Calvin Cycle

Calvin cycle cannot be called “dark reaction” because it is still light-

dependent.

Rubisco

3-phospho-glycerate

3-phospho-glycerate

1. Carbon fixationRibulose 1,5-bisphosphate CO2

RubiscoRibulose 1,5-bisphosphate carboxylase/oxygenaseThe most abundant enzyme on the earth (50% of stromal protein)

Rubisco: 8 large subunits + 8 small subunitsLarge subunit is the catalytic subunit.

2. reduction3-phospho-glycerate ATP

1,3-bisphospho-glycerate

glyceraldehyde 3-phosphate

Triose phosphate isomerase

Dihydroxyacetone phosphate

ADP

NADPH

NADP+

3-phosphoglycerate kinase

Glyceraldehyde 3-phosphate dehydrogenasePi

F 6-P

NADP+1,3-BPGADP3-PGA kinase

Carbohydrates can both synthesized in chloroplast and cytosol by utilizing fixed carbon from chloroplast.

3-PGA

ATPNADPH

G 3-PPi G 3-P

DHAP

F 1,6-BP

DHAPFBPase-1starch

PiDHAP

G 3-P

F 1,6-BPF 6-PSucrose

Pi-triose phosphate antiporter

Triose phosphate isomeraseTrans-

aldolase

G 3-P DH

For cytosolic carbohydrate biosynthesis, triose phosphates needed are transported by Pi-triose phosphate antiporter (translocator)

Pi-Triose phosphate antiporter

This antiporter removes dihydroxyacetone phosphate from stroma into cytosol, importing Pi into stroma to ensure continuous supply of inorganic phosphate for photophosphorylation ATP synthesis.

It will also move NADPH synthesized by photorespiration into cytosol. NADPH will be converted to NADH during this process.

Most of the triose phosphates exported from chloroplast are dihydroxyacetone phosphate.

On the other hand, triose phosphates imported into chloroplast from cytosol are mostly 3-phosphoglycerate.

Fructose 6-phosphate is an important branchpoint. Cell can choose to synthesize starch or regenerate ribulose 1,5-bisphosphate from F-6-P.

Animals do not have these following enzymes so they can not perform photosynthesis:

Sedoheptulose 1,7-bisphosphataseribulose 5-phosphate kinaserubisco

3. Regeneration of RuBP

For every triose phosphate synthesized, 9 ATP and 6 NADPH are consumed.One phosphate from ATP is exported with glyceraldehyde 3-phosphate

Regulation of Carbon metabolism in plants is more complex than animals because of photosynthesis

1.thioredoxin system (photosystem I) 2.variation of H+ and Mg2+ concentration due to light exposure3.Allosteric regulation by intermediates4.covalent modification

Covalent modification

rubisco

Lys CO2

rubisco

Lys-COO

RuBP

rubisco

Lys CO2

Rubisco activase -COO

Although carbamylation at lysine residue of rubisco will activate it, ribulose 1,5-bisphosphate will inhibit this carbamylation at physiological pH. Rubisco activase will promote the ATP-dependent release of ribulose 1,5-bisphosphate, exposing lysine residue for carbamylation.Rubisco is activated after carbamylation activated rubisco will not be inhibited by ribulose 1,5-bisphosphate.

Nocturnal inhibitor also regulate photosynthesis

• Some plants synthesize 2-carboxyarabinitol 1-phosphate in the dark, which is a potent inhibitor of carbamolyated rubisco.

• It will be break down by rubisco activase or by light.

Nocturnal inhibitor is similar to the -keto acid intermediate of

rubisco reaction

p. 766

F 6-P

Light reaction result in H+ transport into stroma. This causes stromal pH to rise.Mg2+ exported from thylakoid into stroma to balance charges.pH, [Mg2+] activate

rubiscofructose 1,6-bisphosphatase

(FBPase-1)

Calvincycle

NADP+NADPHH+

Mg2+

rubisco

G 3-P

FBPase-1

Thioredoxin

/ \ S-S

Fdox

Light will cause disulfide bond reduction of these following enzymes through thioredoxin system :Ribulose 5-phosphate kinaseFructose 1,6-bisphosphataseSedoheptulose 1,7-bisphosphataseGlyceraldehyde 3-phosphate dehydrogenaseIn the dark, these enzymes will slowly re-oxidize.

PSI

FdredFdoxEnzyme

/ \HS SH

Thioredoxin

/ \HS SH

Thioredoxin

/ \ S-S

Enzyme

/ \ S-S

Enzyme

/ \ S-S

O2

p.765

Photorespiration

•Aside from being an carboxylase, Rubisco is also an oxygenase. The oxygenation of ribulose 1,5-bisphosphate produces phosphoglycolate, a metabolically useless product.•Although rubisco does have higher affinity toward CO2 (9M; O2 is 350M), the concentration of O2 (20%) is much higher than CO2 (0.04%).•Rubisco requires aqueous solution of CO2. However, the solubility of CO2 decrease abruptly at higher temperature.•On average, one photorespiration happens for every three photophosphorylation.

RuBP

CO2

O2

RuBP 3-PGA3-PGA3-PGA2-phosphoglycolaterubisco

RuBPO2

3-PGA2-phospho-glycolate

3-PGA

glycolatePi

O2

H2O2glyoxylate

-NH2

glycine

NAD+

NADHNH3CO2Serine

hydroxypyruvate-NH2

NADH

NAD+glycerate

ATP

ADP

Chloroplast Peroxisome Mitochondria

Glycine

p.767

C4 metabolism

C4 metabolism

• Because the initial trapping of CO2 in C4 metabolism involved PEP carboxylase and the production of oxaloacetate (a four carbon compound), it is called C4 metabolism.

• PEP carboxylase utilizes HCO3-, which is

structurally distinct from CO2 and O2.• Moving Calvin cycle to bundle sheath cell

will shield rubisco from any possible exposure to oxygen.

Bundle sheath cell is located in the center of the leaf

Crassulacean acid metabolism (CAM)

• Succulent plants such as cactus and pineapple grow in hot and very dry area have evolved a different strategy for carbon assimilation.

• They also separate the initial trapping of CO2 from Calvin cycle like C4 plants, but the difference between CAM and C4 is CAM separated CO2 trapping and Calvin cycle over time, not space.