6
A. Trebst
is a cytochrome C2 instead of the manganese cluster of the water splitting
system. The basic functional structural unit for the reaction center in the
bacterial photo system catalyzes the oxidation of P870 and reduction of Q B
via QA and pheophytin. It consists of three hydrophobic polypeptides called
L, M, and H (for their relative mobility in the SDS gel electrophoresis). Of
these, the Land M subunits bind the reaction center bacteriochlorophyll
dimer, two monomeric bacterio chlorophylls, two bacterio-pheophytins and
the histidine-bound Fe. QA is bound to the M and QB to the L subunit. In
this composition of redox components, the Dl and D2 protein of photo system
II are very homologous (for review see Barber 1992; Table 1.1).
Low molecular weight polypeptides, attached to the photosystem carry
the antenna bacteriochlorophylls instead of the higher molecular weight
chlorophyll-binding proteins of photosystem II.
The amino acid sequence of all subunits from plant and purple bacteria
systems are known and the genetic origin has been established.
1.3 Structure of Photosystem II
The exact topology of the integral proteins of photosystem It as listed in
Table 1.1 in the membrane is known only in principle from an analysis of
their amino acid sequences. A detailed description of the three-dimensional
folding of their amino acid sequences and the orientation and function of
amino acid residues in the redox reaction is, however, already possible
for the two reaction center polypeptides - the Dl and D2 protein. This
knowledge is due to the homology of the Dl and D2 proteins to the two
reaction center polypeptides Land M of the purple bacteria.
The photosystem of the purple bacteria Rps. viridis and later Rps.
sphaeroidis has been crystalized (Deisenhofer et al. 1985; Allen et al. 1987).
An X-ray structure of about 2.3A resolution was possible for the Rps.
viridis system (Deisenhofer and Michel 1989). This X-ray structure shows
how the three hydrophobic proteins L, M, and H span the membrane
several times in (11) transmembrane helices and how the redox systems are
attached to them. Accordingly, the principal architecture of an integral
membrane protein consists of a cluster of hydrophobic amino acids in the
sequence of the protein. There are five such clusters of about 20 to 25
hydrophobic amino acids in both the Land M subunit. They form a highly
hydrophobic a-helix that has about the length of the hydrophobic part of the
lipid bilayer of a membrane of about 35 to 40 A. There are five helices
in each of the Land M subunits, long enough to span the membrane.
Hydrophilic segments in the amino acid sequence connect the transmembrane helices on either side of the membrane. If short, the hydrophilic
connections will remain in the equivalent phase of the head groups of the
A. Trebst
is a cytochrome C2 instead of the manganese cluster of the water splitting
system. The basic functional structural unit for the reaction center in the
bacterial photo system catalyzes the oxidation of P870 and reduction of Q B
via QA and pheophytin. It consists of three hydrophobic polypeptides called
L, M, and H (for their relative mobility in the SDS gel electrophoresis). Of
these, the Land M subunits bind the reaction center bacteriochlorophyll
dimer, two monomeric bacterio chlorophylls, two bacterio-pheophytins and
the histidine-bound Fe. QA is bound to the M and QB to the L subunit. In
this composition of redox components, the Dl and D2 protein of photo system
II are very homologous (for review see Barber 1992; Table 1.1).
Low molecular weight polypeptides, attached to the photosystem carry
the antenna bacteriochlorophylls instead of the higher molecular weight
chlorophyll-binding proteins of photosystem II.
The amino acid sequence of all subunits from plant and purple bacteria
systems are known and the genetic origin has been established.
1.3 Structure of Photosystem II
The exact topology of the integral proteins of photosystem It as listed in
Table 1.1 in the membrane is known only in principle from an analysis of
their amino acid sequences. A detailed description of the three-dimensional
folding of their amino acid sequences and the orientation and function of
amino acid residues in the redox reaction is, however, already possible
for the two reaction center polypeptides - the Dl and D2 protein. This
knowledge is due to the homology of the Dl and D2 proteins to the two
reaction center polypeptides Land M of the purple bacteria.
The photosystem of the purple bacteria Rps. viridis and later Rps.
sphaeroidis has been crystalized (Deisenhofer et al. 1985; Allen et al. 1987).
An X-ray structure of about 2.3A resolution was possible for the Rps.
viridis system (Deisenhofer and Michel 1989). This X-ray structure shows
how the three hydrophobic proteins L, M, and H span the membrane
several times in (11) transmembrane helices and how the redox systems are
attached to them. Accordingly, the principal architecture of an integral
membrane protein consists of a cluster of hydrophobic amino acids in the
sequence of the protein. There are five such clusters of about 20 to 25
hydrophobic amino acids in both the Land M subunit. They form a highly
hydrophobic a-helix that has about the length of the hydrophobic part of the
lipid bilayer of a membrane of about 35 to 40 A. There are five helices
in each of the Land M subunits, long enough to span the membrane.
Hydrophilic segments in the amino acid sequence connect the transmembrane helices on either side of the membrane. If short, the hydrophilic
connections will remain in the equivalent phase of the head groups of the