Nitrile hydratase (NHase), which catalyzes the hydration of nitriles to amides, has been used in the industrial production of acrylamide and nicotinamide. Recent studies on NHases, which are roughly classified into iron and cobalt types according to the metal involved, have clarified the photoactivation mechanism, the novel ligand structure of the metal-binding sites, the unique mechanism of the enzyme hyper-induction, and the occurrence of an accessory gene involved in cobalt-containing NHase formation. These detailed analyses have led to the development of biotechnological applications of NHase, including biotransformation and bioremediation.

In the form of vitamin B 12 , cobalt plays a number of crucial roles in many biological functions. However, recent studies have provided information on the biochemistry and bioinorganic chemistry of several proteins containing cobalt in a form other than that in the corrin ring of vitamin B 12 . To date, eight noncorrin-cobalt-containing enzymes (methionine aminopeptidase, prolidase, nitrile hydratase, glucose isomerase, methylmalonyl-CoA carboxytransferase, aldehyde decarbonylase, lysine-2,3-aminomutase, and bromoperoxidase) have been isolated and characterized. A cobalt transporter is involved in the metallocenter biosynthesis of the host cobalt-containing enzyme, nitrile hydratase. Understanding the differences between cobalt and nickel transporters might lead to drug development for gastritis and peptic ulceration.Keywords: cobalt; noncorrin; transporter; metal; inorganic chemistry; methionine aminopeptidase; prolidase; nitrile hydratase; glucose isomerase.Cobalt is a transition metal that occupies a position in the periodic table between iron and nickel. It has two naturally occurring oxidation states (Co 2+ and Co 3+ ), but can exhibit oxidation states from ±I to +IV. This metal is rare in comparison with all known essential trace elements except cadmium and tungsten: 0.0025% (w/w) in the Earth's crust and 4 £ 10 ±8 % (w/v) in sea water. Although cobalt is less frequently encountered in metalloenzymes than the other first-row transition metals (e.g. manganese, iron, copper and zinc), it is nevertheless an important cofactor in vitamin-B 12 -dependent enzymes. Vitamin B 12 [1] contains cobalt in a substituted corrin macrocycle (a porphyrin relative). The B 12 coenzyme possesses an axial Co(III)-alkyl (5 H -deoxyadenosine or methyl) group. Much effort has been concentrated on biochemical studies of this corrin cobalt. In contrast, only a few proteins containing noncorrin cobalt have been characterized.Radioactive cobalt ( 60 Co), which is produced by thermal neutron bombardment of the natural isotope 59 Co, has a half-life of 5.3 years and decays by b-and g-emission to nonradioactive nickel ( 60 Ni). It is used as a concentrated source of g radiation in cancer therapy and food sterilization, and as a radioactive tracer in biological and industrial applications [2]. Because cobalt has characteristic spectra correlated with structural properties (such as observed co-ordination numbers for Co 2+ and Co 3+ ), it has also served as a spectroscopic probe in metalloenzymes. Substituting cobalt for zinc has often been a useful tool to investigate the structural basis of catalytic properties in zinc enzymes and the co-ordination environment of active zinc sites in other proteins [3].Non-corrin cobalt is receiving increased interest not only in bioinorganic chemistry but also in biotechnology, and its availability and remarkable chemical versatility makes cobalt an invaluable catalyst in the chemical industry (e.g. hydroformylation as seen in the oxo process [4]). To date, eight noncorrin-cobalt-containing enzymes hav...