Biochemistry, 4th Edition P69. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Is the Molecular Architecture of Photosynthetic Reaction Centers 643 phyll BChl . The e is then transferred via the L bacteriopheophytin BPheo to QA which is also an L prosthetic group. The corresponding site on Mis occupied by a loosely bound quinone QB and electron transfer from QA to QB takes place. An interesting aspect of the system is that no electron transfer occurs through M even though it has components apparently symmetric to and identical to the L e transfer pathway. The reduced quinone formed at the QB site is free to diffuse to a neighboring cytochrome bc1 membrane complex where its oxidation is coupled to H translocation and hence ultimately to ATP synthesis. The use of light energy to drive ATP synthesis by the concerted action of these membrane proteins is called photophosphorylation Figure . Cytochrome c2 a periplasmic protein serves to cycle electrons back to P870 via the four hemes of the reaction center cytochrome subunit. A specific tyrosine residue of L Tyr162 is situated between P870 and the closest cytochrome heme. This Tyr is the immediate e donor to P870 and completes the light-driven electron transfer cycle. The Molecular Architecture of PSII Resembles the R. viridis Reaction Center Architecture Type II photosystems of higher plants green algae and cyanobacteria contain more than 20 subunits and are considerably more complex than the R viridis reaction center. The structure of PSII from the thermophilic cyanobacterium CENGAGENOW ACTIVE FIGURE Photophosphorylation. Photoexcitation of the R. viridis RC leads to reduction of a quinone Q to form QH2. Oxidation of QH2 by the cytochrome bc1 complex leads to H translocation for ATP synthesis by the F- F0-ATP yourself on the concepts in this figure at login. 644 Chapter 21 Photosynthesis CENGAGENOW Go to CengageNOW and click CengageInteractive to explore the R. viridis reaction center a complex scaffold for transduction of light .
