Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Chapter 22 Photosynthesis to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Outline 22.2 The Photoreactivity of Chlorophyll 22.4 The Z Scheme of Photosynthesis 22.7 Light-Driven ATP Synthesis - Photophosphorylation 22.8 Carbon Dioxide Fixation 22.9 The Calvin-Benson Cycle Regulation of Carbon Dioxide Fixation The C-4 Pathway of CO 2 Fixation

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company The Sun - Ultimate Energy 1.5 x kJ falls on the earth each day 1% is absorbed by photosynthetic organisms and transformed into chemical energy 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O tons (!) of CO 2 are fixed globally per year Formation of sugar from CO 2 and water requires energy Sunlight is the energy source!

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Photosynthesis General Aspects Photosynthesis occurs in thylakoid membranes of chloroplasts - structures involving paired folds (lamellae) that stack to form "grana" The soluble portion of the chloroplast is the "stroma" The interior of the thylakoid vesicles is the "thylakoid space" or "thylakoid lumen" Chloroplasts possess DNA, RNA and ribosomes

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Photosynthesis Consists of Both Light Reactions and Dark Reactions The light reactions capture light energy and convert it to chemical energy in the form of reducing potential (NADPH) and ATP with evolution of oxygen The dark reactions use NADPH and ATP to drive the endergonic process of hexose sugar formation from CO 2 in a series of reactions in the stroma

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Water is the electron donor for Photosynthetic NADP + Reduction Equations 22.2 and 22.3 describe the light and dark reactions in green plants, respectively! Equation 22.4 provides a more general version Photosynthetic bacteria use H 2 S, isopropanol or other oxidizable substrates The O 2 we depend upon depends in turn on large amounts of photosynthesis on the earth!

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Chlorophyll Photoreactive, isoprene-based pigment A planar, conjugated ring system - similar to porphyrins Mg in place of iron in the center Long chain phytol group confers membrane solubility Aromaticity makes chlorophyll an efficient absorber of light

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company The Photosynthetic Unit Many chlorophylls but only a single reaction center The "unit" consists of several hundred light-capturing chlorophylls plus a pair of special chlorophylls in the "reaction center" Light is captured by one of the "antenna chlorophylls" and routed from one to the other until it reaches the reaction center See Figure 22.9

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Eukaryotic Photosystems PSI (P700) and PSII (P680) All chlorophyll is part of either LHC, PSI or PSII PSI absorbs at 700 nm PSII absorbs at 680 nm Chloroplasts given light at 680 and 700 nm simultaneously yield more O 2 than the sum of amounts when each is used alone.

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company What does each photosystem do? See Figure PSII oxidizes water (termed “photolysis") PSI reduces NADP + ATP is generated by establishment of a proton gradient as electrons flow from PSII to PSI

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company The Z Scheme An arrangement of the electron carriers as a chain according to their standard reduction potentials PQ = plastoquinone PC = plastocyanin "F"s = ferredoxins A o = a special chlorophyll a A 1 = a special PSI quinone Cytochrome b 6 /cytochrome f complex is a proton pump

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Oxygen evolution by PSII requires accumulation of four oxidizing equivalents PSII (P680) cycles through five oxidation states 1 e - is removed in each of four steps Fifth step involves H 2 O oxidized to O 2 + 4H +

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Structures of Reaction Centers R. viridis is a model! Membrane proteins (as always) are resistant to crystallization (and X-ray diffraction studies) Deisenhofer, Michel and Huber solved R.viridis structure in 1984 (Nobel Prize same year!) Four peptides: L, M, H and cytochrome No electron transfer appears to occur through M See Figures 22.16, 22.18

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company The Quantum Yield Amount of O 2 evolved per photon Four photons per reaction center - 8 total - drive the evolution of 1 O 2, reduction of 2 NADP +, and the phosphorylation of 2 and 2/3 ATP

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Photophosphorylation Light-Driven ATP Synthesis Electron transfer through the proteins of the Z scheme drives the generation of a proton gradient across the thylakoid membrane Protons pumped into the lumen of the thylakoids flow back out, driving the synthesis of ATP CF 1 -CF o ATP synthase is similar to the mitochondrial ATP synthase

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Cyclic Photophosphorylation ATP without NADPH! The photo-excited electron removed from P700 returns to P700 in a pathway indicated by the dashed line in Figure Cyclic photophosphorylation depends only on PSI, not on PSII

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Carbon Dioxide Fixation A unique ability of plants, algae, etc. Melvin Calvin at Berkeley in 1945 showed that Chlorella could take up 14 CO 2 and produce 3-phosphoglycerate What was actually happening was that CO 2 was combining with a 5-C sugar to form a 6-C intermediate This breaks down to two 3-P glycerates

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Ribulose-1,5-Bisphosphate The CO 2 Acceptor Fixation is accomplished by ribulose bisphosphate carboxylase (oxygenase), aka rubisco Probably the world's most abundant protein Study the mechanism in Figure Rubisco is activated when carbamylated (CO 2 added to Lys-201) and with Mg bound RuBP (substrate!) is inhibitor and must be released from inactive rubisco by rubisco activase. Carbamylation and Mg then activate.

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company The Calvin-Benson Cycle aka The Calvin Cycle The set of reactions that transform 3-P- glycerate into hexose sugar The only net CO 2 fixation pathway in nature A disguised gluconeogenesis pathway! With some pentose phosphate pathway reactions thrown in.... See Figure 22.25

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company Regulation of CO 2 Fixation Activities of Calvin cycle enzymes (in the stroma!) are coordinated with photosynthesis Three effects: –Light-induced pH changes –Light-induced generation of reducing power (reduced ferredoxin and NADPH) –Light-induced Mg 2+ Efflux from Thylakoids

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company The C-4 Pathway for CO 2 Fixation aka the Hatch-Slack Pathway Not an alternative to Calvin cycle, nor even a net CO 2 fixation pathway Rather, it is a CO 2 delivery system, which carries CO 2 from the O 2 -rich leaf surface to interior cells where O 2 won't compete in the rubisco reaction Oxaloacetate and malate are the CO 2 transporters Read about Crassulacean acid metabolism

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company