Lecture 8 The O2 Problem PDF

Assimilation of CO2 into Biomass O2 The problem of oxygen 1 Changes in atmospheric O2 over geological time  Early Earth’s atmosphere was anaerobic Rich in hydrogen, methane, CO2 and ammonia Probably essential conditions for the...

Assimilation of CO2 into Biomass O2 The problem of oxygen 1 Changes in atmospheric O2 over geological time  Early Earth’s atmosphere was anaerobic Rich in hydrogen, methane, CO2 and ammonia Probably essential conditions for the evolution of life Haldane proposed that early Earth must have lacked O2 if organic compounds were to be made and persist….. …otherwise organic compounds would have been oxidised to CO2 and H2O Earliest life forms - Methanogenic bacteria Methane producing Archaea Strictly anaerobic (killed by O2) Produce methane from hydrogen and CO2, or acetate CO2 + 4 H2 ATP generation CH4 + 2 H2O CH3COOH ATP generation CH4 + CO2 Acetate and CO2 sustain anaerobic respiration by acting as terminal electron acceptors allowing production of ATP (simple “glycolysis”)  Where methanogens (methane-producing Archaea) live today Grow in habitats closely resembling that of early Earth; an atmosphere rich in CO2 and H2 eg hot volcanic, sulphurous, anaerobic springs,  Some prokaryotes became autotrophic Heterotrophs (Greek: “other-feeder”) Methanogens Selective pressure for self-generation of energy as supplies of energy-rich compounds diminish (~3.5 billion years) Autotrophs (Greek: “self-feeder”) Purple sulphur bacteria  Purple Sulphur Bacteria Photosynthetic bacteria reduce carbon to carbohydrates during photosynthesis but do not release oxygen. Hydrogen sulphide (H2S) plays the same role water plays in oxygenic photosynthesis. H2S is split, and the sulphur accumulates as globules  Cyanobacteria - the evolution of photosynthesis One of a number of organisms which evolved oxygenic photosynthesis; 2H2O O2 + 4H+ + 4e- Anabaena O2 initially absorbed by sediments. Fe2+ Fe3+ Prevented increase in atmospheric O2 Would have been lethal to methanogens Why was this development so important to life on earth? 11  O2 comes at a price – Reactive oxygen species (ROS) ROS short-lived (1 x 10-6 sec) but can extract e- from other molecules – chain reaction, oxidative damage 13 Photosynthesis 14 Detoxification of oxygen radicals Ascorbate peroxidase (APX) H2O2 + Asc H2O + MDHA 15  High light intensity causes over- reduction of photosystems and formation of ROS High energy electrons are unable to be passed to NADP+ Electron leakage to molecular oxygen  Light Absorption & Reactivity Absorption of a photon by a IC ground singlet state (GSS) S1 ISC molecule results in an excited singlet state (ESS) in which one F hv electron at the absorbing ground T1 state (So) is raised to a higher 3O 2 energy level (S1). IC P 1O The excited molecule can 2 dissipate absorbed energy and So return to its ground state in a Electronic Ground state number of ways – via internal So – ground singlet state (GSS) T1 – excited triplet state (ETS) conversion (IC), fluorescence, etc. S1 – excited singlet state (ESS) ISC – intersystem crossing Alternatively, the excited molecule IC – internal conversion ( ) P – phosphorescence ( ) can undergo intersystem crossing F – fluorescence ( ) (ISC) to the excited triplet state (T1) via a ‘forbidden’ change in electron Jablonski Diagram spin. ESS and ETS can transfer excitation energy to nearby acceptor molecules. The problem of oxygen – In a photosynthetic organism, what is level of respiration in the light? 18 CO2 exchange in leaf when placed in darkness Post-illumination burst (PIB) PIB increases with O2 Not saturated until O2 100% 19  W. Ogren: Ribulose bisphosphate carboxylase = bi-functional enzyme – can use O2 as well as CO2 Ribulose bisphosphate carboxylase/oxygenase = RUBISCO 20 Competition between CO2 and O2 21  The Glycolate Pathway of Photorespiration Complex ATP-consuming process for the recovery of C2 fragments from photorespiration. Uses three organelles Loss of C as CO2 by mitochondrial decarboxylation of glycine Ultimately, two moles of P- glycolate are converted to PGA + CO2. 22 Glycine decarboxylase (GDC) and serine hydroxy methyl transferase (SHMT) SHMT 23  Glycine Decarboxylase Is Abundant in Photosynthetic Parts of Plants GDC accounts for up to 30% of mitochondrial protein in some plant leaves 4 subunits P,L,T,H Stoichiometry: P4, H27,T9, L2 24 Mechanism of Glycine Decarboxylase Complex GDC reaction cycle THF = tetrahydrofolate 25  Serine hydroxymethyl transferase (SHMT) CH3- THF + Gly Ser + THF One carbon methylene carried on tetrahydrofolate (THF) is transferred to a second glycine, producing serine. 26 File:Folat.svg Folate  The Anaemia could be reverted by feeding patients yeast extract The ‘Wills Factor’  Discovery of Folic Acid File:Folat.svg Herschell Mitchell, 1941 Isolated from spinach leaves Folic acid from the Latin word folium (leaf) Folate Structure Tripartite molecule C1 units at various oxidation levels are attached to N5 and/or N10 Polyglutamylated in vivo Photolabile Competition between CO2 and O2, Rubisco O2 competes with CO2 for the active site. ~1 in every 3 or 4 turnovers, O2 binds ~ 30% decrease in productivity (e.g. wheat, rice, potato) 31 Is photorespiration essential? High CO2 Ambient CO2 Mutations in photorespiratory pathway are deleterious  Photorespiration Decreases plant productivity approx. 30% Consequence of oxygen in atmosphere Recovers 3 out of 4 C otherwise lost Provides turnover of reducing power and ATP (decreases photoinhibition – removes excess energy) Can it be avoided? Mutations or chemical inhibition in photorespiratory pathway are deleterious High CO2 C4 – photosynthesis (in 2 lectures time…) 33