Calvin cycle Flashcards
Calvin Cycle
Dark reaction
- reduces carbon dioxide to carbohydrates using NADPH and ATP produced during the light reaction of photosynthesis
- composed of 3 stages
1) Fix
2) Reduce
3) Regenerate
3 stages of Calvin cycle
1) Fix
- attach CO2 to ribulose 1,5-bisphosphate forming two molecules of 3-phosphoglycerate
2) Reduce 3-phosphoglycerate to form hexose carbohydrates
3) Regenerate ribulose 1,5-bisphosphate
Another name for rubisco
-random facts
Ribulose 1,5-Bisphosphate carboxylase/oxygenase
- most abundant enzyme on earth
- reaction rate is slow
Rubisco Structure
8 Large (L) subunits -each subunit containing a catalytic and regulatory each 8 small (S) subunits
Rubisco activase
catalyzes the formation of carbamate
-carbamate will form spontaneously but at slower rate
E amino group of R group of Lysine reacts with carbon dioxide forming carbamate
-carbamate binds Mg2+ as part of metal center
Rubisco
-Metal Center
Mg2+ is required for catalysis
Mg2+ attached to rubisco via:
- Glu
- Asp
- Lys carbamate, which requires CO2 other than substrate CO2
- water
Ribulose 1,5-bisphosphate binds and activates
Rubisco Carboxylase activity
Fix CO2 (1C) to Ribulose 1,5-bisphosphate (5C) producing a unstable intermediate (6C) which reacts to form 2 molecules of 3-phosphoglycerate
In Stroma:
-CO2 concentration= 10 uM and O2 concentration=250uM thus favored over oxygenase activitgy
Rubisco oxygenase acitivty
Ribulose 1,5-bisphosphate reacts with O2 and produces 3-phosphoglycerate (3C) and Phosphoglycolate (2C) which is a metabolic dead end
Oxygenase activity increases as temperature increases
-C4 and CAM plants use different mechanisms to overcome this “problem” allowing them to grow efficiency in hot climates
Photorespiratory reactions
- recovers the carbon skeleton for phosphoglycolate
- mitochondria
- chloroplast
- peroxisome (microsome)
Reduction of 3 phosphoglycerate to a hexose requires?
Energy Provided by light reaction:
- ATP and NADPH
- Transketolase and Aldolase enzymes
1) 2 3-Phosphoglycerate molecules converted to 1,3-BPG at the expense of ATP to ADP
2) 2 1,3-BPG molecules converted to 2 molecules of Glyceraldehyde 3-Phosphate DHAP at the expense of NADPH to NADP+
3) 2 molecules G3P is reduced to 1 molecule fructose 1,6-bisphosphate which interconverts to other hexoses in the hexose monophosphate pool
What hexoses are found in the hexose monophosphate pool
Fructose 1,6-BP interconverts to Hexose pool
Fructose 6-Phosphate-> Glucose 6-Phosphate-> Glucose 1-Phospahte
Transketolase
Transfers a COCH2OH (2C) unit of Ketose to an aldose
-Uses coenzyme TPP
Aldolase
Transfers DHAP to aldose of N carbons making a ketose (N+ 3C)
-specific for DHAP but accepts many aldehyde containing sugars
What enzymes are used to regenerate ribulose 1,5-bisphosphate
Phosphopentose isomerase
Phosphopentose epimerase
Phosphoribulose kinase
Ribose 5-Phosphate -> Ribulose 5-phosphate
-What enzyme?
Phosphopentose isomerase
Xyulose 5-phosphate-> Ribulose 5-phosphate
-what enzyme?
Phosphopentose epimerase
Ribulose 5-phosphate-> Ribulose 1,5-bisphosphate
-What enzyme?
Phosphoribulose kinase
What is the plant going to do with the fructose that is bled off?
Store it and convert it to glucose and store it as starch
Starch
Polymer of glucose alpha-1,4 glycosidic bonds and alpha 1,6-branches
- fewer branches than glycogen
- activated precursor ADP-glucose
Synthesis of Sucrose
Table sugar
-synthesized in cytoplasm
Fructose 6-Phosphate + UDP Glucose catalyzed by Sucrose-6-Phosphate Synthase
Fructose 6-Phospahte is synthesized from trioses G3P from ct via a phosphate translocater that utilizes Anitports Phosphate/G3P
Regulation of Calvin Cycle
Light reaction alters chemistry of stroma by:
1) Increased NADPH concentration in Stroma
- reduction of NADP+ by Fd-NADP+ reductase
- activates 2 enzymes (phosphoribulose kinase and Glyceraldehyde 3-Phosphate dehydrogenase) by displacing the inhibitory protein CP12
2) Increases pH from 7 to 8 in thylakoid lumen
- Pumping H+ by cyt bf
- Alkaline pH favors carbamate formation
3) Increase in Mg2+ concentration
- transported due to transport of H+
4) Increase conc in reduced Fd
- which reduces thioredoxin
Thioredoxin structure
Protein
- pair of cysteines that cycle between oxidized disulfides and reduced sulfhydryls
- contain 4Fe-4S cluster which couples 2 single e- reduction into a 2 e- reduction
Regulation by Thioredoxin
Activates biosynthetic pathway by reducing regulatory enzymes of that pathway
-Ferridoxin reduces thioredoxin which reduces regulatory enzymes in pathway
What does The location or timing of the Calvin cycle relative to Carbon fixation reduce? WhY?
Rubisco’s oxygenase activity
Why?
-to increase the local CO2 concentration relative to O2 concentration which favors photosynthesis over photorespiration
