Calvin cycle

Question 1

Calvin Cycle

Answer 1

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

Question 2

3 stages of Calvin cycle

Answer 2

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

Question 3

Another name for rubisco

-random facts

Answer 3

Ribulose 1,5-Bisphosphate carboxylase/oxygenase

• most abundant enzyme on earth
• reaction rate is slow

Question 4

Rubisco Structure

Answer 4

8 Large (L) subunits
-each subunit containing a catalytic and regulatory each
8 small (S) subunits

Question 5

Rubisco activase

Answer 5

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

Question 6

Rubisco

-Metal Center

Answer 6

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

Question 7

Rubisco Carboxylase activity

Answer 7

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

Question 8

Rubisco oxygenase acitivty

Answer 8

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

Question 9

Photorespiratory reactions

Answer 9

• recovers the carbon skeleton for phosphoglycolate
• mitochondria
• chloroplast
• peroxisome (microsome)

Question 10

Reduction of 3 phosphoglycerate to a hexose requires?

Answer 10

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

Question 11

What hexoses are found in the hexose monophosphate pool

Answer 11

Fructose 1,6-BP interconverts to Hexose pool

Fructose 6-Phosphate-> Glucose 6-Phosphate-> Glucose 1-Phospahte

Question 12

Transketolase

Answer 12

Transfers a COCH2OH (2C) unit of Ketose to an aldose

-Uses coenzyme TPP

Question 13

Aldolase

Answer 13

Transfers DHAP to aldose of N carbons making a ketose (N+ 3C)
-specific for DHAP but accepts many aldehyde containing sugars

Question 14

What enzymes are used to regenerate ribulose 1,5-bisphosphate

Answer 14

Phosphopentose isomerase
Phosphopentose epimerase
Phosphoribulose kinase

Question 15

Ribose 5-Phosphate -> Ribulose 5-phosphate

-What enzyme?

Answer 15

Phosphopentose isomerase

Question 16

Xyulose 5-phosphate-> Ribulose 5-phosphate

-what enzyme?

Answer 16

Phosphopentose epimerase

Question 17

Ribulose 5-phosphate-> Ribulose 1,5-bisphosphate

-What enzyme?

Answer 17

Phosphoribulose kinase

Question 18

What is the plant going to do with the fructose that is bled off?

Answer 18

Store it and convert it to glucose and store it as starch

Question 19

Starch

Answer 19

Polymer of glucose alpha-1,4 glycosidic bonds and alpha 1,6-branches

• fewer branches than glycogen
• activated precursor ADP-glucose

Question 20

Synthesis of Sucrose

Answer 20

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

Question 21

Regulation of Calvin Cycle

Answer 21

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

Question 22

Thioredoxin structure

Answer 22

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

Question 23

Regulation by Thioredoxin

Answer 23

Activates biosynthetic pathway by reducing regulatory enzymes of that pathway
-Ferridoxin reduces thioredoxin which reduces regulatory enzymes in pathway

Question 24

What does The location or timing of the Calvin cycle relative to Carbon fixation reduce? WhY?

Answer 24

Rubisco’s oxygenase activity
Why?
-to increase the local CO2 concentration relative to O2 concentration which favors photosynthesis over photorespiration

Question 25

C3 Plant

• What
• When
• Where

Answer 25

Wheat, rice, oats, rose, soybeans

• 3PG is formed first from CO2
• Ribulose 1,5-Bisphosphate +CO2->3PG by ribulose 1,5-bisphosphate carboxylase
• Mesophyll Cells-Carbon Dioxide fixation and Calvin cycle both occur in

Question 26

C4 Plants

• What
• When
• Where

Answer 26

Corn, Sugar Cane, Bermuda

-Oxaloacetate is first forms from CO2
-PEP (C3) + CO2-> Oxaloacete (4C) by PEP carboxylase
Where?
-Mesophyll Cells-Carbon dioxide Fixation
-Bundle Sheath Cells- Calvin Cycle

Question 27

CAM Plants

Answer 27

Crassulacean Acid Metabolism
Succelant desert plants (cacti), pineapple

-Oxaloacetate (4C)-> Malate (4C)
Where? Temporal (time) Seperation
1) Night
-Stomata Open allowing CO2 to enter and carbon fixation occurs
2) Day
-Stomata close to conserve H2O and Calvin cycle occurs