Photosynthesis Lecture 24 Lecture Notes PDF

Lecture 24-Photosynthesis BIOMI 2900 Diverse chlorophototrophic bacteria. From Ann Rev Pl. Biol. 69: 16 (2018) S. Typhimurium uses waste products in a respiratory chain Host Salmonella typhimurium, an enteric pat...

Lecture 24-Photosynthesis BIOMI 2900 Diverse chlorophototrophic bacteria. From Ann Rev Pl. Biol. 69: 16 (2018) S. Typhimurium uses waste products in a respiratory chain Host Salmonella typhimurium, an enteric pathogen, outcompetes other bacteria Salmonella Immune response due to its ability to use tetrathionate (produced from thiosulfate during inflammation) as a terminal electron acceptor for SE Winter et al., Nature 2010  Fermentative cells and F1Fo ATPase Fermentative bacteria like lactic acid bacteria (LAB) make ATP via substrate level phosphorylation (glycolysis) LAB still need a PMF for transport, ion balance, and sometimes motility These cells use F1Fo in “reverse” – as an ATP-driven proton pump (ATPase) ATPase/synthase is reversible Pump versus waterwheel Bioenergetics: A simplified recap OUTSIDE OF THE CELL H+ H+ H+ H+ ATP synthase H+ H+ H+ H+ H+ H+ Electron transpo rt chain Donor e- H+ ADP + Pi ATP H+ e.g. NADH (from glucose) Acceptor e.g. O2 H+ INSIDE THE CELL  Glucose to CO2 overview (focusing on glucose electrons) 4 e- 2 NADH Fermentation 2 x Pyruvate 4e- 2 NADH 2 x Acetyl CoA 6 NADH Electron transport chain 2 x TCA cycle (citric acid cycle) (oxidative phosphorylation), a ton of ATP 2 FADH 16 e- 6 C0+224 electrons (stored in NADH/FADH)Anabolism (synthesis) No extra ATP  Now on to… …photosynthesis!! Using light for ATP synthesis: The simplest phototrophic system, (bacterio)rhodopsins H+ H+ Light H+ H+ Starved E. coli cell containing cloned proteorhodopsin gene The two main functions of photosynthesis 1.ATP generation (through electron transport chain, driven by light) 2. Generation of NAD(P)H for anabolic reactions (most importantly CO2 fixation), also driven by light. CO2 fixation = converting CO2 back into organic matter (e.g. glucose) = reduction of CO2  Chlorophyll a and bacteriochlorophyll a - the key molecules in photosynthesis Purple Many anoxygenic phototrophs Reaction center Antenna Green alga Infrared Ester with 4 isoprenes Often + accessory Fig 14.2 Space filling model of chl a pigments and proteins! Carotenoids - accessory pigments that give access to a wider wavelength spectrum Isoprenoids yet again with conjugated double bonds And many more (Fig 14.13) Beta carotene Lycopene: tomatoes and purple bacteria Right: purple bacterium Chlorobactene: green sulfur bacteria mutant lacking carotenoids Accessory pigments and fall colors Chlorophyll degraded, pigments Chlorophyll = green visible = fall colors The basic ingenuity: how can you use exotic electron donors that are not favorable? - lots of energy in light, can indirectly be used to force unfavorable reaction (like C02 reduction to glucose) - Basic principle: Photons have energy, can be used to ”shoot” electrons out of chlorophylls, essentially forcing an oxidation event that would not occur spontaneously  Oxygenic vs Anoxygenic Oxygenic photosynthesis 6H2O + 6CO2  Anoxygenic C6H12O6 + 6O2 12H2S + 6CO2  C6H12O6 + 6H2O + 12S ∆E0’ = -1.25 v G0’ = +2832 kJ/rxn ∆E0’ = -0.15 v (+118kJ/e-) ∆G0’ = +336 kJ/rxn (+14kJ/e-) ∴ H2O is a lousy electron donor ∴ H2S is an for CO2 fixation unfavorable electron donor for CO2 fixation but way better than H2O  Photosynthetic bacteria Today we know 7 phyla with photosynthetic members Oxygenic – Cyanobacteria (Phylum: Cyanobacteria) Anoxygenic – Purple bacteria(Phylum: Proteobacteria) – Green sulfur bacteria (Phylum: Chlorobi) – Green nonsulfur bacteria (Phylum: Chloroflexi) – Heliobacteria (Phylum: Firmicutes) – Chloracidobacteria (Phylum: Acidobacteria) – Gemmatomonas (Phylum: Gemmatomonadetes)  Purple bacteria In Alpha, Beta, Gamma Proteobacteria Usually carry out photosynthesis under anaerobic conditions PSB Two classical types: Purple sulfur bacteria (PSB) prefer reduced S compounds as e- donors – In Gammaproteobacteria – H2S  internal S0 granules  SO42- after H2S is depleted Purple nonsulfur bacteria (PNSB) prefer organic electron donors or sometimes H2 PNSB – In