View
3
Download
0
Category
Preview:
Citation preview
Assimilation of Carbon in Plants
Module 0210101:
Molecular Biology and Biochemistry of the Cell
Lecture 18
Dale Sanders
17 March 2009
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 end-products:
(a) starch,
(b) sucrose;
4. What photorespiration is, and how it arises;
Reading
Besides the standard big biochemistry textbooks:
Buchanan, BB et al (2000) Biochemistry &Molecular Biology of Plants. pp. 630-675
Assimilation of Carbon in PlantsThe DARK reactions of photosynthesis:
1. CO2 fixation….. Rubisco
CO2 + Ribulose 1,5–bisphosphate 2 (3-phosphoglycerate)
1 + 5 2 x 3
2. Reduction …….. Gluconeogenesis
2 (3- Phosphoglycerate) Fructose 6-Phosphate
2 ATP 2ADP
2 NADPH 2 NADP+
2 x 3 6
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
Energetics of CO2 Assimilation to
Fructose 6-phosphate (Fru 6-P)
From 6 CO2 …
1. 1 ATP consumed in phosphorylating each mole of 3-PGA
Since 6 CO2 + 6 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
3. 1 ATP consumed in regenerating each mole ofRu1,5–BP in the Calvin cycle.
Since 6 Ru 1,5-BP required for fixation of 6 CO2
6 ATP consumed here
i.e. 18 ATP, 12 NADPH consumed/6 CO2 fixed
or 3 ATP & 2 NADPH/CO2 fixed.
The Fate of Fixed CO21. Starch synthesis
Starch: a storage polysaccharide, very similar to glycogenThe 2 component polymers:
• α-amylose: unbranched α 1 4 linked glucose
• amylopectin: Glucose residue backbone with α 1 4 linksbranch points: α 1 6 links
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 + PPi
pyrophosphorylase
starch
ADP-Glu + (glucose)n (glucose)n + 1 + ADP
synthetase
Synthesis favoured when CO2 fixation rate exceeds the utilization rate ofreduced C
2. Sucrose synthesis occurs in the cytosol.
Two stages:
(a) export of reduced CO2 from the stroma
exchanger at inner envelope membrane ensures that P exportedas triose- is replenished (ultimately for phosphorylation ofADP).
Pi (ie DHAP)Triose P
(b) sucrose synthesis from triose- in the cytosol:
Sucrose: The major mobilizable sugar in plants: moved fromleaves developing tissues
roots, as respiratory metabolite
(4)Triose P (2)Fru 6 - P
Glu 1 - P
UDP – Glucose
UDP
UTP
PPi
UDP-Glucosepyrophosphorylase
Sucrose Psynthase
Sucrose 6 – P
Sucrose
PiO O
O
HOCH2
HOCH2
CH OH2
OH
OHOH
OH
HO
aldolase
This is the oxygenase function of Rubisco
The reaction is “unwanted”: an evolutionaryconsequence of [O2 ] in the atmosphere.
Rubisco is not a perfect enzyme -Photorespiration
Besides fixing CO2, Rubisco fixes O2!
Rubisco is not a perfect enzyme -Photorespiration
Ru 1,5-BP
– C – O – P
C = O
– C – OH
– C – OH
– C – O – P
O2
Rubisco+
– C – O – P
C
–O O + H2O + 2H+
Phosphoglycolate
O O–
C
– C – OH
– C – O – P3-PGA
+ H2O
O2 competes with CO2 at active site.
Recovery of C Skeletons Associated withPhosphoglycolate Formation
N.B 1. Processwasteful because C isoxidized and lost asCO2 .
2. However, ¾ of Crecovered as PGAsince glycerate inperoxisome can berecycled tochloroplast andphosphorylated.
Buchanan et al. (2000)
Summary
1. The Calvin cycle regenerates ribulose 1,5-bisphosphate as substrate for Rubisco.
2. The energetic cost of synthesizing hexose-phosphate from CO2 is 18 ATP hydrolysed and12 NADPH oxidized.
3. Fru 6P is the starting point for starch synthesis(chloroplast) and sucrose synthesis (cytosol).
4. Photorespiration involves the recovery of Cskeletons otherwise “lost” as a result ofoxygenase function of Rubisco.
Recommended