The earth's primary energy conversion of sunlight into biomass is oxygeneic photosynthesis, driven by large protein-cofactor complexes in the plasma membrane of photosynthetic bacteria and in thylakoid membranes within chloroplasts of plants. These complexes, photosystem II and photosystem I, capture light energy and act sequentially to raise the energy of electrons. These electrons are utilized in electron transport chains to generate a proton gradient across the membrane as well as NADPH. The electromotive force of the proton gradient is used by ATP Synthase to synthesize ATP. Together, this ATP and the NADPH provide energy to drive the light-independent Calvin Cycle, which fixes carbon from CO2 in organic compounds. See Figure 1 for a schematic of this process.
At left is the the photosystem I complex (PSI) of the thermophilic cyanobacterium, Synechococcus elongatus (Jordan, et al., 2001), which exists as a trimer (relative molecular mass 3 x 356,000). Each of the monomers comprises at least 11 different protein subunits that embed 100+ cofactors. Many of these cofactors serve as a large antennal system that harvests the energy of photons of light and transfers this energy to the core of the reaction center with high effciency. A chlorophyll dimer (P700) in the reaction center contains electrons that are excited by the energy funneled to them. The P700 electrons are passed through an electron transfer chain (ETC), transferred to Ferredoxin and then to NADP+ by Ferredoxin-NADP+ reductase, yielding NADPH. The reaction cycle is completed by re-reduction of the P700 chlorophylls by plastocyanin at the inner side of the membrane. Plastocyanin, in turn, is provided electrons originating in PSII by way of plastoquinones and the cytochrome b6/f complex. See Figure 1 for a schematic of this process.
II. PSI Structure, the P700 Reaction Center, and the Electron Transfer Chain
Each of the PSI monomers comprises at least 11 different protein subunits.
These 11 proteins serve as a scaffold in which 100+ light capturing pigments (chrolophylls and carotenoids) are embedded. These provide the antennal system that harvests light energy and funnels it to the ETC reaction center P700 chlorophylls (see below).
PsaA and PsaB are the largest subunits in each PSI monomer.
PsaA and PsaB display both sequence and structural homologies. Each contains 11 transmembrane helices found in two domains, an amino-terminal domain with 6 helices, and a carboxy-terminal domain with 5 helices. The latter form two interlocked semicircles enclosing the reaction center P700 chlorophylls and multiple ETC cofactors.
In a cutaway view, the reaction center P700 chlorophylls and the remainder of the ETC cofactors (chlorophylls, phylloquinones, iron-sulfur [Fe4S4] clusters) are observed to lie in the heart of a PSI monomer.The secondary chlorophylls and carotenoids of the elliptical antennal system are observed to surround the P700 chlorophylls and ETC cofactors (chlorophylls, phylloquinones, iron-sulfur [Fe4S4] clusters).
The orientation of the P700 chlorophylls and the cofactors of the electron transport chain with respect to the stroma of the chloroplast and the lumen of the thylakoid is such that electrons will be passed from the luminal reaction center to the two stromal-side Fe4S4 clusters that are embedded in protein subunit PsaC of the PSI monomer. PsaC is similar to bacterial 2Fe4S4 ferredoxins, but has an insertion of 10 amino acids that extrudes a prominent loop that connects the two motifs which bind the Fe4S4 clusters. This loop may be engaged in docking of ferredoxin (see below).
One path of the electrons is illustrated, beginning with one of the P700 chlorophylls, with distances between electron transfer cofactors indicated in Angstroms. The magnesium ions of the chlorophylls of the electron transport chain, the Fe4S4 clusters, chlorophylls, phylloquinones, and subunit PsaC are indicated. The homologous pathway starting with the other P700 chlorophyll can transfer electrons at a higher rate, although both pathways are extremely efficient.
The stromal subunits PsaC, PsaD, and PsaE can be seen to form a putative ferredoxin docking site. Note the position of the PsaC loop mentioned above.
Jordan, P., Fromme, P., Witt, H.T., Klukas, O., Saenger, W., and N. Krauss. (2001). Three-dimensional structure of cyanobacterial photosystem I at 2.5 Å resolution. Nature 411: 909-917.