On the mutual orientation of pigments in photosystem I particles from green plants

Photosystem I reaction center from the thermophilic cyanobacterium Mastigocladus laminosus

Photosystem I reaction center was isolated from the cyanobacterium Mastigocladus laminosus. It contained four different subunits with molecular masses (as determined by sodium dodecyl sulfate gels) of about 70,000 (subunit I), 16,000 (subunit II), 11,000 (subunit III), and 10,000 (subunit IV) daltons. The purified reaction center contained about 100 chlorophyll a molecules per P(700); however, they could be readily depleted down to about 50 chlorophyll a per P(700) without loss in the photochemical activities. The reaction center was active in cytochrome c photooxidation, but the photooxidation of an acidic cytochrome, like the Euglena cytochrome 552, required the presence of cations. The purified reaction center was found to be similar in several respects to the photosystem I reaction centers from higher plants and, especially, to the one isolated from green algae. Subunit I appeared on sodium dodecyl sulfate gels in the same position and possessed the same shape of an apparent double band as the corresponding subunits I of green plants and of algae. Subunits I and II of photosystem I reaction centers from Mastigocladus, higher plants, and green algae showed immunological crossreactivity. This observation might serve as biochemical evidence for the common evolution of the photosystem I reaction centers. In higher plants and green algae subunit II is a product of cytoplasmic ribosomes and therefore, a high degree of homology should have been preserved upon transfer of its gene from the prokaryote to the nucleus of the eukaryotes.

Functional and idling rotatory motion within F1-ATPase

ATP synthase mediates proton flow through its membrane portion, F0, which drives the synthesis of ATP in its headpiece, F1. The F1-portion contains a hexagonal array of three subunits alpha and three beta encircling a central subunit gamma, that in turn interacts with a smaller epsilon and with F0. Recently we reported that the application of polarized absorption recovery after photobleaching showed the ATP-driven rotation of gamma over at least two, if not three, beta. Here we extend probes of such rotation aided by a new theory for assessing continuous versus stepped, Brownian versus unidirectional molecular motion. The observed relaxation of the absorption anisotropy is fully compatible with a unidirectional and stepping rotation of gamma over three equidistantly spaced angular positions in the hexagon formed by the alternating subunits alpha and beta. The results strongly support a rotational catalysis with equal participation of all three catalytic sites. In addition we report a limited rotation of gamma without added nucleotides, perhaps idling and of Brownian nature, that covers only a narrow angular domain.

Intersubunit rotation in active F-ATPase

The enzyme ATP synthase, or F-ATPase, is present in the membranes of bacteria, chloroplasts and mitochondria. Its structure is bipartite, with a proton-conducting, integral membrane portion, F0, and a peripheral portion, F1. Solubilized F1 is composed of five different subunits, (alpha beta)3 gamma delta epsilon, and is active as an ATPase. The function of F-ATPase is to couple proton translocation through F0 with ATP synthesis in F1 (ref.3). Several lines of evidence support the spontaneous formation of ATP on F1 (refs 4,5) and its endergonic release at cooperative and rotating (or at least alternating) sites. The release of ATP at the expense of protonmotive force might involve mechanical energy transduction from F0 into F1 by rotation of the smaller subunits (mainly gamma) within (alpha beta)3, the catalytic hexagon of F1 as suggested by electron microscopy, by X-ray crystal structure analysis and by the use of cleavable crosslinkers. Here we record an intersubunit rotation in real time in the functional enzyme by applying polarized absorption relaxation after photobleaching to immobilized F1 with eosin-labelled gamma. We observe the rotation of gamma relative to immobilized (alpha beta)3 in a timespan of 100 ms, compatible with the rate of ATP hydrolysis by immobilized F1. Its angular range, which is of at least 200 degrees, favours a triple-site mechanism of catalysis, with gamma acting as a crankshaft in (alpha beta)3. The rotation of gamma is blocked when ATP is substituted with its non-hydrolysable analogue AMP-PNP.

