Light-independent reactions

The simplified internal structure of a chloroplast

Overview of the Calvin cycle and carbon fixation

The light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of the thylakoid membranes. These reactions take the light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carbon fixation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.

Despite its name, this process occurs only when light is available. Plants do not carry out the Calvin cycle by night. They, instead, release sucrose into the phloem from their starch reserves. This process happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and Crassulacean Acid Metabolism); CAM plants store malic acid in their vacuoles every night and release it by day in order to make this process work.^[1]

Coupling to other metabolic pathways

These dark reactions are closely coupled to the thylakoid electron transport chain as reducing power provided by NADPH produced in the photosystem I is actively needed. The process of photorespiration, also known as C2 cycle, is also coupled to the dark reactions, as it results from an alternative reaction of the rubisco enzyme, and its final byproduct is also another glyceraldehyde-3-P.

Light-dependent regulation

Main article: Light-dependent reactions

Despite its widespread names (both light-independent and dark reactions), these reactions do not occur in the dark or at night. There is a light-dependent regulation of the cycle enzymes, as the third step requires reduced NADP; and this process would be a waste of energy, as there is no electron flow in the dark.

There are 2 regulation systems at work when the cycle needs to be turned on or off: thioredoxin/ferredoxin activation system, which activates some of the cycle enzymes, and the Rubisco enzyme activation, which involves its own activase.

The thioredoxin/ferredoxin system activates the enzymes glyeraldehyde-3-P dehydrogenase, glyceraldehyde-3-P phosphatase, fructose-1,7-bisphosphatase, sedoheptulose-1,7-bisphosphatase, and ribulose-5-phosphatase kinas, which are key points of the process. This happens when light is available, as the ferredoxin protein is reduced in the photosystem I complex of the thylakoid electron chain when electrons are circulating through it.^[2] Ferredoxin then binds to and reduces the thioredoxin protein, which activates the cycle enzymes by severing a cystine bond found in all these enzymes. This is a dynamic process as the same bond is formed again by other proteins that deactivate the enzymes. The implications of this process are that the enzymes remain mostly activated by day and are deactivated in the dark when there is no more reduced ferredoxin available.

The enzyme Rubisco has its own activation process, which involves a more complex process. It is necessary that a specific lysine amino acid be carbamylated in order to activate the enzyme. This lysine binds to RuBP and leads to a non-functional state if left uncarbamylated. A specific activase enzyme, called Rubisco activase, helps this carbamylation process by removing one proton from the lysine and making the binding of the carbon dioxide molecule possible. Even then the Rubisco enzyme is not yet functional, as it needs a magnesium ion to be bound to the lysine in order to function. This magnesium ion is released from the thylakoid lumen when the inner PH drops due to the active pumping of protons from the electron flow. Rubisco activase itself is activated by increased concentrations of ATP in the stroma caused by its phosphorylation.

Further reading

• Rubisco Activase, from the Plant Physiology Online website
• Thioredoxins, from the Plant Physiology Online website

External links

• The Biochemistry of the Calvin Cycle at Rensselaer Polytechnic Institute
• The Calvin Cycle and the Pentose Phosphate Pathway from Biochemistry, Fifth Edition by Jeremy M. Berg, John L. Tymoczko and Lubert Stryer. Published by W. H. Freeman and Company (2002).

References

1. ^ John C. Cushman (2001). "A Plastic Photosynthetic Adaptation to Arid Environments". Plant Physiology 127: 1439–1448. doi:10.1104/pp.010818. PMID 11743087. 
2. ^ Besse, I and Buchanan, B (1997). "Thioredoxin-linked animal and plant processes: the new generation". Bot. Bull. Acad. Sin 38: 1–11.