lecture #18 - oregon state university · lecture #18 “carbon concentrating mechanism” rubisco...
TRANSCRIPT
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Lecture #18 “Carbon Concentrating
Mechanism”
RubisCO is CO2 limited
RubisCO is a carboxylase and an oxygenase with low affinity for CO2
Conditions that drive the evolution of CO2 Concentrating Mechanisms
Earliest fossil evidence of plants with a CO2 Concentrating Mechanism 12 Mya. Kranz anatomy of C4 plants Very little evidence prior to 12 Mya.
Conditions that drive the evolution of CO2 Concentrating Mechanisms
Both the earth system and biology follow Dollo’s
Principle
• “An organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors”
• More generally referred to as contingency
• Both the earth and biology follow this principle
Bridgham, J. T., E. A. Ortlund, AND J. W. Thornton. 2009. An epistatic ratchet constrains the direction of glucocorticoid receptor evolution. Nature 461: 515-519.
C5
C1
2xC3
C3
C1
C4
C3 and “C4 assisted” photosythesis
pyruvate
PPDK
PPDK Pyruvate, Phosphate DiKinase
THE C4 PATHWAY IN PLANTS USES
UBIQUITOUS ENZYMES
CARBOXYLATION DECARBOXYLATION
Pep Carboxylase Malic Enzyme
NAD or NADP
Pep Carboxykinase In a green alga
Pep Carboxykinase
With C4 Intermediate Active Bicarbonate Transport
CO2 Diffusion Active CO2 Transport
HCO3- HCO3
- HCO3-
HCO3-
HCO3-
CO2
CO2 CO2
CO2
CO2 CO2
CO2
CO2
C4 acid
CO2
Biomass Biomass
Biomass Biomass
Chloro- plast CO2
HCO3-
Alternative carbon transport systems
C4 Plants
C3 Plants
Many algae
Does not exist
Carboxylation
Decarboxylation And Fixation by RubisCO
Hatch MD and Slack CR 1966. Photosynthesis by sugar-cane leaves. A new carboxylation reaction and the pathway of sugar formation. Biochemical Journal 101, 103–111
Kranz-less C4 photosynthesis
Bienertia cycloptera CO2
CO2
C4
C3
PPDKp
PEPc
MEm
RubisCO
C3
There are suggestions that single cell C4 may have been the earliest form of CO2 Concentrating Mechanism
C4 Photosynthesis
• Many grasses • Some shrubs • No trees • Many forbs and weeds
• 1% of plant species • 30% of plant photosynthesis
corn
Sugar cane
sorghum
Saltbush (Atriplex)
Pigweed (Amaranthus)
1 2 3 4
Carbonic Anhydrase 5
5
Vasc
ular
Tis
sue
CO2
GAP
Starch
Sucrose
GAP
PEPcase: initial Carbon fixation step; substrate is HCO3 and thus no affinity for O2
C4 Photosynthesis
With C4 Intermediate Active Bicarbonate Transport
HCO3- HCO3
- HCO3- HCO3
-
CO2 CO2
CO2 CO2
C4 acid
CO2
Biomass Biomass
Chloro- plast CO2
HCO3-
Acid Stable Acid Labile
14C 14C
Questions: What is the nature of carbon pool in marine diatoms? Can we determine that the CCM has an organic intermediate pool?
Nota Bene: Inhibit decarboxylation using MPA
Carbon influx Si oil centrifugation
sample of cells
fixative silicon oil
add 14C label and mix
10 sec and centrifuge
freeze in LN2
Scintillation Counter
cut off bottom Acidify half
Varied fixative and added MPA
Difference = organic carbon
About 70% of the carbon taken up in 10 s is stored as organic carbon
With C4 Intermediate Active Bicarbonate Transport
HCO3- HCO3
- HCO3- HCO3
-
CO2 CO2
CO2 CO2
C4 acid
CO2
Biomass Biomass
Chloro- plast CO2
HCO3-
What is the nature of carbon pool in marine diatoms? Organic Carbon
Question: • What is the importance of C4 carbon fixation in diatom photosynthesis? Straightforward methodology. Inhibit carbon fixation with DCDP (3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-
propenoate)
-OOC -OOC OPO32- CH2PO3
2-
C C C
Cl Cl CH2 PEP DCDP
control
+DCDP
HCl +DCDP
T. weissflogii (diatom)
Chlamydomonas sp. (C3 Chlorophyte)
control
Effect of PEPCase
inhibition by DCDP*
on diatom photosynthesis
control
+DCDP
+DCDP high CO2
+DCDP low O2 control
+DCDP
T. pseudonana PEPC1 (mito) ++ PEPC2 (CER/PPS) ++ ME 0 PEPCK 0 T. weissflogii ME 0 PEPCK 0
Key transcripts upon shift from high to low CO2
Quantitative Proteomics MudPIT
Tryptic Lys-C Digest HPLC fractions
T. pseudonana PEPC2 (CER/PPS) ++ ME 0
PEPCK - CA (3) ++ Pyruvate Carboxylase ++ plastid targeted
Proteins under low CO2
C3 & C4 Photosynthesis
Calvin Cycle
Rubisco CO2 CO2
stomata
Calvin Cycle
Rubisco CO2 CO2
stomata
C3
C4 C4 acid
• C4 has significant impacts on the photosynthesis-irradiance relationship and water use efficiency
• Minimal loss from photorespiration • Better water use efficiency (for a given CO2 uptake, C4 plants can keep
stomata more closed)
• Enhanced high-light photosynthetic rate
Advantages of C4 Photosynthesis
note
C3 C4
Advantages of C4 Photosynthesis
4
5
Vasc
ular
Tis
sue
CO2
GAP
Starch
Sucrose
GAP
Disadvantages of C4 Photosynthesis Pyruvate,orthophosphate dikinase (PPDK) in C4 plants is cold sensitive - mechanism of inactivation not fully resolved. [Dramatic difference in the responses of phosphoenolpyruvate carboxylase to temperature in leaves of C3 and C4 plants, Chinthapalli, Murmu and Raghavendra. Journal of Experimental Botany, 2003 54:707±714]
C3 C4
Disadvantages of C4 Photosynthesis
C4 Photosynthesis
CO2
CO2
HCO3 C4 CO2
NADPH
C3
NADPH
C3
ATP
Advantages • Minimal loss from
photorespiration • Better water use
efficiency
Disadvantages • Temperature
sensitivity • Still dependent on gas
exchange during the day
CAM Plants CAM: Crassulacean acid metabolism
The ‘other C4’ or ‘C4-PM’
Basic principles: • Another ‘patch’ on typical C3
photosynthesis • As with C4, initial carbon fixation in CAM is
through PEPCase • The primary objective of CAM, however, is
to allow the stomata to be CLOSED for most (or all) of the day – thus addressing a weakness of standard C4
• This is achieved by storing carbon at night
CAM Photosynthesis
C3, C4, & CAM
Calvin Cycle
Rubisco CO2 CO2
stomata
Calvin Cycle
Rubisco CO2 CO2
stomata
C3
C4 C4 acid
Calvin Cycle
Rubisco CO2 CO2
stomata CAM C4 acid
Night Day
Night
CAM Photosynthesis
The quantity of acid produced at night can be very high (up to 1.4 M H+) – the cytoplasm could not function properly at the pH this would create
CAM Photosynthesis
Day One major difference between C4 and CAM is that CAM does not require separation of pathway between different cells. It all happens within a single cell.
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Time for a break…