Alphaproteobacteria and Betaproteobacteria – Versatile: many also grow as aerobic PNSB heterotrophs Are red, pink, orange, or purple due to carotenoids Carry out photosynthesis in infolds in cell membrane Most contain bcl a absorbing at 870 nm in the reaction center Fig16.2 bacteria in their natural habitat: Green Lakes State Park near Sy ergetics of purple bacteria RC II RC= reaction center Purple bacteria fix CO2 via the reverse electron flow reductive pentose H2S  NADH  NADPH phosphate (RPP) aka Calvin cycle C III – Same pathway as plants and cyanobacteria ATP To reduce 6CO2  C6H12O6 via the RPP cycle requires: – 18 ATPs (3/CO2 - Fig 14.16 Cashing in light energy in purple bacteria Antennas Complex III Q loop + Q cycle Figure 3.25a  Time for a neighborly discussion: CCCP is a so-called “protonophore” (also called an “uncoupler”), which makes the membrane permeable to protons. Which process would be affected more, photosynthesis or oxidative phosphorylation? Or both?  Green sulfur bacteria (GSB) GSB are in the phylum Chlorobi – Phase contrast a sister to the Bacteroidetes micrograph of GSB are anaerobes and use H2S Chlorobium showing or other inorganic electron extracellular S0 donors like thiosulfate or Fe2+ granules They deposit S0 extracellularly They have powerful membrane- Culture of Chlorobium associated antenna complexes called chlorosomes Chlorosomes allow some GSB to to Chlorosome grow in deep waters or in sediments at light intensities 10-6 that at the surface Found in similar habitats to purple sulfurs but can grow at lower light Fig 14.11b Fig 15.15 and tolerate higher H2S  Electron flow in GSB In GSB, the electrons flow through an FeS protein to ferredoxin (Fd) an FeS electron carrier with a potential near -0.4 v Fd is stronger reductant than NADPH (-0.32 v) NADPH It provides electrons for a CO2 CO2 fixation pathway called the reverse tricarboxylic (citric) acid cycle GSB were the first organisms found to fix CO2 by a pathway different from the RPP cycle bchla GSB reaction centers (RC1) resemble PS I in cyanobacteria and chloroplasts  The electron donor crisis Anoxygenic photosynthetic bacteria made use of electron donors available on early earth However, they were limited by the amount of electron donor (H2S, Fe2+) in their environment H2O is an abundant electron donor (55 M) in water but a terrible one (O2/H20 = +0.82 v) Solved by organisms using both PS I and PS II, engaging in oxygenic photosynthesis Descendants are today’s cyanobacteria and chloroplasts Oxygenic photosynthesis Electron flow in oxygenic photosynthesis (chloroplast) RPP (Calvin) cycle Cytoplasm 4.67 H+? Q loop Q cycle Cyt b3f Complex III Outside The Great Oxidation Event  Cyanobacteria Chromophoric group on Originally called blue green algae protein Are in the phylum Cyanobacteria Carry out oxygenic photosynthesis Found in diverse waters and soils Have diverse morphologies Most have accessory pigments called phycobilins – Phycocyanin blue, phycoerythrin – red – Antenna pigments for PS II Fig 14.14  Cyanobacteria can be found in soils and rocks Cyanobacteria can be found in soils including in some harsh environments – Desert crusts- dried cyanobacteria that photosynthesize upon wetting – Cryptpodendolithic cyanobacteria- grow under surface of rocks- Found in deserts including Antarctica dry valleys-model for Mars? Layer of cyanobacteria under surface Desert crust- from U. of Edinburgh web site of rock- U. Guelph web site  Prochlorococcus Prochlorococcus contains chl b, like plants - instead of phycocyanin or phycoerythrin It is abundant between 40o N and 40o S at depths of 20-200 m (likes water ≥ 15o C) Most numerous photosynthetic genus on Earth (1027 cells) Is the dominant primary producer at mid latitudes and is responsible for one out of 20 breaths of oxygen we take MIT  Comparing photosystems /Plants Non cyclic electron flow Complex III Mn4Ca site RC II RC I Sippewissett Marsh, Cape Cod, MA – Microbial mats  Microbial mats: Dividing up the spectrum Diatoms - chla Cyanobacteria chla and phycocyanin Purple sulfur bacteria bcl aPurple sulfur SRB  H2S bacteria bcl a Purple sulfur bacteria bcl b Green sulfur bacteria bcl c&e in chlorosomes All have carotenoids