Figure 2. Alternative natural carbon fixation pathways.

Schematic of the enzymes and intermediates of 4 of the 5 alternative (non-Calvin cycle), natural carbon fixation pathways utilized by autotrophic and chemolithotrophic microorganisms. All pathways use a common strategy of cycling between succinyl-CoA (center) and acetyl-CoA (circumference) in generate C2 or C3 carbon products which exit the cycle (shown in blue; acetyl-CoA, pyruvate). These pathways partially overlap one another with regard to metabolic intermediates and enzyme usage, allowing them to be diagramed as ‘spokes’ on a circle, centered on succinyl-CoA. The carboxylating steps are indicated by the incorporation of carbon dioxide (yellow circles) or bicarbonate (green circles). Reactions requiring enzymes or co-factors with partial or complete oxygen sensitivity are indicated in red (O₂), although some alternative oxygen-resistant enzymes exist for some of these reactions (4).