 |
INTRODUCTION |
The electron transfer processes of oxygenic photosynthesis, as
observed in cyanobacteria, eukaryotic algae, and higher plants, involve
two distinct types of photosynthetic reaction centers located in the
thylakoid membrane. Photosystem II catalyzes the light-driven luminal
oxidation of water and the reduction of plastoquinone near the stromal
side of the photosynthetic membrane. Photosystem I
(PSI)1 luminally oxidizes the
soluble electron donor plastocyanin (alternatively cytochrome
c6) and stromally reduces the extrinsic electron
acceptor ferredoxin or flavodoxin. The reduced ferredoxin induces the
reduction of NADP+, a reaction catalyzed by
ferredoxin:NADP+ reductase. Photosystem I receives
electrons from photosystem II via an intermediate plastoquinone pool,
the cytochrome b6/f complex, and
water soluble electron carriers. The difference in proton concentration
across the thylakoid membrane, which results from the proton pumping of
the plastoquinone pool and the cytochrome b6/f complex, the stromal consumption
of protons by NADP+ reduction, and the luminal release of
protons following water oxidation, is used by the ATP-synthase for
phosphorylation of ADP to ATP (1, 2).
Cyanobacterial PSI consists of 11 subunits referred to as PsaA to PsaF
and PsaI to PsaM. An x-ray structural model of a cyanobacterial PSI
complex from the thermophile Synechococcus elongatus has
been postulated on the basis of an electron density map calculated at
4-Å resolution (3, 4). Despite the comparatively low resolution, it
was possible to suggest an assignment of 43
-helices to the
individual subunits of PSI by correlating the information provided by
the electron density map with available biochemical and biophysical
data. Furthermore, the electron density map allowed the positions of 89 Chl a molecules, constituents of both the core antenna
system and electron transfer system, one phylloquinone, and three
iron-sulfur clusters to be modeled.
The electron transfer reactions of PSI are initiated through excitation
of the primary electron donor P700 positioned near the luminal side of
the membrane-integral complex. Structurally, P700 consists of a
chlorophyll a dimer
(eC1/eC1'), whose mutually parallel
dihydroporphyrin ring planes are aligned with the membrane normal. Upon
excitation, P700* passes an electron to the primary electron acceptor A
(probably eC2 or eC2'; see below). Spectroscopically, the first electron acceptor has been
identified as A0, in all probability one (though possibly either) of the pair of Chl a monomers denoted
eC3 and eC3' in the
structural model of PSI (3, 4). This process occurs with a rate
constant of about 5·1011 s
1 (5). The charge
separation P700
A
0 is spatially extended across the
membrane by electron transfer from the radical A
0 to the next
electron acceptor spectroscopically referred to as A1; the
rate constant is estimated to be 2-5·1010
s
1 (for a review, see Ref. 5). A1 is now
generally agreed to be a phylloquinone (6, 7). Due to the difficulty of
locating the small phylloquinone molecules in low resolution electron
density maps and because of the stability of the radical state
P700
A
1, the position and orientation of
A1 relative to the PSI holocomplex has recently received
increased attention, especially by improved EPR techniques. These have,
inter alia, determined the distance between A
1 and
P700
to be ~25.4 Å (8, 9, 10). A relative position for
A1 was derived through orientation-dependent pulsed EPR measurements on PSI single crystals (10). Geometrically, this
position was found to correspond to QK, a single
phylloquinone assigned to a well defined pocket in the earlier electron
density map (4). The assignment of this position, however, remained internally uncorroborated, since an expected pseudosymmetrically positioned second phylloquinone could not be identified at the time.
The three terminal cofactors of the electron transfer system are
iron-sulfur centers, FX being closest to P700, followed by F1 and F2 (we retain this
nomenclature, for the present, to emphasize the remaining structural
ambiguity in their assignment to the known cofactors FA and
FB, although see Refs. 11-13 as well as Ref. 14 for recent
results correlating FA with F1 and
FB with F2).
