Photosystem I

The PSI antenna/RC complex appears as a native trimeric unit in the membrane of cyanobacteria, in contrast to the monomeric PSI complex occurring in higher plant species. Despite substantial differences in the number of total polypeptides and differences in structural details, including the different macroorganisation of PSI in the different organisms, there probably exist many similarities in the structure of their core antenna, as judged on the basis of the sequence homology. No detailed structure is as yet available from a higher-plant PSI complex, although a 4.5 A X-ray structure of LHI-PSI is in press at the time of writing; see also Kargul et al. (2003).
 

The PSI complex of S. elongatus consists of 12 protein subunits and contains 96 Chls (95 Chl a and one Chl a’ ’, which is located in the ‘special pair’ of the RC), 22 carotenoids (mostly pcarotene), two phylloquinones and the [4Fe-4S] centres involved in electron transfer as electron acceptors.
 

Figure 2.4 (p. 83) shows the structure of Photosystem I (only one monomer of the trimeric structure is shown for clarity) to a resolution of 2.5 8, (Fromme and Witt, 1998; Fromme et al., 2001; Jordan et al., 2001) at near atomic resolution. These studies have for the first time revealed the orientations of all the chlorin ring systems, enabling more rigorous theoretical calculations of energy-transfer properties based on the distances and transition-dipole orientations of the Chls. A salient feature of the PSI complex is that a large number of the antenna pigments (89 Chls), as well as the 6 RC Chls, are bound to the same two polypeptides, namely the highly homologous psaA and psaB units forming the core of the structure. These two polypeptides are related by a pseudo-C2 axis located at the centre of the PSI monomer. The organic cofactors of the electron-transfer chain of RCs are arranged in two branches along this pseudo-C2 axis.
 

Figure 2.5 (p. 84) shows the arrangement of the pigments in one monomer of the Photosystem I trimer. Viewed from above (upper diagram), the antenna Chls form an elliptically distorted, cylindrical ring structure around the six central RC Chls. Except for the two ‘linking Chls’, which may possibly connect energetically to the antennae and the RC pigments, the distance between the nearest antenna pigments and the RC pigments is relatively large (>18 A). The antenna Chls have centre-to-centre distances of 7-16 A, with a maximum in the distance distribution around 10 A, well within the range allowing ultrafast energy transfer. The side view of the pigment arrangement (lower diagram of Fig. 2.5) shows that in the greater part of the PSI antenna, except close to the RC, the Chls are arranged in two layers located near the two membrane surfaces, with large distances between the two. Thus energy transfer is expected to occur preferentially within the layers, while layer-layer transfer will occur only near the RC. Thus a large part of the PSI antenna is quasi-two-dimensional with respect to the arrangement of the Chls.
 

There are three regions in the antenna structure (highlighted in red in Fig. 2.5) where close stacking of two or three Chls occurs. These arrangements should lead to a substantial excitonic coupling, conferring special spectroscopic properties on these Chls. In all probability, these stacking regions contain the so-called special ‘red Chls’ that are present in all PSI complexes to various extents. This term denominates those groups of Chls that absorb beyond 700 nm, above the absorption maximum of the RC, which is located around 700 nm. These ‘red Chls’ are particularly prominent in cyanobacterial PSI and are believed to play a special limiting role in the overall energy-transfer process to the special pair.
 

Energy transfer in PSI core complexes of cyanobacteria and green algae has been extensively studied, both experimentally and theoretically, but no general agreement has so far been reached as to the rates of the various energy-transfer steps and ratelimiting processes. Most experimental studies have been performed on the core complexes of either cyanobacteria or green algae and higher plants, while relatively few detailed studies are available for intact higher-plant PSI complexes that also carry the light-harvesting I complexes (LHCI) of the outer antenna. These studies have been interpreted either qualitatively or in more detail in terms of so-called compartment models. In these simpler models, energy and electron-transfer processes between groups of pigments are analysed, rather than those between each pair of pigments in each complex.

Figure 2.6 summarises various compartment models for the energy-transfer processes in the PSI core. These models differ in the relative rates of energy transfer from the core antenna to the RC and the effective charge-separation rates. Figure 2 . 6 ~ shows the so-called trap-limited scheme, where the energy transfer from the core antenna to the RC is much faster than the charge-separation lifetime. This model has been adopted by several groups (for reviews see Karapetyan et al., 1999 and Melkozernov, 2001). A contrasting model is the so-called transfer-to-trap limited model, which is primarily based on data from cyanobacterial core complexes (for reviews, see Gobets and van Grondelle, 2001 and Gobets et al., 2001). A common theme in these models, shown in Figs. 2.6b-d, is the overall transfer time from the core antenna pigments to the RC, which is the rate-limiting step, having a lifetime of -20 ps. The models differ, however, in the charge-separation lifetimes within the RC, with variations from 1 ps to 10 ps. Individual energy-transfer steps between groups of pigments are very rapid, in the range of 100-200fs, leading to a very fast subpicosecond energy equilibration within the core antenna. A complication in the kinetics arises from the presence of the special ‘red pigments’ in PSI of cyanobacteria, which generally slow down energy transfer to the RC if they are located in the antenna.
 

