C4 carbon fixation

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C₄ carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960s discovery by Marshall Davidson Hatch and Charles Roger Slack.^[1]

C₄ fixation is an addition to the ancestral and more common C₃ carbon fixation. The main carboxylating enzyme in C₃ photosynthesis is called RuBisCO, which catalyses two distinct reactions using either CO₂ (carboxylation) or oxygen (oxygenation) as a substrate. RuBisCO oxygenation gives rise to phosphoglycolate, which is toxic and requires the expenditure of energy to recycle through photorespiration. C₄ photosynthesis reduces photorespiration by concentrating CO₂ around RuBisCO.

To enable RuBisCO to work in an environment where there is a lot of carbon dioxide and very little oxygen, C₄ leaves generally contain two partially isolated compartments called mesophyll cells and bundle-sheath cells. CO₂ is initially fixed in the mesophyll cells in a reaction catalysed by the enzyme PEP carboxylase in which the three-carbon phosphoenolpyruvate (PEP) reacts with CO₂ to form the four-carbon oxaloacetic acid (OAA). OAA can then be reduced to malate or transaminated to aspartate. These intermediates diffuse to the bundle sheath cells, where they are decarboxylated, creating a CO₂-rich environment around RuBisCO and thereby suppressing photorespiration. The resulting pyruvate (PYR), together with about half of the phosphoglycerate (PGA) produced by RuBisCO, diffuses back to the mesophyll. PGA is then chemically reduced and diffuses back to the bundle sheath to complete the reductive pentose phosphate cycle (RPP). This exchange of metabolites is essential for C₄ photosynthesis to work.

Additional biochemical steps require more energy in the form of ATP to regenerate PEP, but concentrating CO₂ allows high rates of photosynthesis at higher temperatures. Higher CO₂ concentration overcomes the reduction of gas solubility with temperature (Henry's law). The CO₂ concentrating mechanism also maintains high gradients of CO₂ concentration across the stomatal pores. This means that C₄ plants have generally lower stomatal conductance, reduced water losses and have generally higher water-use efficiency.^[2] C₄ plants are also more efficient in using nitrogen, since PEP carboxylase is cheaper to make than RuBisCO.^[3] However, since the C₃ pathway does not require extra energy for the regeneration of PEP, it is more efficient in conditions where photorespiration is limited, typically at low temperatures and in the shade.^[4]