In the following, we describe an improved model of the electron
transfer system of PSI based on the present electron density map at
4-Å resolution (14). This map reveals the position of the second
phylloquinone molecule and allows the spatial positioning of all 11 cofactors of the electron transfer system of PSI. The positions and
orientations of individual cofactors are discussed and compared with
structural information derived from spectroscopic data.
 |
EXPERIMENTAL PROCEDURES |
Calculation of an Improved Electron Density Map--
The phases
for the electron density map presented here were derived using
essentially the same data described previously (4), although a new
native data set with a resolution of 3.5 Å and an additional mercury
derivative data set have been included (14). Using the program SHARP
(15) instead of the earlier combination VECREF/MLPHARE (16, 17) and
including a total of five heavy atom derivative data sets, it was
possible, by incorporating new minor sites, to derive a significantly
improved heavy atom model. The program SOLOMON (16) has been employed
in the solvent flattening procedure. Due to the low diffraction quality
of heavy atom derivative crystals, experimentally obtained phase
information is still limited to a resolution of 4 Å. Since no
additional phase information at higher resolution could be achieved by
phase extension using density modification techniques, the electron
density map was calculated at a resolution of 4 Å. It reveals more
detailed information on the polypeptide chain folding than previous
maps as well as the complete cofactor set of PSI. For the detailed
procedure and statistics for the determination of this electron density
map, see Ref. 14.
Model Building--
The previously reported model of the
electron transfer system (3, 4) has been used as a basis for the
present cofactor model. The Chl a head groups are visible as
almost quadratically flat density pockets. The positions and
orientations of the Chl a molecules are modeled by 4-fold
symmetrical porphyrin moieties, since the present resolution does not
permit their asymmetric features to be defined unambiguously.
Similarly, the phylloquinone molecules are represented by their
naphthalene moieties to interpret the corresponding elongated
ellipsoidal electron density. Neither the phylloquinone side chains nor
the oxygen atoms have been included in the model.
Chl a cofactors of the electron transfer system were placed
into the electron density using the program O (18), and their positions
were optimized using the real space refinement procedure as provided by
this program.
Distances between Cofactors and Associated Errors--
For Chl
a molecules center-to-center distances were calculated
between the central Mg2+ ions, while for iron-sulfur
clusters and phylloquinones the centroid of the cluster and naphthalene
model, respectively, have been used. Edge-to-edge distances of
cofactors important for the kinetics of electron transfer were
determined between the outer atoms of the porphyrin, naphthalene, and
iron-sulfur cluster models, respectively. For iron-sulfur clusters,
edge-to-edge distances have been determined between the iron and sulfur
atoms of the clusters, as modeled.
The estimated errors for center-to-center and edge-to-edge distances
are on the order of ±1 and ± 2 Å, respectively, the latter reflecting the larger uncertainties in the orientations of the planar
cofactors within their molecular planes.
 |
RESULTS |
Two Symmetrically Arranged Density Pockets Assigned to the
Positions of the Phylloquinones--
In our previous x-ray structural
model of PSI (4), only a tentative positional description of a single
phylloquinone was included, assigned to an electron density pocket
located between FX and eC3. The lack
of a second, pseudosymmetrically positioned phylloquinone, however,
prevented an internal corroboration of this identification.
The new electron density map now reveals two such electron density
structures symmetrically positioned on either side of the pseudo-2-fold
rotation axis C2(AB) and located between
eC3 and FX. These have
been assigned to the phylloquinone electron acceptors
QK and QK' (Fig.
1). The latter is equivalent to the
position QK identified previously (4). Note that following our earlier convention of priming cofactors coordinated by
primed
-helices, the position previously denoted
QK will be renamed QK'
(coordinated by
-helices m'-n'), while the new second
phylloquinone will be referred to as QK (coordinated by m-n).

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Fig. 1.
Stereo view of the electron density
structures interpreted as QK and QK'. They
are largely separate from the surrounding electron density
corresponding to -helices, inter-helical loops and
eC3, eC3'. This
figure was produced using O (18).
