Photosynthesis & Photorespiration PDF

The Carbon Reaction & Photorespiration A Photosynthesis Road Map Light Stroma NADP+ Stack of thylakoids Light reactions = capture the energy from the sun and use that energy to convert ADP + Pi ATP, NADP + H+ NADPH Carbon reactions = use that ATP and NADPH to build simple carbohydrates (a 3-C m...

The Carbon Reaction & Photorespiration A Photosynthesis Road Map Light Stroma NADP+ Stack of thylakoids Light reactions = capture the energy from the sun and use that energy to convert ADP + Pi ATP, NADP + H+ NADPH Carbon reactions = use that ATP and NADPH to build simple carbohydrates (a 3-C molecule called PGAL) ADP +P Light reactions Calvin cycle Chloroplast Sugar used for • Cellular •respiration Cellulose • Starch • Other organic compounds Carbon Reactions • Occur at same time as Light Reactions • Cease soon if light energy is not available to make light reaction products http://www.ualr.edu/~botany/photosynthesis.gif Rubisco – Calvin cycle’s main player • Ribulose 1,5-Bisphosphate Carboxylase Oxygenase • Fixes CO2 & O2 • Enzyme in Calvin Cycle (1st step) • Most abundant protein on Earth – Ca. 25% total leaf protein • It’s one of the major reasons that plants require a lot of N (it’s a huge protein, and there is a lot of it in the cell) “RubisCO is found in all three domains of life: bacteria, archaea and eukaryotes. The enzyme makes up 30-50% of the soluble protein in plant leaf and it has been estimated that for every person on earth there is 5 kg of RubisCO” 10.1016/j.copbio.2017.07.017 RuBisCO consists of 8 large subunits (green and purple) and 8 small subunits (red) Rubisco is the most abundant protein on the planet; up to 40% of all protein in plant cells In all Archaeplastids, the genes for the small subunits of Rubisco have relocated to the nucleus; the plastid genome still contains the genes for the large subunits. The Carbon Reactions use the ATP and NADPH from the Light Rxns to build carbohydrates (CH2O) Substrate: RuBP (ribulose 1,5-bisphosphate, a 5-C sugar) RuBP + CO2 à 2 molecules of 3PGA G3PàRuBP 3PGAà G3P 3 overall steps Substrate: RuBP (5-C sugar) STEP 1: RuBP + CO2 à an unstable 6carbon compound (fixation) which breaks apart into two 3PGA molecules STEP 2: 3PGAàG3P (reduction) STEP 3: G3PàRuBP (regeneration) Rubisco needs an induction period à occurs after few minutes of illumination Phase 1 of the Calvin cycle: Carbon fixation The enzyme RuBisCO takes a CO2 and adds the carbon to a 5-C RuBP molecule, resulting in an unstable 6-C compound that immediately splits into two 3-C molecules (3-Phosphoglycerate, or 3PGA) Phase 2 of the Calvin cycle: Reduction This is the phase that uses the energy stored in the bonds of ATP and the high energy electron in NADPH to build the carbohydrate. ATP phosphorylates 3PGA (it transfers a Pi to it), and NADPH reduces the phosphorylated molecule into G3P (Glyceraldehyde-3phosphate), a 3-C molecule that can combine with a second G3P to form glucose. Carbonyl Carboxyl (3PGA, or PGA) (G3P,or PGAL) Phase 3 of the Calvin cycle: Regeneration of RuBP Not all G3P goes to build glucose. Some are regenerated back into RuBP (through phosphorylation) to begin the cycle again. GA3P in stroma GA3P in cell cytosol converted to sucrose (transport sugar) or glucose (metabolic sugar) 3 CO2 + 3 ribulose 1,5-biphosphate + 3 H2O + 6 NADPH + 6 H+ + 6 ATP 6 Triose phosphates + 6 NADP+ + 6 ADP + 6 Pi 5 Triose phosphates + 3 ATP + 2 H2 O 3 ribulose 1,5biphosphate + 3 ADP + 2 Pi 1 Triose phosphate used for glucose & others OVERALL EQUATION 3 CO2 + 5 H2O + 6 NADPH + 9 ATP Glyceraldehyde 3-phosphate + 6 NADP+ + 9 ADP + 8 Pi DURING THE DAY STARCH: accumulates in Chloroplasts SUCROSE: