Dynamics in Photosystem II Structure and Function
9
for reaction center chlorophyll binding (his198) and the four histidines for
Fe binding in both the D1 (his215, his272) and D2 (his215, his 270) subunits
- a detailed model of the three-dimensional structure of photo system II was
proposed (Trebst 1986) (Fig. 1.2). This was particularly successful for the
QB-binding niche in PS II.
Instrumental in the support of the model were studies with herbicidetolerant mutants of higher plants and algae. Inhibitors, some of them herbicides, were known to displace the plastoquinone from the QB site and
occupy the place on the D1 protein instead (for review on herbicides in PS
II see Draber et a!. 1991). The molecular biology of herbicide tolerance
identified those amino acids in the D1 protein - by sequencing the mutated
psbA gene that encodes it - that participate in herbicide binding (Rochaix
and Erickson 1988). Their location in the amino acid sequence could be well
rationalized by the proposed folding model and in this way described the QB
binding niche in great detail (Fig. 1.3). The folding of the DlID2 protein
model proved to be sufficiently accurate to allow a prediction of the
tyrosine(s) implicated by the ESR signal of the radical to be the electron
donor Y z for P 680+. Sitedirected mutagenesis in Cyanobacteria indeed
identified tyrosin 161 in the D1 protein as the primary electron donor in the
active arm and tyrosin 161 in the D2 protein as the one in the inactive arm
which is = Y D of electron flow through photosystem II (Debus et a!. 1988;
Verma as et a!. 1988; Fig. 1.1).
Further herbicide-tolerant mutants of higher plants, green algae, and
cyanobacteria, isolated after site-selected screening or site-directed mutagenesis, led to the identification of many amino acid substitutions in the
QB binding niche (Fig. 1.3). Photo affinity labeling with radioactive azido
derivatives of herbicides followed by protease digestion and protein sequencing indicated further amino acids in herbicide binding. Antibodies
with site-directed epitops and the trypsin accessibility of arg238 and of the
cleavage site in the "rapid turnover' of the Dl protein and photoinhibition
( see below) are further methodical approaches and provided further insights
into the three-dimensional folding of the D1 protein.
Accordingly, these amino acids of the D 1 protein are of significance in
the QB and herbicide binding: phe211 , met214, his215, leu218, va1219,
tyr237, arg238, ile248, va1249, tyr254, phe255, gly256, ser264, asn266,
leu271, his272, leu275.
His215 and his272 are ligands to Fe binding and his215 and ser264 form
hydrogen bridges in quinone binding, deduced from the homology to the
purple bacteria system and its X-ray structure (Deisenhofer et a!. 1985;
Deisenhofer and Michel 1989). Asn247, his252 and ser264 appear to participate in the proton channel in the protonization of Q B when it becomes
reduced to quino!. They are at homologous positions to those amino acids in
the proton channel in the bacterial QB reduction.
Herbicide-tolerant mutants with a specific point mutation in the amino
acid sequence of the D1 protein can be further analyzed. For example, the
9
for reaction center chlorophyll binding (his198) and the four histidines for
Fe binding in both the D1 (his215, his272) and D2 (his215, his 270) subunits
- a detailed model of the three-dimensional structure of photo system II was
proposed (Trebst 1986) (Fig. 1.2). This was particularly successful for the
QB-binding niche in PS II.
Instrumental in the support of the model were studies with herbicidetolerant mutants of higher plants and algae. Inhibitors, some of them herbicides, were known to displace the plastoquinone from the QB site and
occupy the place on the D1 protein instead (for review on herbicides in PS
II see Draber et a!. 1991). The molecular biology of herbicide tolerance
identified those amino acids in the D1 protein - by sequencing the mutated
psbA gene that encodes it - that participate in herbicide binding (Rochaix
and Erickson 1988). Their location in the amino acid sequence could be well
rationalized by the proposed folding model and in this way described the QB
binding niche in great detail (Fig. 1.3). The folding of the DlID2 protein
model proved to be sufficiently accurate to allow a prediction of the
tyrosine(s) implicated by the ESR signal of the radical to be the electron
donor Y z for P 680+. Sitedirected mutagenesis in Cyanobacteria indeed
identified tyrosin 161 in the D1 protein as the primary electron donor in the
active arm and tyrosin 161 in the D2 protein as the one in the inactive arm
which is = Y D of electron flow through photosystem II (Debus et a!. 1988;
Verma as et a!. 1988; Fig. 1.1).
Further herbicide-tolerant mutants of higher plants, green algae, and
cyanobacteria, isolated after site-selected screening or site-directed mutagenesis, led to the identification of many amino acid substitutions in the
QB binding niche (Fig. 1.3). Photo affinity labeling with radioactive azido
derivatives of herbicides followed by protease digestion and protein sequencing indicated further amino acids in herbicide binding. Antibodies
with site-directed epitops and the trypsin accessibility of arg238 and of the
cleavage site in the "rapid turnover' of the Dl protein and photoinhibition
( see below) are further methodical approaches and provided further insights
into the three-dimensional folding of the D1 protein.
Accordingly, these amino acids of the D 1 protein are of significance in
the QB and herbicide binding: phe211 , met214, his215, leu218, va1219,
tyr237, arg238, ile248, va1249, tyr254, phe255, gly256, ser264, asn266,
leu271, his272, leu275.
His215 and his272 are ligands to Fe binding and his215 and ser264 form
hydrogen bridges in quinone binding, deduced from the homology to the
purple bacteria system and its X-ray structure (Deisenhofer et a!. 1985;
Deisenhofer and Michel 1989). Asn247, his252 and ser264 appear to participate in the proton channel in the protonization of Q B when it becomes
reduced to quino!. They are at homologous positions to those amino acids in
the proton channel in the bacterial QB reduction.
Herbicide-tolerant mutants with a specific point mutation in the amino
acid sequence of the D1 protein can be further analyzed. For example, the