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QK (QK') is situated
slightly luminally of and close to the N terminus of
-helix n (n')
and immediately adjacent to the loop connecting
-helices m and n (m'
and n'). The corresponding electron density is clearly separated from
that of the neighboring
-helices (Fig. 1). Facing away from the loop
m-n (m'-n'), each phylloquinone is additionally delimited by the long
loop n-o (n'-o') connecting the C-terminal end of n (n') to the stromal
end of o (o').
In addition to QK and
QK', a significantly more symmetrical
arrangement of
-helices and connecting loops on either side of the
pseudo-2-fold axis C2(AB) is now apparent in it
vicinity as compared with the previously published electron density
map. Whereas the earlier model of the
-helix m almost passed through
the position now assigned to QK, the stromal end
of m now has a comparable inclination relative to the membrane normal
as its pseudosymmetric partner m'. The loops m-n (m'-n') connecting
-helix m (m') to the "surface"
-helix n (n') are similar in
both shape and length (Fig. 2).

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Fig. 2.
The core of the reaction center of
PSI. a, view direction perpendicular to the
C2(AB)-axis onto median plane of all cofactors of the
electron transfer system. The palisade of -helices surrounding the
electron transfer system and the loops connecting these -helices are
rendered in black and gray. The luminal loop
i-j could not unambiguously be
located in the electron density map. The naphthalene backbone of the
modeled phylloquinone orientation as well as all remaining cofactors of
the electron transfer system are depicted in white.
b, view direction from the stromal side onto the membrane
plane. The pseudo-C2(AB)-axis passes through the center of
the Fe4S4 cluster FX. For clarity,
only the stromal loop region j-k
coordinating the iron sulfur cluster FX is shown. This
figure was produced using Setor (37).
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The electron densities of both QK and
QK' are elongatedly ellipsoidal (Fig. 1). As a
result, the long molecular axis of the naphthoquinone moiety may be
identified with some confidence. However, the plane orientation as well
as the quinone oxygen atoms remain indeterminate. As a result, the
phylloquinone molecules have been modeled by their naphthalene
backbones only. These naphthalene models were placed into the electron
density optimizing their positions and the orientations of their long molecular axes. The molecular plane of QK was
then rotated around the long axis to align the molecular plane with the
vector eC1-QK, to account for the
observation that the carbonyl O-O-axis is
approximately aligned with the vector P700
-A
1 (19). Since the electron spin density is primarily located on either eC1 or eC1' (20)
(although which one remains to be clarified), the procedure was
repeated to align the molecular plane of QK with
the vector eC1'-QK. Similarly, two
plane orientations were obtained for QK'.
The long molecular axes of both QK and
QK' are observed to be inclined by 13 ±5°
relative to the membrane plane (equivalent to the crystallographic
a,b-plane). Projected onto the
a,b-plane, the long molecular axis of
QK (QK') describes an
angle of 18° (60°) to the crystallographic a-axis. The
axes of QK and QK' form
an angle of 42° with each other.
The Iron-Sulfur Clusters--
The positions of the iron-sulfur
clusters correspond to the highest electron density observed (21).
FX was tentatively modeled by fitting a
Fe4S4 cluster into the electron density.
Contouring the electron density map at 11 S.D. above the mean density
reveals a tetrahedrally distorted electron density structure associated with FX (Fig. 3). The most
likely explanation is, that this tetrahedron is equivalent to the
arrangement of the four iron atoms of the Fe4S4
cluster. Modeling a Fe4S4 cluster into this
tetrahedral shape results in a good structural match, while the four
sulfur atoms lie outside the contour, in agreement with their lower
density of electrons. Interestingly, the derived orientation of
FX upholds the 2-fold symmetry of C2(AB), a
fact that had been assumed on grounds of symmetry yet had remained
unsubstantiated. The observation that the gXX
principal axis of the g tensor of reduced FX is
oriented perpendicular to the thylakoid membrane (22) now favors one of
two alternative assignments of g tensor axes to the
distorted cubane structure of Fe4S4 clusters.
