User talk:Cole Faulkner
From Proteopedia
Photosynthetic Pathway
Photosynthesis is the pathway that uses light energy (photons) to drive the production of Adenosine Triphosphate. As photons hit the organism, a small percentage hit the photo-reaction center of P680 (Photosystem II) and P700 (Photosystem I) to excite either P680 (2AXT) or P700 (2O01) which then drives electron transport until the final products are reduced ferradoxin and NADPH.
Photosynthesis is a multistage process that occurs in the cytoplasm of a plant, comprising of multiple enzymes that work in tandem. The process begins with the photosynthetic oxygen-evolving center, an asymmetric (C-1) hetero 19-mer residing in the thylakoid membrane. This enzyme oxidizes water during the light reactions to provide protons for photosystem II [1] [A]. The photosynthetic oxygen evolving center is coupled with a redox active tyrosine Yz which assists in oxygen evolution. Yz is reduced by a proton coupled electron transfer and formed by a chlorophyll cation [2]. Once water is oxidized, it is fed into photosystem II, a large protein complex imbedded in the thylakoid membrane of plants. Inside of the protein resides P680, a chlorophyll dimer, which is excites an electron when hit by a photon of light [3]. The excited electron is captured by
To excite P680 in photosystem II, four electrons from Yz (electron donor) and four photons interact with P680. This causes electronic excitation of P680 (at nearly -0.8 volts), which then causes a cascade of oxidation-reduction reactions. Furthermore, with each oxidation-reduction reaction, the redox potential increases to nearly +0.4 volts at the end of photosystem II (1).
The cascade of oxidation-reduction reactions starts off with an electronically excited P680, which donates electrons to a pheophytin acceptor (Ph). From there, the pheophytin acceptor donates electrons to QA (which is a protein-bound plastoquinone), and then QA donates its electrons to QB (another protein-bound plastoquinone). Next, QB donates the electrons to QH2 (reduced plastoquinone), which then donates the electrons to the cytochrome b6f complex. When this occurs, the cytochrome b6f complex donates it electrons to plastocyanin (PC) and also releases eight protons into the thylakoid membrane. Finally, the plastocyanin transfers its electrons to P700 in order to start photosystem I. The overall reaction for photosystem II is 2H2O (with 4 photons excitation)-> 4H+ + 4e- +O2 (2).
Photosystem I (P700) whose PDB ID is 2O01 works as in transferring electrons and driving electron transport through the photosystem until the final product NADPH is synthesized. The overall equation is4e^-+2H^++2〖NADP〗^+→2NADPH. The reaction is driven by 4 photons of light which excite photosystem I and make it move forward. The A0 complex known as Chlorophyll A0 is an early electron acceptor. It accepts the electrons from the excitation of P700 before passing them along to the next early electron acceptor.
The A1 complex known as Phylloquinone A1 is the next early electron acceptor. It oxidizes A0 to receive the electron and then reduces the Fx (Iron-Sulfur complex) to pass the electron the next two Iron-Sulfur complexes (FA and FB).(3,4) FA and FB (Iron-Sulfur complexes) serve as the electron relays. They are bound to protein subunits of photosystem I. Meanwhile Fx is tied to the photosystem I complex.5
Next, Fd which is known as Ferredoxin is the soluble protein that performs the reaction that reduces NADP+ to NADPH. The main role of Fd is to transfer the electron from an Iron-Sulfur complex to Ferredoxin-NADP+ Reductase (FNR).6 FNR is a certain enzyme that transfers the electron from the reduced ferredoxin to NADP+ to form NADPH. The overall reaction for this entire pathway is 2H_2 O+2〖NADP〗^+→2H^++O_2+2NADPH.7
References: (1) Tommos, C.; Tang, X.-S.; Warncke, K.; Hoganson, C. W.; Styring, S.; Mccracken, J.; Diner, B. A.; Babcock, G. T. Spin-Density Distribution, Conformation, and Hydrogen Bonding of the Redox-Active Tyrosine Yz in Photosystem II from Multiple Electron Magnetic-Resonance Spectroscopies: Implications for Photosynthetic Oxygen Evolution, Journal of the American Chemical Society1995, 117(41), 10325–10335. (2) Rappaport, F.; Guergova-Kuras, M.; Nixon, P. J.; Diner, B. A.; Lavergne, J. Kinetics and Pathways of Charge Recombination in Photosystem II, Biochemistry 2002, 41 (26), 8518–8527. (3) Rutherford, A. W., and Heathcote, P. (1985) Primary photochemistry in photosystem-I. Photosynthesis Research 6, 295-316. (4) Itoh, S., and Iwaki, M. (1989) Vitamin K1 (phylloquinone) restores the turnover of FeS centers in the ether-extracted spinach PS I particles. FEBS Letters 243, 47-52. (5) Palace, G.P., Franke, J. E., and Warden, J. T. (1987) Is phylloquinone an obligate electron carrier in photosystem I? FEBS Letters 215, 58-62. (6) Vassiliev, I. R., Antonkine, M. L., and Golbeck, J. H. (2001) Iron-sulfur clusters in type I reaction centers. Biochimica et Biophysica Acta (BBA)- Bioenergentics 1507, 139-160 (7) Forti, G., and Grubas, P. M. G. (1985) Two sites of interaction of ferredoxin with thylakoids. FEBS Letters 186, 149-152 (8) Madoz, J., Fernandez-Recio, J., Gomez-Moreno, C., and Fernandez Victor M. (1998) Investigation of the diaphorase reaction of ferredoxin-NADP reductase by electrochemical methods. Bioelectrochemistry and Bioenergetics 47, 179-183.
