Fundamentals of Biochemistry Third Edition Fundamentals of Biochemistry Third Edition Chapter 19 Photosynthesis Chapter 19 Photosynthesis Copyright 2008 by John Wiley & Sons, Inc. Donald Voet Judith G. Voet Charlotte W. Pratt The basic equations Overall: 6 CO H 2 O C 6 H 12 O O 2 Two stages: The dark reaction is also known as the Calvin cycle or the reductive pentose phosphate cycle (Melvin Calvin, James Bassham and Andrew Benson, UC Berkeley (1950), The path of carbon in photosynthesis, J. Biol. Chem. 185) Figure 19-1 The chloroplast is the site of the light reaction. This organelle is similar to the mitochondrion in having its own DNA. The lumen is on the inside of the thylakoid. Figure 19-2 Chlorophyll is the principle photoreceptor. It has a heme-analogous heterocycle, with a magnesium ion stabilizing the structure. The various chromophores (R groups) alter the individual pigments maximum absorption wavelength. Figure 19-3 Other pigments fill in absorption holes of chlorophyll Figure 19-4 Photosynthetic antenna complex located on the thylakoid membrane Dark green circles are the reaction centers where Photosystem I and II are present Figure 19-5a A light-harvesting complex (LHC) that combines proteins (red and blue) with a chlorophyll pigment (green) and a lycopene carotenoid pigment (yellow) Page 645 Figure 19-6 Much of the point of the LHC is to excite electrons within the pigment, thus increasing the energy of the molecule. This energy can then be transferred to other molecules. Recall that E = h c/ and that = c/. Figure 19-7 Er, yeah. Figure 19-8 Purple photosynthetic bacterial reaction complexes (PbRC) Figure 19-9 The structure of the PbRC allows the absorption of a particular wavelength of light, e.g., P960 Figure The energy cycle of a PbRC 3 picoseconds Transfers within the LHC This process does not result in a net oxidation- reduction, since the excited electrons return to P960. The energy is used to translocate protons across the thylakoid membrane using Q B. Membrane- bound ubiquinone s Page 650 In plants and cyanobacteria, the light reaction is not cyclic. There are two photosystems (I and II), where electrons from II into I. This herbicide blocks that flow and causes photosynthetic O 2 production to cease. Figure The thylakoid membrane protein complexes are similar to those found in the mitochondrial inner membrane. There are proton translocators, and an ATP synthase, as well as electron transporters. Page 651 How the electron transporters work Figure Energy level diagram of the light reaction in plants and cyanobacteria Figure 19-13b PS II structure note the proximity of electron transporters and the oxygen-evolving center (OEC). Figure Protein-OEC and protein-electron carrier interactions Where in relation to the membrane do these reactions take place? Figure Pierre Joliet (1969, Institut de Biologie Physio- chimique, Paris) and Bessel Kok (1970, Research Institute for Advanced Studies) showed that the OEC worked in stages, since oxygen is not evolved uniformly Figure Kok cycle Figure OEC model, showing an interesting manganese ion (magenta), oxygen ion (purple) and calcium ion (blue) complex. Abstract The resolution of Photosystem II (PS II) crystals has been improved using isolated PS II from the thermophilic cyanobacterium Thermosynechococcus vulcanus. The new 1.9 resolution data have provided detailed information on the structure of the water-oxidizing complex (Umena et al. Nature 473: 55-61, 2011). The atomic level structure of the manganese-calcium cluster is important for understanding the mechanism of water oxidation and to design an efficient catalyst for water oxidation in artificial photosynthetic systems. Here, we have briefly reviewed our knowledge of the structure and function of the cluster. Biological water-oxidizing complex: a nano-sized manganese-calcium oxide in a protein environment. Najafpour MM, Moghaddam AN, Yang YN, Aro EM, Carpentier R, Eaton-Rye JJ, Lee CH, Allakhverdiev SI. Photosynth Res Oct;114(1):1-13. Epub 2012 Sep 2. Box 19-1 Bottom line: about 4 ATP and 2 NADPH (total of 9 ATP equivalents) are made for each oxygen molecule evolved, which translates to about one ATP per photon absorbed. Page 663 Dark reaction keys off of this molecule Figure carbon 5 carbons 2 times 3 carbons Figure Sugars Figure part 1 Note that step 2 will be the rate-determining, irreversible step Table 19-1 Hey, you can mess with concentrations to make reactions more favorable! Figure part 2 Figure part 3 Steps 7 and 10 also have a large negative G, and so are rate-determining steps as well. Figure part 4 If the Calvin cycle is allowed to complete, it converts five 3- carbon sugars (3PG or GAP) into three 5-carbon sugars (Ru5P). Figure 19-27c Calvin cycle is controlled by light, indirectly, through changes in pH, [Mg 2+ ] and the synthesis of a transition-state analog 2-carboxyarabinitol-1- phosphate. Page 671 Figure The enzyme ribulose-1,5,- bisphosphate carboxylase (step 2) fixes carbon. It has a very low catalytic efficiency but there are a lot of these molecules. Figure The mechanism of Ru5P carboxylase involves base catalysis (first step) Cleavage of the -keto acid is exergonic and drives the reaction. Figure The products of photosynthesis (basically, GAP) can be converted to F6P, then G1P, then starch (as shown). Note the similarity to glycogen synthesis. Page 669 Sucrose is the major transport sugar in plants to non- photosynthesizing cells. Synthesized in cytosol, so precursor transport out of the chloroplast is needed. Figure By regulating FBPase (step 7) and SBPase (step 10) using the mechanism to the right note the light sensitivity the Calvin cycle can be turned off when there is darkness, which prevents the Calvin cycle from using ATP and NADPH produced by glycolysis. Figure Plants have an alternative method of generating CO 2 and using O 2 distinct from respiration it is called photorespiration; O 2 was found to compete as a substrate for RuBP carboxylase (step 2). Figure The peroxisome is the organelle where the oxygen is used during photorespiration to yield water. This process, due to its repeated reductions and phosphorylations, uses up ATP and NADPH made by the light reaction, and thus qualifies as a futile cycle. The metabolic reason for photorespiration is unknown. The existence of RuBP oxygenase activity (NE Tolbert (1971), Annual Review of Plant Physiolology 22) was shown by the use of the then-new gas phase IR spectrometer. Figure Since photorespiration is futile, most plants have evolved a way of concentrating CO 2 so that O 2 doesnt compete for RuBP carboxylase. These plants are called C 4 plants because they use a four-carbon intermediate (incorporating a CO 2 from the atmosphere, as opposed to C 3 plants that use only GAP. C 4 plants are found more in the tropics, where heat and light are more prevalent, so photorespiration would be more of a threat.