According to EPR studies on Fe4S4 model
compounds (23, 24), our structural model and the EPR results are in
agreement with the assignment, where each of the three principal
magnetic axes is normal to one of the mutually orthogonal faces of the
distorted Fe4S4 cube (25).

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Fig. 3.
Stereoscopic depiction of the electron
density of FX contoured at 1.2 and at 11 above mean density. The
tetragonal shape of FX in the 11 contour reveals the
absolute orientation of FX observed to comply with the
pseudosymmetry of C2(AB) (arbitrarily oriented axis
C2(AB) not shown). For F1 and
F2, such tetrahedrally shaped density structures
are not evident. This figure was produced using BobScript
(38).
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The orientations of F1 and
F2 have been inferred from the
2Fe4S4 ferredoxin structure from
Peptostreptococcus asaccharolyticus (26) used as a model for
PsaC (27). They are in agreement with those derived independently by
EPR experiments on PSI single crystals (28). For
F1 and F2, such tetragonally shaped density structures as observed in the case of
FX are not evident as the electron density "outside"
the membrane-integral region is less well defined.
 |
DISCUSSION |
Overall Cofactor Arrangement--
The electron transfer system of
PSI constitutes the innermost cylindrical core of the larger,
membrane-integral photosynthetic reaction center complex. A set of 10
-helices, five from each of the two central subunits, PsaA and PsaB,
tightly encloses the electron transfer system, separating it from the
surrounding antenna system (4). The electron transfer system itself
consists of two symmetrically arranged cofactor branches (Fig. 2). Now
that all cofactors of the electron transfer system have been
identified, this pseudosymmetry is seen to encompass the whole of the
membrane-integral region, extending from the pair
eC1/eC1' near the luminal side to
QK/QK' near the stromal
side (Fig. 2). The iron-sulfur cluster FX is located on the
pseudo-2-fold axis C2(AB), completing the symmetrical
arrangement at the stromal edge of the membrane-integral subunits.
Merely the two stromal iron-sulfur clusters F1
and F2 (FA and FB),
coordinated by the extrinsic subunit PsaC, do not adhere to this 2-fold symmetry.
In the direction parallel to the membrane normal, the membrane-integral
cofactors divide the membrane into four sections of roughly comparable
width, here denoted
eC1-eC2, eC2-eC3,
eC3-QK, and
QK-FX. The height
difference for
eC1-eC2, eC2-eC3,
eC3-QK, and
QK-FX amount to 5.9, 8.6, 7.8, and 8.8 Å, respectively, while the total distance
eC1-FX is 31.1 Å. This corresponds to fractional distances of 0.19, 0.28, 0.25, and 0.28, respectively (Fig. 4a).

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Fig. 4.
Schematic representations of the
electron transfer system. a, cofactor distribution along the
membrane normal showing distances in Å and fractional distances. The
observed center-to-center distances (±1 Å) (b) and the
edge-to-edge distances (±2 Å) (c) are indicated. Except
for the interplane distance between eC1 and
eC1' (3.6 Å) the values of the other
edge-to-edge distances have been determined with an accuracy of 0.5 Å.
d, the individual distances and angles between the cofactors
eC1, eC1',
QK, and QK' are shown.
They correspond to the center of the phylloquinones and are independent
of the phylloquinone plane orientations. Present distances and angles
are largely in agreement with those published previously (4). Note the
schematic nature of these diagrams; true distances are supplied but may
not be measured directly.
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Photovoltage measurements on oriented PSI thylakoid membranes estimated
values of fractional dielectrically weighted transmembrane distances of
0.62 for P700-A0 (compare with
eC1-eC3, 0.47), 0.16 for A0-A1 (compare with
eC3-QK, 0.25), and 0.22 for A1-FX (compare with
QK-FX, 0.28) (29, 30).
