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 The Hausinger/Karplus Proposed Mechanism 

The mechanism proposed by Hausinger and Karplus attempts to revise some of the issues apparant in the Blakely and Zerner pathway, and focuses on the positions of the side chains making up the urea-binding pocket. From the crystal structures from K. aerogenes urease, it was argued that the general base used in the Blakely mechanism, His320, was too far away from the Ni2-bound water to deprotonate in order to form the attacking hydroxide moiety. In addition, the general acidic ligand required to protonate the urea nitrogen was not identified. Hausinger and Karplus suggests a reverse protonation scheme, where a protonated form of the His320 ligand plays the role of the general acid and the Ni2-bound water is already in the deprotonated state. The mechanism follows the same path, with the general base omitted (as there is no more need for it) and His320 donating its proton to form the ammonia molecule, which is then released from the enzyme. While the majority of the His320 ligands and bound water will not be in their active forms (protonated and deprotonated, respectively,) it was calculated that approximately 0.3% of total urease enzyme would be active at any one time. While logically, this would imply that the enzyme is not very efficient, contrary to established knowledge, usage of the reverse protonation scheme provides an advantage in increased reactivity for the active form, balancing out the disadvantage. Placing the His320 ligand as an essential component in the mechanism also takes into account the mobile flap region of the enzyme. As this histidine ligand is part of the mobile flap, binding of the urea substrate for catalysis closes this flap over the active site and with the addition of the hydrogen bonding pattern to urea from other ligands in the pocket, speaks to the selectivity of the urease enzyme for urea.