STRUCTURAL AND FUNCTIONAL CONSEQUENCES OF A GLU L212-]LYS MUTATION INTHE Q(B) BINDING-SITE OF THE PHOTOSYNTHETIC REACTION-CENTER OF RHODOPSEUDOMONAS-VIRIDIS
I. Albert et al., STRUCTURAL AND FUNCTIONAL CONSEQUENCES OF A GLU L212-]LYS MUTATION INTHE Q(B) BINDING-SITE OF THE PHOTOSYNTHETIC REACTION-CENTER OF RHODOPSEUDOMONAS-VIRIDIS, Biochemistry, 33(37), 1994, pp. 11355-11363
The properties of the quinone acceptor complex in the photosynthetic r
eaction center of the atrazine-resistant Rhodopseudomonas viridis muta
nt A2 (Glu L212 --> Lys) were studied by EPR spectroscopy and by photo
electric measurements. The EPR signal attributed to the semiquinone-ir
on (Q(B)(-)Fe(2+)) was significantly different from wild type and rese
mbled that found in PS II. Essentially normal oscillations of Q(B)(-)F
e(2+) were observed upon flash illumination. The kinetics of the first
and the second electron transfer from Q(A) to Q(B) were characterized
by a photoelectric double-flash method. Compared to wild type, the ra
te of the first electron transfer in the large majority of reaction ce
nters was decreased drastically from k(1) = (18 mu s)(-1) in the wild
type to (70 ms)(-1) in the mutant, whereas the second electron transfe
r was only slightly slowed down with a rate of k(2) = (260 mu s)(-1) c
ompared to (65 mu s)(-1) in wild type (pH 7). When the pH was raised a
bove 10, in a major fraction of the reaction centers a fast kinetics o
f the first electron transfer, like that in wild type, reappeared. The
experimental results are interpreted as an effect of the positive cha
rge on the lysine causing a significant structural change of the QB bi
nding pocket and a strongly diminished affinity for ubiquinone. The sl
ow Q(A)(-) --> Q(B) electron transfer kinetics are thus attributed to
ubiquinone binding, which is rate limiting. The possible role of the r
esidue Glu L212, which is conserved in all purple bacteria, in electro
n and proton transfer to Q(B) is discussed.