Figure 2.7 gives the absorption spectra of some PSI and PSII preparations, showing the much larger red tail in the absorption of the PSI as compared with the PSII particles. The tail is particularly pronounced for Spirulina platensis PSI because of the extreme content of red pigments. The various cyanobacterial PSI complexes are believed to contain different numbers of red pigment molecules, ranging from about 2 in Synechocystis to an extreme of about 6-8 in Spirulina platensis. They also differ in their spectral signatures, ranging from 708 nm absorption (Ch1708) in Synechocystis, through 7 18 nm in Synechococcus elongatus, to 735 nm in Spirulina. The latter seems to contain the longest-wavelength absorbing Chls of any PSI complex, giving rise to 760 nm fluorescence at low temperature (Karapetyan et al., 1999). The red pigments are believed to exchange energy with the core antenna in about 5-10 ps (Gobets and van Grondelle, 2001). This relatively slow energy exchange slows down the overall energy-transfer rate to the RC and severely complicates the observed kinetics. The time constant of the overall energy-trapping process (as measured by the kinetics of formation of the charge-separated states) in cyanobacterial core complexes at room temperature ranges from about 23 ps for Synechocystis, a species with minimal redpigment content, to about 35 ps in s. elongatus with an intermediate amount of red pigments, to a maximum of about 50 ps in Spirulina platensis. This slowing down in the overall trapping rate has been ascribed to the effects of the red pigments (Gobets et al., 2001).

A problem for all these models at present is that until recently no detailed spectra of the intermediate species involved in the kinetics have been resolved. Based on such data, Muller er ul. (2003) have recently proposed the new model shown in Fig. 2.8. This does not so far take into account the effects of the very long-wavelength red pigments, but is limited to PSI particles with low red pigment content. such as the green algae Cltlnrnyrloiiioiius reitlhurdtii and the cyanobacterium Synechocystis (Milller EI nl.. 2003). It is essentially a trap-limited model where energy equilibration within the core is subpicosecond, the transfer between the core antenna and the RC is very rapid (at most a few ps), and the effective charge-separation step is still fast (about 6-9 ps) but significantly slower than the energy equilibration between the core antenna and RC. Energy equilibration within the RC itself is also ultrafast, typically about 200 fs. In this model, the intrinsic charge-separation step in the RC is estimated to occur on a sub-picosecond time scale. This model is the first time to provide consistent spectra of the various intermediates, along with a description of the dynamics of the core antenna processes of PSI. Eventually it will have to be extended lo include the effects of the more extreme red pigments.
 

These data imply that PSI is the fastest RC known, featuring an intrinsic initial charge-separation step taking about 0.5-0.8 ps, which is a factor of five faster than bacterial RCs. with their charge-separation time of about 3 ps. Despite disagreements in the literature about the relative rates of antenndRC equilibration and charge separation, there is general agreement that energy equilibration within the PSI core, barring the slower transfer to/from the small number of red pigments, is very fast, typically in the time range of 200-600 fs (Melkozernov et al., 2000; Kennis et al., 2001; Gibasiewicz et al., 2001; Muller et al., 2003). Such fast equilibration is also in agreement with the detailed structure-based modelling studies mentioned above.
 

We have already noted that S. elongatus contains several close-lying groups of Chls that could be assigned to the special red Chl pigments. This is borne out by data showing that these red pigments derive their bathochromic shift from excitonic coupling rather than a special protein environment. Several groups have tried to model the spectral and kinetic properties of the S. elongatus PSI in detail, based on crystallographic information (Beddard, 1998; Byrdin et al., 2002; Sener et al., 2002; Damjanovic et al., 2002). However, even a very precise high-resolution structure does not provide the exact spectroscopic properties of a specific Chl in a protein complex. This energetic position is determined by the detailed interaction of the pigments with the environment, which can be obtained only by a full quantum-mechanical calculation based on an exact structure. PSI is particularly variable in this respect: the core antenna contains only Chl a, but the absorption maxima of the various Chls range from about 640nm up to 735 nm, indicating a wide range of pigment environments. Much of this range is due to pigment-protein interactions, but pigment-pigment interactions such as charge-transfer and excitonic interaction also have an effect. Thus theoretical descriptions usually treat the spectral properties of the individual pigments as a fitting parameter ((Byrdin et al., 2002; Sener et al., 2002).
 

One attempt has been made (for PSI) to calculate the energetic locations of the pigments quantum-mechanically, taking into account the interaction of each specific Chl with its environment (Damjanovic et al., 2002). At present, the conclusions of these studies, based on the information available for the PSI complex of S. elongatus, vary substantially. More work is needed to arrive at final conclusions about the specific location of the red pigments as well as many other details of the spectral and kinetic properties of PSI of S. elongatus. However, irrespective of the outcome of such calculations, it can already be stated that the exact details of the pigment arrangement in the antenna and the distribution of spectral forms across the antenna do not have any decisive influence on the overall kinetics, because of the quasistatistical averaging of pigment properties that occurs in such a large antenna array. Thus the functioning of the PSI antenna system seems to be fairly robust against even relatively drastic changes in spectral distribution and other properties. By contrast, the particular spectral and kinetic properties of the reaction centre, as well as its electrontransfer rate, seem to be much more decisive for the overall trapping kinetics and the total yield of charge separation. This is not surprising since these parameters directly

influence the key steps in the energy equilibration between antenna and RC and the overaIl energy flow toward the RC. This can be understood in more detaiI from the scheme in Fig. 2.8. Photosynthetic organisms have probably used these parameters to fine-tune PSI function in different organisms.
 

