Kinetic Determination of the Fast Exchanging Substrate Water Molecule in the S3 State of Photosystem II,

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Abstract
In a previous communication we showed from rapid isotopic exchange measurements that the exchangeability of the substrate water at the water oxidation catalytic site in the S3 state undergoes biphasic kinetics although the fast phase could not be fully resolved at that time [Messinger, J., Badger, M., and Wydrzynski, T. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 3209−3213]. We have since improved the time resolution for these measurements by a further factor of 3 and report here the first detailed kinetics for the fast phase of exchange. First-order exchange kinetics were determined from mass spectrometric measurements of photogenerated O2 as a function of time after injection of H218O into spinach thylakoid samples preset in the S3 state at 10 °C. For measurements made at m/e = 34 (i.e., for the mixed labeled 16,18O2 product), the two kinetic components are observed: a slow component with k1 = 2.2 ± 0.1 s-1 (t1/2 ∼ 315 ms) and a fast component with k2 = 38 ± 4 s-1 (t1/2 ∼ 18 ms). When the isotopic exchange is measured at m/e = 36 (i.e., for the double labeled 18,18O2 product), only the slow component (k1) is observed, clearly indicating that the substrate water undergoing slow isotopic exchange provides the rate-limiting step in the formation of the double labeled 18,18O2 product. When the isotopic exchange is measured as a function of temperature, the two kinetic components reveal different temperature dependencies in which k1 increases by a factor of 10 over the range 0−20 °C while k2 increases by only a factor of 3. Assuming simple Arrhenius behavior, the activation energies are estimated to be 78 ± 10 kJ mol-1 for the slow component and 39 ± 5 kJ mol-1 for the fast component. The different kinetic components in the 18O isotopic exchange provide firm evidence that the two substrate water molecules undergo separate exchange processes at two different chemical sites in the S3 state, prior to the O2 release step (t1/2 ∼ 1 ms at 20 °C). The results are discussed in terms of how the substrate water may be bound at two separate metal sites.