These relative distances, especially for the pair P700-A0,
do not correspond to the x-ray structural model distances eC1-eC3 as
well as one might have expected. Because the distances
A0-A1
(eC3-QK) and
A1-FX
(QK-FX) are comparable
(29), matching our observations, the distance P700-A0
(eC1-eC3) has clearly been overestimated by the photovoltage measurements relative to the other distances. Possibly, the fast rate of charge separation results in a significant error for the distance
eC1-eC3 (alternatively, the dielectric constant around P700 may differ substantially from that nearer the middle of the membrane), giving rise
to the observed distortion.
Intercofactor Distances from X-ray Structure and Spectroscopic
Studies--
Comparisons of structural and spectroscopic data have
recently been published based on models of PSI derived at 4.5- and 4-Å resolution (4, 5). These studies, however, included none or, in the
latter case, a single phylloquinone position designated QK (now renamed QK').
Here we will include the latest structural results and compare these to
the available spectroscopic data.
The Moser-Dutton "ruler" (31) (an empirical first order
relationship between electron transfer rates and shortest edge-to-edge distances of the cofactors involved) provides a simple tool to estimate
the "optimal" electron transfer rates from structural data, the
optimal electron transfer rate being achieved when the sum of the
standard reaction free energy and reorganization energy is essentially
zero (32). In Table I, the edge-to-edge
distances and the optimal (i.e. fastest theoretically
possible) electron transfer rates derived, using the above
relationship, are listed.
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Table I
The averaged edge-to-edge distances between the cofactor planes and the
"optimal" electron transfer rates derived using the Moser-Dutton
relationship, log kET = 15 0.6R 3.1( G° + )2/ (31).
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Chlorophyll a Cofactors--
The Chl a molecules,
eC1, eC1',
eC2, eC2',
eC3, and eC3' constitute
the luminal half of the electron transfer system.
eC1 and eC1' have been
identified as structural components of the spectroscopically identified
primary electron donor P700; eC2 and
eC2' are referred to as the accessory
chlorophylls; while either or both of eC3 and
eC3' have been assigned to the spectroscopically identified primary electron acceptor A0.
As noted (4, 5), the edge-to-edge distance between
eC1 and
eC3 of 13.3 Å is too long to be
compatible with the electron transfer rate of 5·1011
s
1 (for a review, see Ref. 5) determined for the primary
electron transfer step (see Table I, Fig. 4c). It therefore
seems likely that neither eC3 nor
eC3' (the spectroscopically identified electron
acceptor A0), but one or both of eC2 and eC2', represent an additional intermediate
(i.e. true primary electron acceptor) with a calculated
optimal electron transfer rate from
eC1 to
eC2 of 4·1012
s
1 and eC2 to
eC3 of 1·1012
s
1 (see Table I).
The Phylloquinone Electron Acceptors--
The average
center-to-center distance between QK
and the neighboring cofactor eC3
amounts to 8.7 Å. Combining data from pulsed EPR experiments on the
position of the quinone cofactor (relative to P700 and the
crystallographic a-axis) with the x-ray structural model of
PSI yielded a comparable center-to-center distance estimate of 7.5 ± 2 Å for eC3-QK
(10).
The center-to-center distance between
QK and FX has similarly
been estimated to be 14 ± 2 Å by EPR measurements (10), matching
that of the x-ray structural model
(QK/QK')-FX = 14.3 ± 1/14.1 ± 1 Å (Fig. 4b). Although the orientation
of the molecular plane of QK about
the naphthalene long axis could not be determined from the electron
density map, we determined the edge-to-edge distances to the
surrounding cofactors. Compared with the overall errors estimated for
edge-to-edge distances (±2 Å), the distances observed for the two
QK plane orientations modeled prove insignificant.
The averaged edge-to-edge distance
eC3-QK of
4.8 ± 2 Å is somewhat shorter than the value of
A0-A1
7.8 Å estimated from pico-nanosecond
laser spectroscopy (33) (Table I). The difference between these values
is possibly due to the inequivalence of the reaction free energy and
reorganization energy, causing the distance to be overestimated (33).