The PSI core antemst of higher plants and green algae binds various amounts of LHCI light-harvesting ncomplexes, whose structures may be similar to those of LHCU (discussed below) but are not known in any structura1 detail. These peripheral antenna complexes in higher plants also contain red pigments (Melkozernov, 2001). The average energy of the Chls in the LHCI complexes is above 700 nm, i.e. above the RC absorption maximum. The overall energy equilibration within the PSI antenna systems of higher plants seem however to be very rapid, with lifetimes from a few ps to about 12 ps, depending on the amount and type of peripheral antenna complexes present. The overall trapping times in the intact complex range from -5Ops to -100 QS at room temperature: see Melkozernov (2001) for a detaiIed review. Much longer lifetimes may be observed at low temperatures because of trapping on longwavelength pigments Iocated far from the RCs. Thus the overall trapping times in PSI of higher plants are generaliy dower than in cyanobacterial PSI by a factor of 2-5. However, the ovem1l trapping times are still fast, allowing a high yield of >90% for charge separation. The slower trapping in higher-plant PSI is mainly a consequence of the larger number of pigment moIecuIes per RC but partly due to the distant location of red pigments in the peripheral antenna. Thus in the PSI compIexes of higher plants with associated peripheral light-harvesting complexes, there may be a mixed situation. with trap-Iimited kinetics in the core, but diffusion-limited kinetics in at least part of the peripheral LHC compkxes (Jennings ef al., 1998; Croce ef al., am).

The contribution of red pigments to the kinetics of the cyanobacterial core complexes could also perhaps be described using such a mixed model. Some structural details of the arrangement of the peripheral antenna complexes in PSI of higher plants and green algae have recently been obtained by electron microscopy (Boekema et al., 2001b; German0 et al., 2002; Kargul et al., 2003). It was found that the LHCI complex binds on one side of the PSI core only. There are, however, substantial differences in the size and arrangement of the outer antenna complexes between different organisms. Figure 2.9 (p. 84) shows the arrangements of the various PSI complexes in the green algae Chlamydomonas reinhardtii, spinach and the cyanobacterial trimeric PSI core. In contrast to PSII (discussed below), there does not seem to be high symmetry in the antenna PSI complexes of green algae and higher plants.
 

As regards the carotenoids, about 60 of the antenna chlorin heads in the PSI structure of Synechococcus are in van der Waals contact with the 22 carotenoids (Jordan et al., 2001), which should provide excellent conditions for light harvesting through the carotenoids as well as photoprotection of the antenna by Chl triplet quenching. Little experimental information is available at present on carotenoid-to- Chl energy transfer in the core antenna of PSI as compared with PSII. However, one can conclude from simple fluorescence excitation spectra that the carotenoids contribute significantly (typically above 50% yield, but there are exceptions: see below) to the light-harvesting function in the wavelength range 450-650 nm where the Chls do not absorb well.
 

A very interesting case of regulation involving a major restructuring of the PSI antenna has recently been found in cyanobacteria (Bibby et al., 2001a; Boekema et al., 2001a) and Prochlorococcus (Bibby et al., 2001b), the most abundant photosynthetic organism in the oceans. Iron deficiency, which is often the limiting factor for growth of photosynthetic organisms in aquatic ecosystems, leads to the induction of additional proteins around the PSI core such as IsiA in cyanobacteria and a similar protein in Prochlorococcus. IsiA has been implicated in chlorophyll storage, energy absorption and protection against excessive light. However, it has now been shown that a PSI-IsiA supercomplex is abundant under conditions of iron limitation. Electron microscopy has revealed that this supercomplex consists of trimeric PSI surrounded by a giant closed ring of 18 IsiA proteins binding about 180 chlorophyll molecules (Bibby et al., 2001a and 2001b; Boekema et al., 2001a; Nield et al., 2003). Figure 2.10 (p. 85) shows the structure of this giant ring.
 

Energy transfer within this supercomplex has been recently studied by timeresolved absorption and emission spectroscopy (Melkozernov ef al., 2003), showing that the ring is energetically tightly coupled to the core, and energy equilibration  within the ring occurs on a sub-picosecond time scale. One should realise that this time constant probably does not reflect energy transfer through the whole giant ring, but simply energy equilibration between directly neighbouring subunits. Energy transfer along the outer ring cannot be resolved by time-resolved spectroscopy since it occurs among spectrally identical subunits. Energy transfer from the outer ring to the nearest Chl molecules in the central core occurs with a time constant of -1.7 ps, while overall energy transfer from the ring to the core takes -10 ps.