Unformatted text preview: Lecture 8: Photosynthesis
Dark Reactions: Fixing CO2 to Make Sugars
1. PCR cycle [photosynthetic carbon reduction; Calvin cycle]
2. Rubisco and Photorespiration
3. Two different adaptations of plants to reduce photorespiration
A. C4 metabolism
B. CAM metabolism
4. Glyceraldehyde-3-phosphate (GA3P) to starch/sucrose The PCR cycle consists of
1. Carboxylation (CO2 fixation)
2. Reduction of 3PGA-->GA3P
3. Regeneration of RuBP The Photosynthetic
Carbon Reduction (PCR)
[The Calvin Cycle] 1. The PCR cycle condenses 1 CO2 molecule with a 5-carbon sugar (Ribulose1,51.
bisphosphate [RuBP]) to produce 2 molecules of a 3-carbon sugar (3-phosphoglycerate)
2. ATP and NADPH, generated in the light reaction, are required to reduce the product
and regenerate the 5-carbon CO2-acceptor molecule (RuBP).
and This CO2 fixation reaction is catalyzed by the enzyme
ribulose-1,5-bisphosphate carboxylase-oxygenase or RUBISCO,
the most abundant protein on earth (50% of the soluble proteins in leaves)
the [Transient and unstable]
[Transient In order for the chloroplast to continue to take up CO2, two conditions must be met
1. The product molecule 3PGA must be continually removed
2. RuBP must be regenerated to maintain an adequate supply of the acceptor molecule. RUBISCO (L8S8)
RUBISCO 1. 4X(LS)2 [L, 55kDa+S, 13kDa)
2. The L gene is on the chloroplast genome while
the S gene is encoded by the nuclear genome
3. The active site is the interface between two
of its L chains.
4. Turning over ~1011 tons of CO2 with an
extremelylow turnover rate (3/sec)
5. Providing the only link between inorganic and
organic carbon in the biosphere Regulation of Rubisco Activity
1. pH: Rubisco is more active in higher pH 2.
4. Mg2+ is absolutely required for rubisco activity
CO2 not only as a substrate but also an activator (carbamylation)
requires an additional protein: Rubisco activase (and ATP)
uncouples RuBP from decarbamylated active sites, thus promoting the access of CO 2
and Mg2+ for the carbamylation reaction Step 2: Reduction.
This reduction phase is a 2-step process that couples ATP hydrolysis
with the reduction of 3-phosphoglycerate (3PGA) to glyceraldehyde 3-phosphate (GA3P).
A. An enzyme phosphorylates ( adds a phosphate) to 3-phosphoglycerate (3PGA)
by transferring a phosphate from the ATP to form 1-3-bisphosphoglycerate (BPG).
B. Electrons from the NADPH reduce the carboxyl group of the 1-3bisphosphoglycerate to the aldehyde group of glyceraldehyde-3-phosphate (GA3P).
C. For every 3 CO2 molecules that enter the Calvin cycle,
6 GA3P are produced, only one can be counted as
a net gain. The other 5 (5 X 3C) are used to regenerate 3 molecules of RuBP (3 X 5C).
Net result: 3 CO2 ------> 1 3C glyceraldehyde-3-phosphate (GA3P) activation
activation reduction CO2
Reduction carbon currency 3. RuBP
Regeneration [dihydroxy-acetone-3-P] For making sugar
--> 1/2 glucose 6C
7C 3 CO2 + 9 ATP + 6
------> 1/2 GLUCOSE
3 ATP + 2 NADPH
are needed to fix
1 CO2 ribulose-5-P
epimerase transketolase xylulose-5-P fructose-1,6-bisphosphate isomerization
isomerase aldolase hydrolysis dihydroxyacetone-3-P
aldolase erythrose-4-P hydrolysis
ribose-5-P transketolase sedoheptulose-7-P complete oxidation of C6H12O6 to CO2 releases 2804 kJ
complete synthesis of 1 mol of
12 NADPH (12 x 217 kJ/mol)
18 ATP (18 x 29 kJ/mol) the thermodynamic efficiency
is ~ 90% [(2804/3126) x100] Photorespiration occurs when the CO2 levels inside a leaf become low.
