Carbon Fixation

Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms. See also [1].

Organisms that grow by fixing carbon are called autotrophs, which include photoautotrophs (which use sunlight), and lithoautotrophs (which use inorganic oxidation).

Seven autotrophic carbon fixation pathways are known.

Calvin cycle

3 CO₂ + 6 NADPH + 6 H⁺ + 9 ATP + 5 H₂O → glyceraldehyde-3-phosphate (G3P) + 6 NADP⁺ + 9 ADP + 8 P_i (P_i = inorganic phosphate) (See also [2])

Step 1

The enzyme RuBisCO catalyses the carboxylation of , RuBP, a 5-carbon compound, by carbon dioxide (a total of 6 carbons) in a two-step reaction. The product of the first step is enediol-enzyme complex that can capture CO2 or O2. Thus, enediol-enzyme complex is the real carboxylase/oxygenase. The CO2 that is captured by enediol in second step produces an unstable six-carbon compound called 2-carboxy 3-keto 1,5-biphosphoribotol (CKABP) (or 3-keto-2-carboxyarabinitol 1,5-bisphosphate) that immediately splits into 2 molecules of 3-phosphoglycerate, a 3-carbon compound.