Histidine residues urease

The initially proposed mechanism of urease activity (Scheme 1) assumed that urea is first polarized and activated by monodentate O-coordination to one Ni(II) ion, in conjunction with extensive hydrogen bonding within the active site pocket of the protein. The retained water on Ni2 becomes deprotonated, while a nearby histidine residue becomes protonated. The resulting Ni2-bound terminal hydroxide then acts as the nucleophile and attacks the carbonyl C of urea. The reaction proceeds through a tetrahedral intermediate, from which NH3 is released, assisted... 

Nickel is required for the synthesis of active urease in plant and other cells. The enzyme catalyzes the hydrolysis of urea to carbon dioxide and ammonia, via the intermediate formation of carbamate ion (equation 46). The molecular weight has been redetermined recently as 590 000 30 000, with six subunits. Each subunit has two nickel centres and binds one mole of substrate. The activity of the enzyme is directly proportional to the nickel content, suggesting an essential role for nickel in the enzyme. Several approaches, including EXAFS measurements, suggest that histidine residues provide some ligands to nickel, and that the geometry is distorted octahedral. There appears to be a role for a unique cysteine residue in each subunit out of the 15 groups present. Covalent modification of this residue blocks the activity of the enzyme. 

Ligand and complex design in biomimetic systems is diverse but a few general concepts are normally followed (i) the metal ions employed are often the same as in the native systems e.g. Ni(II) in urease models [68] and (ii) pyridine or pyrazole residues are often used to mimic the histidine residues in the enzymes phenol, carboxylate, pyrazolate or water molecules serve as mimics for bridging residues like aspartate, lysine or water/hydroxide and (iii) dinucleating ligands are used to bring the two metal ions into close proximity. 

A clearer picture emerged when the crystal structure of K. pneumoniae urease was determined [27], The nickel atoms in the center, Ni-1 and Ni-2, are 3.5 A apart. They are bridged by a carbamyl group, formed from C02 and a lysine residue, explaining the requirement for hydrogen carbonate in reconstitution. The other ligands are two histidines for Ni-1 and an aspartate and two histidines for Ni-2. 

A for the two histidines. The other two Cu" ions are found in surface exposed sites coordinated by two histidine e nitrogens and one or two water molecules. The putative cysteine ligand identified by spectroscopy and mutagenesis " does not bind Cu" in the structure and is quite distant from the metal-binding sites. It may be that this interaction occurs in solution between a cysteine residue from one dimer and a Cu" ion from a second dimer, and is precluded in the structure by crystal packing. The dimer interface site is proposed to deliver Ni" ions one at a time to the urease active site, and the other two sites are proposed to play a more secondary role, serving as reservoirs for Ni". ... 

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