The Calvin Cycle

The key enzyme in the Calvin cycle is ribulose bisphosphate carboxylase; it can make up 15% of a leaf cell's protein. Ribulose bisphosphate carboxylase is a very large enzyme; it has a mass of 550 kdalton and consists of 16 subunits. It is unusual among enzymes utilizing oxidized, inorganic carbon; it uses gaseous CO2 as its substrate (not bicarbonate):

ribulose-1,5-bisphosphate + CO2 ----> enediol intermediate ----> hydrated intermediate ----> 2 X 3-phosphoglycerate (DG = -51.9 kJ/mol)

This reaction is followed by a phosphorylation and a reduction using the ATP and NADPH produced in the light reactions.

3-PG + ATP ----> 1,3-BPG + ADP

1,3-BPG + NADPH ----> glyceraldehyde-3-P + NADP+

These reactions are the reverse of reactions found in glycolysis (which came first?!)

Six of these trioses (glyceraldehyde-3-P) will be used to form three hexoses (fructose bisphosphate); this reaction is also the reverse of a reaction found in glycolysis (aldolase). One of these fructose bisphosphates is drawn off and converted to product (glucose):

FBP ----> F6P ----> G6P ----> G1P + ATP ----> amylose

Regeneration reactions are required for the Calvin cycle, just as they are in the TCA cycle and pentose phosphate pathway. Ribulose bisphosphate is regenerated via a series of transketolase and transaldolase reactions acting on the remaining two fructose bisphosphates:

2 F 6-P + 2 G 3-P ----> 2 xylulose 5-P + 2 E 4-P

2 E 4-P + 2 DHAP ---> 2 sedoheptulose 1,7-bisphosphate

2 sedoheptulose 1,7-bisphosphate + 2 G 3-P---> 2 ribose 5-P + 2 xylulose 5-P

The xyluloses and riboses are converted to ribulose-5-phosphate, which is then phosphorylated:

6 ribulose 5-P + 6 ATP ----> 6 ribulose bisphosphate

Summary the stoichiometery of the dark reactions:

6 CO2 + 18 ATP + 12 NADPH + 12 H2O ----> C6H12O6 + 18 ADP +

18 Pi + 12 NADP+ + 6 H+ + O2 + 6 H2O

Summary the stoichiometery of the light reactions:

Producing one mole of hexose (glucose) from carbon dioxide and water requires 2870 kJ of energy.

48 photons are required to make 12 NADPH

(2 photons per electron and 2 electrons per NADPH yields 48 photons per 12 NADPH)

48 Einsteins of light (an einstein is a mole of photons) of wavelength 650 nm contain 8000 kJ of energy (this value is obtained from the equation E = hn).

Regulation of photosynthesis:

Of all of the variety of ways in which enzyme activity can be regulated, regulation of photosynthesis is unusual; its regulation is based on the redox state of the chloroplast.

Under conditions of strong radiation, the levels of reduced ferredoxin (in photosystem-I) increase. The redox capacity in the ferredoxin can be transferred to a protein called a thioredoxin; this reaction is catalyzed by the enzyme ferredoxin-thioredoxin reductase.

The reduced thioredoxin, in turn, reduces disulfides in a series of enzymes involved in the dark reactions of photosynthesis.

reduced thioredoxin ----> reduces disulfides in:

ribulose bisphosphate carboxylase,

sedoheptulose-1,7-bisphosphatase,

glyceraldehyde-3-phosphate dehydrogenase

ribulose-5-phosphate kinase

Thereby activating these enzymes!!

Photorespiration: the two competing reactions

Ribulose 1,5-bisphosphate carboxylase binds carbon dioxide gas as one of its substrates. The energy of this binding is modest. Oxygen is similar in size and electronic configuration to carbon dioxide, which means it can be bound by ribulose 1,5-bisphosphate carboxylase. When this occurs, we call the process photorespiration:

ribulose 1,5-bisphosphate + O2 ----> 2 phosphoglycolate

The C-4 plants (see below) maintain higher levels of CO2 in the chloroplasts to reduce photorespiration.

C-4 Plants:

C-4 plants; such as sugarcane, maize, other tropical grasses; have devised a system for pumping carbon dioxide into the tissue where the Calvin cycle is located.

The process used by C-4 plants was discovered by carrying out labeling experiments just like those used to discover the Calvin cycle:

a. incubate plants with 14CO2

b. isolate and identify the labeled compounds (they turned out to be malate and aspartate - not 3-phosphoglycerate!)

c. these are C-4 compounds, not C-3 as observed in Calvin cycle, hence the name of these plants

Reactions that occur in the mesophyll cell:

a. pyruvate + ATP ----> phosphoenolpyruvate + AMP + 2Pi

b. phosphoenolpyruvate + CO2 ----> oxaloacetate

c. oxaloacetate + NADPH ----> malate + NADP+

Reactions that occur in the bundle-sheath cell:

a. malate ----> pyruvate + CO2

Shuttle movements: malate moves from the (outer) mesophyll cells to the (inner) bundle-sheath cells; pyruvate moves in the opposite direction.

C-4 plants expend 12 additional moles of ATP per hexose relative to C-3 plants (6 CO2 x 2 ATP each = 12 ATP). This energy acts just to increase the CO2 concentration in the bundle sheath cells! There are two reasons why the use of this energy is reasonable:

a. CO2 is rate-limiting at high levels of illumination (as in the tropics)