Urease was the first enzyme to be crystallized and is the first enzyme containin
ID: 268147 • Letter: U
Question
Urease was the first enzyme to be crystallized and is the first enzyme containing nickel(II) ions to be characterized. A. Below is the active site of the enzyme with a bound urea molecule. How many ligands does each Ni(II) ion have, what are they, and what are the roles of the two histidine and one carbonyl residue from elsewhere in the protein? NH(Lys) N(His) (His) NH2 (Asp) HN proteopedia.org / mages/0/06/Schem B. Below is a proposed reaction mechanism for the first step in breaking down urea. Explain the role of each entering and leaving group. For example 3 H2O come in on left from bottom to top. Why? Nw2m ??? 2 H2O 136 NO34 NH2 NH2 NH2 http://www.cup.uni de/ac/kluefers/homepage L e/L/BAC/cyclus ur.gif -muenchen C. What is the net reaction for this process? D. What are the final breakdown products for urea? E. Why is nickel a good metal for this process?Explanation / Answer
Answer A:
Two nickel (II) centers of enzyme Urease each coordinated by two nitrogens from histidines, one water molecule, and a bridging carbamylated lysine through the O atom. And the second Ni is further coordinated by O-atom from aspartic acid. So, one of the nickels is penta-coordinated whereas the other is hexa-coordinated.
Two His residues participate in substrate (urea) binding and its catalysis. His residues provide support for the binding of the urea carbonyl oxygen to Ni1.
Answer B: The hydroxyl group bound to Ni-2 attacks urea and polarized carbonyl group by coordination to Ni1and releases ammonia with His which acting as a general acid. The bridging hydroxyl group attacks urea and the hydroxyl proton transfers to the released ammonia. In elimination it forms a carbamate which spontaneously decomposes.
Answer c: Net reaction for the process is:
CO(NH2)2 + H2O urease 2NH3 + CO2
Answer D: Final breakdown products of Urea are ammonia and Carbon dioxide.
Answer E: Nickel is good transition metal for this process because Ni2+ ions are characterized by higher affinity toward nitrogenous-based ligands thus it stabilizes the binding of a urea. The Ni2+ ions possessing multiple available binding sites due to their preference for octahedral coordination sphere
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