C4 carbon fixation

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.

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 a cellular environment where there is a lot of carbon dioxide and very little oxygen, the leaves of C4 plants 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.