyanate permease. Activity of cyanase is also dependent on the concentration of its substrates, cyanate and bicarbonate. Because bicarbonate is itself a recycling substrate and the kinetics of cyanase include a rapid and random equilibrium, bicarbonate can also act as an enzyme inhibitor. At low concentrations, bicarbonate shows uncompetitive inhibition, where it binds to the one of enzyme's anionic binding sites, and inhibit cyanate binding, or bicarbonate can bind once more so that the enzyme complex is bound to two bicarbonates in its double anion active site. At higher concentrations, the trend moves toward non-competitive inhibition, where the incorrect, dead-end complex must decompose to the initial separated units before the binding action can begin again. This random equilibrium of cyanate and bicarbonate complexing with the enzyme causing substrate inhibition is referred to as ping pong inhibition. Functional Relevance Cyanase is an important enzyme for plants and microbacteria as cyanate is a toxic substance that is prevalent in many environments. Further cyanase activity has been found to be encoded in cynH, a gene only found in marine cyanobacteria. There is a dodecapeptide, a partial region near the N-terminus of cyanate, that as a novel cationic α-helical antimicrobial peptide, known by CL 14-25. Cyanase is also important as an energy metabolizer to fix Nitrogen in nitrifiers. Cyanate is an important source of reduced nitrogen in aquatic and terrestrial ecosystems, and although it is a toxic ion, it can be converted to ammonium and carbon dioxide by