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 [2]
3 CO2 + 6 NADPH + 6 H+ + 9 ATP + 5 H2O → glyceraldehyde-3-phosphate (G3P) + 6 NADP+ + 9 ADP + 8 Pi (Pi = inorganic phosphate) (See also [3])
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 , a 3-carbon compound.
Step 2
The enzyme phosphoglycerate kinase catalyzes the phosphorylation of 3-PGA by ATP (which was produced in the light-dependent stage). (glycerate-1,3-bisphosphate) and ADP are the products.
Step 3
The enzyme glyceraldehyde-3-phosphate dehydrogenase catalyzes the reduction of by NADPH (which is another product of the light-dependent stage). (also called G3P, GP, TP, PGAL, GAP) is produced, and the NADPH itself is oxidized and becomes NADP+. Again, two NADPH are utilized per CO2 fixed. This reaction is opposite to that in the glycolysis.
