Assimilation of Carbon in Plants

Transcription

1 Module : Molecular Biology and Biochemistry of the Cell Lecture 18 Assimilation of Carbon in Plants Dale Sanders 17 March 2009

2 Objectives By the end of the lecture you should understand: 1. How ribulose 1,5-bisphosphate can be regenerated after C-fixation; 2. The energetic cost of fixation of inorganic C to fructose 6-phosphate; 3. The pathways for metabolism of fructose 6-phosphate into the endproducts: (a) starch, (b) sucrose; 4. What photorespiration is, and how it arises;

3 Reading Besides the standard big biochemistry textbooks: Buchanan, BB et al (2000) Biochemistry & Molecular Biology of Plants. pp

4 Assimilation of Carbon in Plants The DARK reactions of photosynthesis: 1. CO 2 fixation.. Rubisco CO 2 + Ribulose 1,5 bisphosphate 2 (3-phosphoglycerate) x 3 2. Reduction.. Gluconeogenesis 2 (3- Phosphoglycerate) Fructose 6-Phosphate 2 ATP 2ADP 2 NADPH 2 NADP + 2 x 3 6

5 3. Regeneration of Ribulose 1,5 bisphosphate. occurs via the CALVIN CYCLE Analogy with Pentose phosphate pathway: making a 5C sugar from 6C + 3C sugars. Enzymes involved Transketolase: transfers a 2C unit from a ketose to an aldose Aldolase: Condensation between DHAP + an aldehyde These reactions are + at equilibrium Cycle driven by reactions with large ve ΔG: Rubisco; Fructose 1,6-bisphosphatase; Phosphoribulose kinase

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7 Energetics of CO 2 Assimilation to Fructose 6-phosphate (Fru 6-P) From 6 CO ATP consumed in phosphorylating each mole of 3-PGA Since 6 CO Ru1,5-BP 12 PGA. 12 ATP consumed here 2. 1 NADPH consumed in reducing each mole of 1,3-BPG 12 NADPH consumed here

8 3. 1 ATP consumed in regenerating each mole of Ru1,5 BP in the Calvin cycle. Since 6 Ru 1,5-BP required for fixation of 6 CO 2 6 ATP consumed here i.e. 18 ATP, 12 NADPH consumed/6 CO 2 fixed or 3 ATP & 2 NADPH/CO 2 fixed.

9 The Fate of Fixed CO 2 1. Starch synthesis Starch: a storage polysaccharide, very similar to glycogen The 2 component polymers: α-amylose: unbranched α 1 4 linked glucose amylopectin: Glucose residue backbone with α 1 4 links branch points: α 1 6 links

10 Starch synthesis is mainly in the chloroplast stroma Fru 6-P Glu 6-P Glu 1 P Phosphogluco Phosphogluco ISOMERASE MUTASE The 2 key-reactions in starch synthesis: ADP-glucose Glu 1 P + ATP ADP Glu + PP i pyrophosphorylase ADP-Glu + (glucose) n starch (glucose) n ADP synthetase Synthesis favoured when CO 2 fixation rate exceeds the utilization rate of reduced C

11 2. Sucrose synthesis occurs in the cytosol. Two stages: (a) export of reduced CO 2 from the stroma Pi Triose P (ie DHAP) exchanger at inner envelope membrane ensures that P exported as triose- is replenished (ultimately for phosphorylation of ADP).

12 (b) sucrose synthesis from triose- in the cytosol: (4)Triose P aldolase (2)Fru 6 - P HOCH 2 OH HO Sucrose: O OH HOCH2 O UTP PPi OH O Glu 1 - P UDP-Glucose pyrophosphorylase UDP Glucose OH CH OH 2 UDP Sucrose P synthase Sucrose 6 P Sucrose The major mobilizable sugar in plants: moved from leaves developing tissues Pi roots, as respiratory metabolite

13 Rubisco is not a perfect enzyme - Photorespiration Besides fixing CO 2, Rubisco fixes O 2! This is the oxygenase function of Rubisco The reaction is unwanted : an evolutionary consequence of [O 2 ] in the atmosphere.

14 Rubisco is not a perfect enzyme - Photorespiration O 2 competes with CO 2 at active site. C O P C O P Phosphoglycolate O 2 + C = O C OH + H 2 O Rubisco C O O + H 2 O + 2H + C OH C O P Ru 1,5-BP O O C C OH C O P 3-PGA

15 Recovery of C Skeletons Associated with Phosphoglycolate Formation N.B 1. Process wasteful because C is oxidized and lost as CO However, ¾ of C recovered as PGA since glycerate in peroxisome can be recycled to chloroplast and phosphorylated. Buchanan et al. (2000)

16 Summary 1. The Calvin cycle regenerates ribulose 1,5- bisphosphate as substrate for Rubisco. 2. The energetic cost of synthesizing hexosephosphate from CO 2 is 18 ATP hydrolysed and 12 NADPH oxidized. 3. Fru 6P is the starting point for starch synthesis (chloroplast) and sucrose synthesis (cytosol). 4. Photorespiration involves the recovery of C skeletons otherwise lost as a result of oxygenase function of Rubisco.