Deuterium Kinetic Isotope Effects in the p-Side Pathway for Quinol Oxidation by the Cytochrome b 6  f Complex

Plastoquinol oxidation and proton transfer by the cytochrome b 6 f complex on the lumen side of the chloroplast thylakoid membrane are mediated by high and low potential electron transport chains. The rate constant for reduction, k bred, of cytochrome b 6 in the low potential chain at ambient pH 7.5−8 was twice that, k fred, of cytochrome f in the high potential chain, as previously reported. k bred and k fred have a similar pH dependence in the presence of nigericin/nonactin, decreasing by factors of 2.5 and 4, respectively, from pH 8 to an ambient pH = 6, close to the lumen pH under conditions of steady-state photosynthesis. A substantial kinetic isotope effect, k H 2 O/k D 2 O, was found over the pH range 6−8 for the reduction of cytochromes b 6 and f, and for the electrochromic band shift associated with charge transfer across the b 6 f complex, showing that isotope exchange affects the pK values linked to rate-limiting steps of proton transfer. The kinetic isotope effect, / ≈ 3, for reduction of cytochrome b in the low potential chain was approximately constant from pH 6−8. However, the isotope effect for reduction of cytochrome f in the high potential chain undergoes a pH-dependent transition below pH 6.5 and increased 2-fold in the physiological region of the lumen pH, pH 5.7−6.3, where / ≈ 4. It is proposed that a rate-limiting step for proton transfer in the high potential chain resides in the conserved, buried, and extended water chain of cytochrome f, which provides the exit port for transfer of the second proton derived from p-side quinol oxidation and a “dielectric well” for charge balance.