Carbamic acid bridging coordination

So far there is no known specific biochemical function for nickel in animal organisms. In plants and microorganisms, some metal-loenzymes containing nickel have been found. Such an enzyme is urease, found in soybeans, other legumes (this enzyme was first isolated from the seeds of the Jack bean, Canavalia ensiformis), rice and tobacco. Molecule of urease (580 kDa) contains 12 nickel atoms in six subunits. In the active enzyme centre, two nickel atoms are coordinated by four histidyl residues and one residue of e-N-carbamoyl lysine, while the carbamoyl group and one molecule of water form a bridge between the two nickel atoms. Urease catalyses the hydrolysis of urea to ammonia and carbamate (carbamic acid), which is hydrolysed spontaneously to hydrogen carbonate (bicarbonate) and ammonium ions ... 

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