flows from leaves to sink tissues Two big photosynthetic predicaments: 1- Opening stomata to take up CO2 also allows water to escape 2- RuBisCO (ribulose 1,5-bisphosphate carboxylase oxygenase) is an oxygenase too!! Meaning, it can also grab onto an O2 instead of a CO2 and tack it onto the RuBP, resulting in a 5 C molecule with an O2. Therefore the cell must expend a lot of ATP to restore the RuBP à “Photorespiration” When stomata close, CO2 is rapidly depleted, increasing the probability of O2 being used instead. Photorespiration • Rubisco molecule can work both ways (“carboxylase-oxygenase”) – Catalyses formation of {CH2O}n from CO2 (assimilation) – Catalyses destruction of {CH2O}n to form CO2 (photorespiration) • Unlike “dark” respiration, photorespiration has been seen as a wasteful loss of energy but it’s not entirely! • Relative rates of carboxylation vs. oxygenation determined by competition between CO2 and O2 molecules for active sites on Rubisco enzyme Photorespiration • When Rubisco fixes O2, not CO2 – Lose 1/2 C as CO2; costs 2.5 extra ATP – Take up O2 – Only occurs in light – Occurs 1 out of 4 reactions under todayʼs atmospheric [CO2] • Rate increases with temperature, increased O2, and decreased CO2 Good times… • Carboxylation highly efficient when ample CO2 – Calvin cycle efficiency almost 90% Good times……..Bad times But once CO2 starts getting depleted (e.g. when the stomata close), there is an increased probability of Rubisco grabbing onto an O2 instead!!! “The Calvin cycle utilizes a notoriously inefficient carboxylating enzyme, RuBisCO, featuring sluggish catalytic rates and an oxygenation reaction that forms toxic phosphoglycolate, which must be routed through photorespiratory pathways, releasing previously fixed carbon (Fig. 1). Consequently, reengineering RuBisCO has been a primary objective for enhancing photosynthesis (45), although most of these efforts have met with limited success.” Ducat & Silver, Curr Opin Chem Biol. 2012 Aug; 16(3-4): 337–344. 10.1016/j.copbio.2017.07.017 Rubisco can gain or lose carbon • Carboxylase – Reacts with CO2 to produce sugars – Leads to carbon gain • Oxygenase (C2 pathway) – – – – Reacts with O2 to convert sugars to CO2 Respires 20-40% of fixed carbon Known as photorespiration Photoprotection mechanism • High light, salt stress, drought – Links to other pathways – Involves 3 organelles If RuBisCO grabs an O2, it fixes it and produces phosphoglycolate *Note: this wasn’t a problem for millions of years because atmospheric O2 was relatively scarce in the atmosphere and CO2 was high. To regenerate phosphoglycolate back into RuBP, it has to go through multiple steps involving multiple organelles, requiring ATP and NADH Why is photorespiration not a complete waste? Why is photorespiration not a complete waste? • Movement of N, C and O atoms – Recovers 1 molecule NH4+ – 3 molecules of O2 are reduced – Salvages part of the C by the C2 oxidative pathway (recovers glycolate) • Also found in cyanobacteria – Mechanism to cope with increasing intercellular O2 When does photorespiration occur? • All the time, to some degree. • Rubisco affinity for CO2 80x greater than for O2, so usually photoresp. is low (~25%) • But increased photorespiration happens when: – Mesophyll CO2 concentrations get low relative to O2 levels (e.g., stomata closed) – Hot, dry conditions – Crowded plant canopies (e.g., corn field) Agricultural issues • Photorespiration lowers productivity of crops, although plants without it don’t survive. – Approaches using genetic engineering to release respired CO2 into chloroplasts à release it close to Rubisco – Plants grow faster, more biomass, more soluble sugars (less ATP spent)