Dynamics in Photo system II Structure and Function
5
on the D1 protein with tryptophan 254 of the D2 protein channeling the
electrons.
Two excitation cycles and two protons (from the matrix space) are needed
for the two electron reduction of Qs to the hydroquinone. A total of four
excitation cycles are needed to oxidize two molecules of water to oxygen
and four protons released inside.
4H+ out + 2H20 + 2Qs + 4hu ~ O2 + 4H+in + 2Qs H2'
A histidine-bound Fe with no obligatory redox changes participates in the
reaction from QA to Qs . Four bound Mn atoms cluster in the oxygen
evolution system. The manganese cluster changes valency four times when
oxidized by consecutive excitations in the reaction center. Two water
molecules are then split to one O2, possibly via a bound peroxide. The
reduced manganese cluster reduces the oxidized P 680+ via a bound tyrosine
radical called Y z (tyrosine at position 161 in the sequence of the D1 protein). An equivalent tyrosine in the D2 protein, called Yo) is also oxidized
but much slower. The role of the cytochrome bss9 in the photosystem II
complex remains to be clarified.
The photosystem II complex furthermore contains about 30 core antenna
chlorophylls (only chlorophyll a) that channel excitation energy to the
reaction center chlorophylls. The additional light harvesting system (LHCP)
of about 300 chlorophylls (a and b) and with accessory pigments are functionally and structually attached to the core antenna polypeptides.
The minimum polypeptide composition of a functional oxygen evolving
PS II complex consists of seven hydrophobic integral proteins (of 47 and 43
molecular weight, the D1 and D2 proteins, two polypeptides subunits of
cytochrome bss9 and the product of the psbI gene) (Barber 1992; Satoh
1992) and three hydrophobic peripheral polypeptides. The core composition
of a photosystem II still able to perform the primary charge separation of
P680 is five polypeptides (D1 and D2 protein, the two subunits of cytochrome
bss9 and the product of the psbI gene) (Nanba and Satoh 1987). A preparation of just the D1 and D2 protein has been reported that can still
oxidize P680 (Satoh 1992). This then shows that just the two polypeptides
D1 and D2 carry the reaction center P680, its primary electron acceptor
(pheophytin) and the primary donor (the tyrosin radical Y z), as well as the
two bound plastoquinones QA and Qs and the histidine bound Fe.
The reaction center of photosystem II is homologous in its functional
mechanism and cofactor composition to that of purple bacteria and of the
green nonsulfur bacteria Chlorofiexus (Barber 1992; Blankenship 1992).
These also reduce quinones (usually ubiquinone in both the Q A and Q s site)
with bacteriochlorophyll in the reaction center and bacterio-pheophytin as
the primary acceptor. They also contain histidine-bound Fe. However, the
bacterial systems do not contain a cytochrome b ss9 nor the manganese
cluster. The electron donor in the bacterial system is, of course, different
from that of photosystem II. As bacteria cannot oxidize water to oxygen, it
5
on the D1 protein with tryptophan 254 of the D2 protein channeling the
electrons.
Two excitation cycles and two protons (from the matrix space) are needed
for the two electron reduction of Qs to the hydroquinone. A total of four
excitation cycles are needed to oxidize two molecules of water to oxygen
and four protons released inside.
4H+ out + 2H20 + 2Qs + 4hu ~ O2 + 4H+in + 2Qs H2'
A histidine-bound Fe with no obligatory redox changes participates in the
reaction from QA to Qs . Four bound Mn atoms cluster in the oxygen
evolution system. The manganese cluster changes valency four times when
oxidized by consecutive excitations in the reaction center. Two water
molecules are then split to one O2, possibly via a bound peroxide. The
reduced manganese cluster reduces the oxidized P 680+ via a bound tyrosine
radical called Y z (tyrosine at position 161 in the sequence of the D1 protein). An equivalent tyrosine in the D2 protein, called Yo) is also oxidized
but much slower. The role of the cytochrome bss9 in the photosystem II
complex remains to be clarified.
The photosystem II complex furthermore contains about 30 core antenna
chlorophylls (only chlorophyll a) that channel excitation energy to the
reaction center chlorophylls. The additional light harvesting system (LHCP)
of about 300 chlorophylls (a and b) and with accessory pigments are functionally and structually attached to the core antenna polypeptides.
The minimum polypeptide composition of a functional oxygen evolving
PS II complex consists of seven hydrophobic integral proteins (of 47 and 43
molecular weight, the D1 and D2 proteins, two polypeptides subunits of
cytochrome bss9 and the product of the psbI gene) (Barber 1992; Satoh
1992) and three hydrophobic peripheral polypeptides. The core composition
of a photosystem II still able to perform the primary charge separation of
P680 is five polypeptides (D1 and D2 protein, the two subunits of cytochrome
bss9 and the product of the psbI gene) (Nanba and Satoh 1987). A preparation of just the D1 and D2 protein has been reported that can still
oxidize P680 (Satoh 1992). This then shows that just the two polypeptides
D1 and D2 carry the reaction center P680, its primary electron acceptor
(pheophytin) and the primary donor (the tyrosin radical Y z), as well as the
two bound plastoquinones QA and Qs and the histidine bound Fe.
The reaction center of photosystem II is homologous in its functional
mechanism and cofactor composition to that of purple bacteria and of the
green nonsulfur bacteria Chlorofiexus (Barber 1992; Blankenship 1992).
These also reduce quinones (usually ubiquinone in both the Q A and Q s site)
with bacteriochlorophyll in the reaction center and bacterio-pheophytin as
the primary acceptor. They also contain histidine-bound Fe. However, the
bacterial systems do not contain a cytochrome b ss9 nor the manganese
cluster. The electron donor in the bacterial system is, of course, different
from that of photosystem II. As bacteria cannot oxidize water to oxygen, it