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 [2]

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 [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.

• RuBisCO
• Rubisco and Crop Output
• RuBisCO (Hebrew)

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.