Module 0210101:
Molecular Biology and Biochemistry of the Cell
Lecture 18
Assimilation of Carbon in Plants
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 endproducts:
(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 Plants
The DARK reactions of photosynthesis:
1.
CO2 fixation….. Rubisco
CO2 + Ribulose 1,5–bisphosphate
1
+
2 (3-phosphoglycerate)
5
2x3
2. Reduction …….. Gluconeogenesis
2 (3- Phosphoglycerate)
Fructose 6-Phosphate
2 ATP
2ADP
2 NADPH
2 NADP+
2x3
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 of
Ru1,5–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 CO2
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
branch points: α 1 6 links
4 links
Starch synthesis is mainly in the chloroplast stroma
Fru 6-P
Glu 6-P
Phosphogluco
ISOMERASE
Glu 1–P
Phosphogluco
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 of
reduced C
2. Sucrose synthesis occurs in the cytosol.
Two stages:
(a) export of reduced CO2 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).
(b) sucrose synthesis from triose-
in the cytosol:
aldolase
(2)Fru 6 - P
(4)Triose P
Glu 1 - P
UTP
UDP-Glucose
pyrophosphorylase
PPi
UDP – Glucose
UDP
Sucrose P
synthase
Sucrose 6 – P
HOCH 2
O
HOCH2
O
OH
Pi
OH
O
HO
OH
Sucrose
CH2OH
OH
Sucrose: The major mobilizable sugar in plants: moved from
leaves
developing tissues
roots, as respiratory metabolite
Rubisco is not a perfect enzyme Photorespiration
Besides fixing CO2, Rubisco fixes O2!
This is the oxygenase function of Rubisco
The reaction is “unwanted”: an evolutionary
consequence of [O2 ] in the atmosphere.
Rubisco is not a perfect enzyme Photorespiration
O2 competes with CO2 at active site.
Phosphoglycolate
–C–O– P
C=O
O2 +
– C – OH + H2O
–C–O– P
C
Rubisco
–
O
O
O
O–
– C – OH
–C–O–P
+ H2O + 2H+
C
– C – OH
Ru 1,5-BP
–C–O– P
3-PGA
Recovery of C Skeletons Associated with
Phosphoglycolate Formation
N.B 1. Process
wasteful because C is
oxidized and lost as
CO2 .
2. However, ¾ of C
recovered as PGA
since glycerate in
peroxisome can be
recycled to
chloroplast and
phosphorylated.
Buchanan et al. (2000)
Summary
1. The Calvin cycle regenerates ribulose 1,5bisphosphate as substrate for Rubisco.
2. The energetic cost of synthesizing hexosephosphate from CO2 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.