Longitudinal (T1 -1) and transverse (T2 -1) nuclear magnetic resonance (NMR) relaxation rates of 1H and 17O of water molecules were measured in solutions of oxidized Rhus vernicifera laccase in order to investigate the structural and dynamic aspects of water coordination to its type 2 Cu(II) site. The magnitude and negative temperature dependence of the paramagnetically induced proton T1 -1 and T2 -1 showed that the proton fast-exchange mechanism is operative. The field dependence of T1, pmg -1 gave an apparent correlation time of 2.7 ns for the 9-60-MHz range. T1, pmg -1 is pH dependent, giving two dissociation steps with pK = 6.2 and 8.6 which are attributed to the hydrolysis of a H2O molecule and a protein hydroxyl group coordinated to the type 2 Cu(II), respectively. Relaxation rates of 17O were not paramagnetically enhanced. Neither T1 -1 nor T2-1 values of both 1H and 17O were significantly affected by the binding of fluoride or azide ions to type 2 Cu(II). Our data lead to the following interpretation: The water oxygen equatorially bound to type 2 copper does not exchange on the NMR time scale. The paramagnetic relaxation via the fast exchange mechanism originates from proton transfer between the Cu(II)-bound water molecule or hydroxyl group and the buffered solution, acid-base catalyzed by a protein residue. The relative inertness of the water oxygen is consistent with the structural concept of a cavity which accommodates the laccase type 2 (and type 3) copper sites, the active centers involved in O2 reduction, and is connected to the bulk solvent via an orifice which in the native oxidized enzyme is only penetrable by protons. A tentative model of the type 2 Cu coordination structure is presented.
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