Orientation of Photosystem-I pigments: low temperature linear dichroism spectroscopy of a highly-enriched P700 particle isolated from spinach

The linear dichroism of Photosystem I particles containing 10 chlorophylls per P700 has been investigated at 10 K. The particles were oriented by uniaxial squeezing of polyacrylamide gels. The oxidation state of P700 was altered either by incubation of the gels with redox mediators or by low temperature illumination. The QY transitions of the primary electron donor P700, of the remaining unoxidized chlorophyll in P700(+) and of a chlorophyll molecule absorbing at 686 nm, which presumably corresponds to the primary electron acceptor A0, are all preferentially oriented perpendicular to the gel squeezing direction. The QY transition of the chlorophyll forms absorbing at 670 and 675 nm appear tilted at 40 ± 5° from this orientation axis. This orientation of the various chlorophylls is compared to that previously reported for more native Photosystem I particles.

Photosystem I complex

Photosystem I is an integral component of the thylakoid membrane which catalyzes the photoreduction of ferredoxin using plastocyanin or cytochrome c as electron donor. In higher plants, the photosystem I complex is composed of eight protein subunits, chlorophyll a, carotenoids, phylloquinone and bound iron sulfur clusters. The molecular biology and biochemistry of the complex are discussed in relation to the structure and function of the individual components. The mechanisms involved in the assembly of the components into a functional complex are also discussed.

Photosystem I reaction centers from Chlamydomonas and higher plant chloroplasts

A photosystem I reaction center has been isolated from Chlamydomonas chloroplasts and compared with the photosystem I reaction center from higher plants. While the higher plant reaction center is active in cytochrome 552 photooxidation, the Chlamydomonas preparation was not active unless salts were included in the assay medium or the pH was lowered to 5. Subunit III-depleted photosystem I reaction center from higher plants is also inactive in cytochrome 552 photooxidation in the absence of salts. As with the Chlamydomonas reaction center, salts induced its activity. Subunit I of the photosystem I reaction center has tentatively been identified as the binding site of cytochrome 552.

Role of beta-carotene in the reaction centres of photosystems I and II of spinach chloroplasts prepared in non-polar solvents

Spinach chloroplasts have been prepared nonaqueously using non-polar solvents (n-hexane, CCl4, n-hepatane) and the beta-carotene content extracted in a controlled manner. This procedure is reproducible and does not result in large structural or spectral changes of the chloroplasts. The organisation of the chlorophyll-proteins is unaltered, as fragmentation with digitonin results in the appearance of the same fractions as found previously for aqueously-prepared chloroplasts, including the pink zone containing cytochromes f and b6 in the ratio 1 : 2. The chloroplasts possess both Photosystem I activity (P-700 photo-bleaching, and NADP+ photoreduction) and Photosystem II activity (parabenzoquinone reduction with Mn2+ as electron donor, and chlorophyll fluorescence induction). Use of moderate intensity red illumination has allowed a study of the role of beta-carotene in photochemistry separate from its roles in energy transfer and photoprotection. Removal of the fraction of beta-carotene closely associated with the Photosystem I reaction centre caused the rate of NADP+ photoreduction to fall to a low, but significantly non-zero level. Thus, in the complete absence of beta-carotene, photochemistry can still be observed, however the specific association of beta-carotene with the reaction centre is required for maximal rates. We propose that beta-carotene bound at the reaction centre decreases the rate of transfer of excitation energy away from the reaction centre, and increases the rate of photochemistry. It is possible that this occurs via formation of an exciplex between ground state beta-carotene and chlorophyll in the first excited state.

The field of possible structures for the chlorophyll a dimer in photosystem I of green plants delineated by polarized photochemistry

Photoselection experiments with immobilized photosystem I particles have been done to determine the mutual orientation of pigments in the reaction centre. When these particles are excited and interrogated with linearly polarized light, the flash-induced transient absorption changes (mainly from the chlorophyll a dimer) reveal linear dichroism, which yields information on the mutual orientation between the excited and the interrogated transition moments. The interpretation of the data, however, is ambiguous, (1) for reasons of principles inherent in the photoselection technique when applied to complex systems and (2) because of incomplete knowledge about the relative contribution of x- and of y-polarized transitions of chlorophyll a to absorption or to absorption changes at a given wavelength. We find it impossible to attribute any particular structure to the photooxidizable dimer based on photoselection data alone. Instead we present a field of possible structures, imposing constraints on proposed models for the dimer structure.