When the CO2 levels inside the leaf drop to around 50 ppm,
Rubisco starts to combine O2 with RuBP instead of CO2.
The net result of this is that instead of producing 2 molecules of 3C 3PGA,
only one molecule of 3PGA
and a toxic 2C molecule called phosphoglycolate are produced.
phosphoglycolate High Temperature Favors Oxygenation
High CO2 and O2 compete for binding to the same site on RUBISCO
When [CO2] = [O2], the carboxylation rate is 80 x of the rate of oxygenation, the
In an aqueous solution [CO2]/[O2] = 0.0416
Thus, carboxylation outruns the oxygenation by a 3-1 factor
When temperature increases,
1. The affinity of RuBP with O2 increases
2. The aqueous [\O2]/[CO2] ratio also increases
Oxygenation outruns carboxylation The plant must get rid of the phosphoglycolate (TOXIC)!
1. First it immediately gets rid of the phosphate group, converting it to glycolic acid (glycolate)
(happens in chloroplasts).
2. The glycolate is then transported to the peroxisome and there converted to a glycine.
3. Two glycine are then transported into mitochondria where they are converted into a serine
(with the loss of a CO2 and 1 NH3 and generation of 1 NADH).
4. The serine is then converted back to 3PGA in 3 reactions (using 1 ATP and 1 NADH) .
All these conversions cost the plant energy and results in the net lost of CO2 from the plant. α-ketoglutarate
Glutamate photosynthetic carbon
2 RuBP + 4 O2 + 2 glutamate + α-ketoglutarate --> 3 PGA + 2 α-ketoglutarate + O2 + CO2 + NH3 + glutamate
2 RuBP + 3 O2 + glutamate --> 3 PGA + α-ketoglutarate + CO2 + NH3 (costing 1 ATP)
NH (costing NH3+α-ketoglutarate --> Glutamate
Cost 1 ATP and 1 NADH or Fd red
recovering 3 C from 2 phosphoglycolate
Costs 2 ATP and 1 FdRed (NADPH)
Photorespiration depends on the
Photosynthetic electron transport system Can genetic engineering create
a ideal RUBISCO enzyme? Why photorespiration???
Why 3% [CO2]
3. Ambient [CO2] Recover 75% carbon from glycolate (not essential)
Protection by consuming ATP/NADPH(FdRed)
Important for nitrate assimilation? Many plants developed ways to circumvent the problems of photorespiration
by concentrating CO2 in the Rubisco environment 1. CO2 and HCO3- pumps (in cyanobacteria and green algae)
CO 2. The C4 photosynthesis:
To prevent the wasteful effects of photorespiration, some plant like corn and
sugarcane that grow in hot dry climates have evolved a different system for fixing CO2.
The anatomy of these plants leaves is different from normal leaves.
They are said to exhibit KRANZ anatomy. Spatial separation of initial CO2 fixation and the calvin cycle
1. CO2 fixation by the carboxylation of (PEP) using HCO3- in the mesophyll cells to form a C4 acid
2. Transport of the C4 acids to the bundle sheath cells
3. Decarboxylation of the C4 acids within the bundle sheath cells and generation of CO2
that is then reduced to carbohydrates via the PCR (calvin) cycle
4. Transport of the C3 acid back to mesophyll cells and regeneration of the CO2 acceptor
phosphoenolpyruvate The concentration of CO2 in bundle sheath cell has an energy cost
requires 2 ATP molecules and 2 NADPH molecules for moving one CO2 molecule
The affinity of PEP carboxylase for its substrate HCO3- is very high
Because the substrate is HCO3-, O2 is not a competitor for the initial CO2 fixation reaction
Spatial separation of the initial CO fixation and Calvin cycle. 3. CAM (Crassulacean acid metabolism), formation of the C4 acids is both
temporally and spatially separated from decarboxylation and the PCR cycle.
CAM is associated with anatomical features that minimize water loss
such as thick cuticles, low surface-volume ratios, large vacuoles
CAM plants lose 50-00 g water for 1 g of CO2 fixed.
CAM The photosynthetic assimilation of CO2 by leaves yields sucrose and starch as end products of
Two gluconeogenic pathways that are physically separated:
Starch is synthesized in the chloroplasts while sucrose is synthesized in the cytosol
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