Ph. Mcpherson et al., LIGHT-INDUCED PROTON UPTAKE BY PHOTOSYNTHETIC REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES R-26.1 .2. PROTONATION OF THE STATE DQ(A)Q(B)(2-), Biochimica et biophysica acta, 1144(3), 1993, pp. 309-324
Proton uptake associated with the two-electron reduction of Q(B) was i
nvestigated in reaction centers (RCs) from Rhodobacter sphaeroides R-2
6.1 using pH-sensitive dyes. An uptake of two protons was observed at
pH less-than-or-equal-to 7.5, consistent with the formation of the dih
ydroquinone Q(B)H-2. At higher pH, the proton uptake decreased with an
apparent pK(a) of approx. 8.5, i.e., to 1.5 H+/2 e- at pH 8.5. A mole
cular model is presented in which the apparent pK(a) is due to the pro
tonation of either the carbonyl oxygen on Q(B) or of an amino acid res
idue near Q(B) (e.g., His-L190). Experimental evidence in favor of the
protonation of the oxygen is discussed. The kinetics of the electron
transfer from Q(A)-Q(B)- to Q(A)Q(B)2- and the associated proton uptak
e were compared at several pH values and temperatures. At pH 8.5 (21.5
-degrees-C) the rate constants for the proton uptake and electron tran
sfer are the same within the precision of the measurement. At lower pH
, the proton uptake rate constant is smaller than that for electron tr
ansfer. The difference between the rate constants is temperature depen
dent, i.e., it varies from 12 +/- 4% at 21.5-degrees-C (pH 7.5) to 28
+/- 4% at 4.0-degrees-C (pH 7.5). We show that the kinetics can be exp
lained by a previously proposed model (Paddock, M.L., McPherson, P.H.,
Feher, G. and Okamura, M.Y. (1990) Proc. Natl. Acad. Sci. USA 87, 680
3-6807) in which the uptake of two protons by doubly reduced Q(B) occu
rs sequentially, one concomitant with and the other after electron tra
nsfer.