The averaged edge-to-edge distance between
QK and FX is 11.3 Å ± 2 Å, which agrees well with the 10.7 Å suggested previously (34)
(Table I).
The discrepancy between the electron transfer rate derived from the
edge-to-edge distance
eC1-QK and
those rates reported for the charge recombination reaction P700
A
1
P700A1 proves to be slightly
more problematic; the latter is roughly 4·103
s
1 (5) (Table I). According to the Moser-Dutton
approximation (31), the optimal (i.e. fastest possible)
transfer rate for a direct recombination through the distance
eC1-QK (20.5 Å) would be in the range of 5.0·102 s
1.
The reason for the observed rates being faster than that calculated from the corresponding edge-to-edge distance for this pair of cofactors
is unclear, although an intermediate step in recombination could
provide an explanation of this difference.
Correlation of A1 with QK or
QK'--
The identification of two phylloquinones and
their introduction to the x-ray structural model gives new impetus to
the question of which one of QK or
QK' corresponds to the spectroscopically identified cofactor A1. To analyze the current
possibilities, we derived two orientational models for each
phylloquinone (see "Results"). The orientation and position of each
of these models (two for each QK and
QK') are quantified by the parameters shown in
Fig. 5. They are successively compared
with the corresponding values derived mainly from EPR experiments on oriented PSI particles or PSI single crystals (Table
II).

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Fig. 5.
Schematic illustration of the geometric
parameters used to describe the orientation of the phylloquinone
molecules QK and QK'. a, side
view (i.e. parallel to the membrane plane). , (vector
P700 -A 1; c-axis). In addition to the
phylloquinone centroid, the centers of the individual naphthalene model
rings are used as reference points (dotted
lines), since the electron spin density of A 1 is
presumably located exclusively on the ring bearing the oxygen atoms.
Since these have not been located, both rings are considered.
b, oblique view. , (phylloquinone
O-O-axis; c-axis); , (phylloquinone
C2-Cmethyl bond; c-axis) (both
orientations directed toward the stromal or luminal side are
considered; the smallest value is given); , (phylloquinone
molecular plane; a,b-plane); , orientation of
the phylloquinone long molecular axis relative to the
a,b-plane. c, top view onto the
a,b-plane. µ*, (phylloquinone long
molecular axis; a-axis); *, (phylloquinone long
molecular axis; vector P700 -A 1); , (phylloquinone O-O-axis; vector P700 -A 1);
*, (vector P700 -A 1; a-axis); *,
angles in projection onto the a,b-plane.
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The distance between P700
and A
1 has been placed at
25.4 ± 0.3 Å (8, 9). The center-to-center distances of all four combinations (eC1-QK,
eC1'-QK,
eC1-QK', and
eC1'-QK') are compatible with the
EPR data (Table II, Fig. 4d).
--
The inclination of P700
-A
1 relative to
the membrane normal (c-axis) is 27 ± 5° (10, 35).
The corresponding values for eC1-QK,
eC1'-QK,
eC1-QK', and
eC1'-QK' are 23, 28, 31, and 29 ± 2°, respectively (Table II, Fig. 4d; see also
in
Fig. 5). A unique assignment of P700
A
1 is thus not
obtained, although eC1'-QK and
eC1'-QK' give the closest agreement.
--
The angle between the projection of
P700
-A
1 onto the a,b-plane and
the a-axis (
in Fig. 5c) is put at 0 ± 10° by EPR simulations (10). It should be noted that because of the
inherent D3 symmetry of these EPR techniques, the above
values do not only hold for the crystallographic a-axis but
also for the equivalent directions described by the vectors b and (a + b). In the following, the
term "a-axis" therefore includes the true
crystallographic a-axis, as well as its repetitions at 60°
intervals in the a,b-plane. The corresponding
values for
derived from the x-ray structural model are 12°
(eC1-QK), 10°
(eC1'-QK), 21°
(eC1-QK'), and 5°
(eC1'-QK'). Here
eC1'-QK' yields the best agreement,
although eC1'-QK and to a lesser
degree eC1-QK cannot (entirely) be excluded.
