C3 Carbon Reduction Cycle


The C3 carbon reduction cycle is the primary pathway of carbon fixation in all photosynthetic organisms, reducing carbon dioxide from the atmosphere to form carbohydrates, and in higher plants, it takes place in the chloroplast stroma. The carboxylation, reduction and regeneration phases of the Calvin–Benson cycle require several enzymes whose properties have been the focus of intense study. The intermediates of the Calvin–Benson cycle serve as links with several other pathways, and photosynthetically fixed carbon is exchanged among the pathways. Control of Rubisco involves several regulation mechanisms, and some are similar to those of other enzymes of the Calvin–Benson pathway, such as thioredoxin redox control as well as multiprotein complexes. All of these enzymes and control mechanisms, as well as plant canopy structure, are being studied with the aim of improving photosynthesis for increased plant production to feed a growing population.

Key Concepts

  • Rubisco is the key carbon-fixing enzyme in photosynthesis. Its activity is inefficient for several reasons, including the competition between its carboxylase and oxygenase activities, its kinetic properties, affinity for substrate, and the temperature sensitivity of Rubisco activase.
  • The oxygenase activity of Rubisco causes photorespiration, which results in a loss of fixed carbon, especially under high temperatures. Other plants and some bacteria have carbon concentrating mechanisms or photorespiratory CO2 trapping mechanisms that allow for greater efficiency of carbon fixation. These provide opportunities for engineering of more efficient plant production.
  • The C3 cycle produces carbon compounds that are used in plant growth and development, and so is linked to several other pathways. The result is flexibility in responding to environmental changes and plant needs, but also interesting challenges in engineering photosynthesis apparatus and Calvin-Benson cycle enzymes.
  • Activities of several of the enzymes of the C3 cycle are regulated by light, which allows coordination of the pathway as a whole. The light regulation through redox processes also allows coordination with the electron transport chain. Light also has a central role in control of gene expression for enzymes, along with other environmental factors.
  • The need to increase food production has stimulated research on photosynthetic and Calvin-Benson cycle limitations. Modelling based strategies have allowed analysis of these limitations for potential areas of improvement.

Keywords: Calvin cycle; Calvin–Benson cycle; photosynthesis; light regulation; thioredoxin; Rubisco; metabolism

Figure 1. Reactions of the C3 cycle. The enzymes catalysing each step are ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco), phosphoglycerate kinase (PGK), glyceraldehydes 3‐phosphate dehydrogenase (GAPDH), triose phosphate isomerase (TPI), aldolase (ALD), fructose 1,6‐bisphosphatase (FBP), transketolase (TK), sedoheptulose 1,7‐bisphosphatase (SBP), ribulose phosphate epimerase (RPE), ribose 5‐phosphate isomerase (RPI) and phosphoribulokinase (PRK).
Figure 2. Ferredoxin/thioredoxin‐mediated light regulation of enzyme activity. Ferredoxin is reduced by electrons from the photosynthetic electron transport chain, and in turn the enzyme ferredoxin/thioredoxin reductase brings about the reduction of thioredoxin f. Finally, reduced thioredoxin f activates the target enzyme by reducing a disulfide bridge between two cysteine residues, forming two thiol groups.


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Further Reading

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Adam, Neal R(Apr 2017) C3 Carbon Reduction Cycle. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001314.pub3]