|MadSci Network: Biochemistry|
Hi Lauren, This is actually a very good question. At first, I thought that the answer should be easy to find in any decent Biochemistry textbook. Then I looked in mine. Pq, Pc, and Fd are all abbreviations for molecules that ferry electrons between different compounds. The reason that they are drawn in circles in your book is that they are recycled in the process. Pq stands for plastoquinone, just one member in a family of compounds known as quinones. One member of this family is so widely found that it was named ubiquinone (from ubiquitous). Plastoquinone is a lipid soluble compound found diffused in the thylakoid membrane of chloroplasts in plants. It binds reversibly to photosystem II and accepts electrons that have been energized by light. It has a ring structure with two ketone groups, one on each end. When it takes up electrons the ketone groups are reduced to alcohol groups. In this form, quinones are called quinols. It also takes up two hydrogen ions. It then moves away from photosystem II to the cytochrome b6/f complex. There it donates its electrons to reduce cytochrome b6. In the process it is recycled to its original state as a quinone. The cytochrome b6/f complex then uses the energy of the electrons to pump protons across the thylakoid membrane. The electrons are then passed to Pc, or plastocyanin. Plastocyanin is a protein that can accept electrons by virtue of a coordinated copper ion that is reduced from Cu2+ to Cu+. Plastocyanin functions much like cytochrome c of the mitochondrial electron transport chain and indeed substitutes for it in some photosynthetic bacteria and algae. Plastocyanin then donates the electrons to the chlorophyll of photosystem I, which has been oxidized by a previous round of photosynthesis. Photosystem I then reenergizes the electrons, which have lost some of their spunk after all this shuttling and pumping of protons, and passes them to Fd, or ferredoxin. Ferredoxin is an iron-sulfur protein that diffuses around on the stroma face of the thylakoid membrane. It transfers the electrons to an enzyme called ferredoxin-NADP oxireductase. This enzyme uses the energized electrons to reduce NADP+ to NADPH. NADPH is a molecule that is used as a basic currency within the cell to carry out all kinds of different functions. The ferredoxin molecule is oxidized to its previous state by this and can go back and get some more electrons from photosystem I. If you want to know more about photosynthesis, there is an excellent web site on it at http://www.life.uiuc.edu/govindjee/paper/gov.html I am currently working in Dr. Whitmarsh's lab. He is one of the authors of this paper. Thanks for the question. It was an opportunity for me to refresh my knowledge of the details of photosynthetic electron transfer processes. Sincerely, J. Todd Holland Graduate student in Biophysics At the University of Illinois at Urbana-Champaign Fellow in the Integrative Photosynthesis Research Training Grant Hollandtodd@hotmail.com
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