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The mechanism proposed by Ciurli and Manganicite-ref-pmid21542631-20-0[20] is one of the more recent and currently accepted views of the mechanism of urease and is based primarily on the different roles of the two nickel ions in the active site.cite-ref-benini-s-1999-15-2[15] One of which binds and activates urea, the other nickel ion binds and activates the nucleophilic water molecule.cite-ref-benini-s-1999-15-3[15] With regards to this proposal, urea enters the active site cavity when the mobile ‘flap’ (which allows for the entrance of urea into the active site) is open. Stability of the binding of urea to the active site is achieved via a hydrogen-bonding network, orienting the substrate into the catalytic cavity.cite-ref-benini-s-1999-15-4[15] Urea binds to the five-coordinated nickel (Ni1) with the carbonyl oxygen atom. It approaches the six-coordinated nickel (Ni2) with one of its amino groups and subsequently bridges the two nickel centers.cite-ref-benini-s-1999-15-5[15] The binding of the urea carbonyl oxygen atom to Ni1 is stabilized through the protonation state of Hisα222 Nԑ. Additionally, the conformational change from the open to closed state of the mobile flap generates a rearrangement of Alaα222 carbonyl group in such a way that its oxygen atom points to Ni2.cite-ref-benini-s-1999-15-6[15] The Alaα170 and Alaα366 are now oriented in a way that their carbonyl groups act as hydrogen-bond acceptors towards NH2 group of urea, thus aiding its binding to Ni2.cite-ref-benini-s-1999-15-7[15] Urea is a very poor chelating ligand due to low Lewis base character of its NH2 groups. However the carbonyl oxygens of Alaα170 and Alaα366 enhance the basicity of the NH2 groups and allow for binding to Ni2.cite-ref-benini-s-1999-15-8[15] Therefore, in this proposed mechanism, the positioning of urea in the active site is induced by the structural features of the active site residues which are positioned to act as hydrogen-bond donors in the vicinity of Ni1 and as acceptors in the vicinity of Ni2.cite-ref-benini-s-1999-15-9[15] The main structural difference between the Ciurli/Mangani mechanism and the other two is that it incorporates a nitrogen, oxygen bridging urea that is attacked by a bridging hydroxide.cite-ref-zimmer-18-1[18]

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