--
The axis defined by the phylloquinone oxygen atoms has
been reported to be essentially parallel (±10°) to the vector
P700
-A
1 (19). Although the x-ray models of
QK (QK') were derived by
aligning the phylloquinone plane with the vector
eC1-QK or
eC1'-QK
(eC1-QK',
eC1'-QK'), corresponding to
P700
-A
1, this leaves a possible deviation between the
O-O-axis and
eC1-QK
within the phylloquinone plane (
in Fig. 5c). This
deviation is observed to be 15 and 14 ± 10° (15 and 1°),
respectively, for the vectors listed above, if the center of the
phylloquinone is taken to represent the center of spin density of
A
1. These values change by as much as 3° if the electron
spin density is assumed to be centered on either one of the
naphthoquinone model ring systems (Table II). Here eC1'-QK' clearly shows the best
correspondence to the EPR data.
--
Because EPR results indicate the phylloquinone
O-O-axis to be aligned with P700
-A
1 (see
above), the angle between the O-O-axis and the
c-axis (denoted
in Fig. 5 and Table II) is assumed to be
equivalent in size to the angle
( 208 (P700
-A
1;
c-axis), Fig. 5a), i.e. 27°. Because
of the observed deviation between the modeled O-O-axis and
eC1-QK in
the x-ray model, however,
( 208 (O-O-axis;
c-axis)) deviates by as much as 10° from 27° in the case
of eC1'-QK (17°) and
eC1-QK' (35°), although the
agreement is quite good for both
eC1-QK (27°) and
eC1'-QK' (28°).
--
The phylloquinone plane of A
1 is reported to be
inclined by 76 ± 10° (35) relative to the
a,b-plane (
in Fig. 5, Table II). For all
x-ray structurally derived models, the inclinations are in the range
50-60°; the match between EPR- and x-ray structural values,
therefore, is less than satisfactory.
--
In the EPR model of Kamlowski et al. (36), the
methyl group of the phylloquinones either faces the lumen or the
stroma, while the C2-Cmethyl bonds subtend an
angle of 35 ± 20° with respect to the c-axis (36).
The equivalent value for the x-ray structural model phylloquinone plane
orientations (
in Fig. 5b, Table II) are in the range of
45-53°. Since the error of this value may be as large as 10°, the
values do not necessarily contradict the EPR estimate of 35 ± 20° (36). This criterion, however, does not aid in distinguishing the
four cofactor pairs of the x-ray structural model.
--
In the present x-ray structural model, the angles between
the longest molecular axis of QK
(QK') and the plane defined by the
c-axis and the vectors equivalent to
P700
-A
1 (eC1-QK,
eC1'-QK,
eC1-QK', and
eC1'-QK') are 30, 68, 81, or 68°,
respectively (
in Fig. 5, Table II). At present, no estimate for the
angle
is available from spectroscopic investigations.
Combining the conclusions from the geometric comparison of the
spectroscopic data and x-ray structural model indicates that eC1'-QK' shows the closest
correspondence with the pair P700
-A
1. Nevertheless,
the pairs eC1-QK and
eC1'-QK similarly mostly remain
within the error limits. Overall,
eC1-QK' is most poorly correlated
with the EPR data.
 |
CONCLUSION |
A complete and internally consistent set of cofactors of the
electron transfer system of PSI has been modeled into an improved electron density map at 4-Å resolution. The corrected model of the
-helices in the vicinity of the phylloquinone molecules leads to a
better 2-fold symmetrical arrangement in this region of the PSI core.
The distances for QK and
QK' to other cofactors compare favorably with
those suggested from spectroscopic measurements. Although the
orientation of the phylloquinones remains undetermined about the long
axis of the molecular plane, the pair P700
-A
1 is best
correlated with eC1'-QK' followed by eC1-QK and
eC1'-QK.