Improving carbon fixation pathways

Abstract

A recent resurgence in basic and applied research on photosynthesis has been driven in part by recognition that fulfilling future food and energy requirements will necessitate improvements in crop carbon-fixation efficiencies. Photosynthesis in traditional terrestrial crops is being reexamined in light of molecular strategies employed by photosynthetic microbes to enhance the activity of the Calvin cycle. Synthetic biology is well-situated to provide original approaches for compartmentalizing and enhancing photosynthetic reactions in a species independent manner. Furthermore, the elucidation of alternative carbon-fixation routes distinct from the Calvin cycle raises possibilities that novel pathways and organisms can be utilized to fix atmospheric carbon dioxide into useful materials.

Figure 1. The reductive pentose phosphate pathway (Calvin cycle) and regulatory control points

Schematic of the enzymes and intermediates of the Calvin cycle. The carboxylating step involving Ribulose 1, 5-bisphosphate carboxylase oxygenase (RuBisCO) is highlighted in red, with the inorganic carbon source (carbon dioxide) highlighted in yellow. Enzymes with known or suspected rate-limiting control over Calvin cycle activity under some natural conditions are shown in bold with purple text. The oxygenase reaction of RuBisCO is displayed (dashed arrow) and the canonical plant phosphoglycolate-processing photorespiratory pathway is shown in brown. An alternative, chloroplast-specific pathway engineered into plants (23) is shown in dashed red arrows. Reactions requiring or generating ATP (orange circles) or NADPH (blue circles) are designated (although in some organisms, NADH may be substituted for NADPH in some reactions). Entry or exit of carbon dioxide (yellow circles) or oxygen (red) is denoted. Major exit points for intermediates of the Calvin cycle are designated by green arrows.

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).