Photosynthesis and Carbon Fixation

C4 photosynthesis cycle

The C4 pathway is a spatial addition to the Calvin cycle. CO_2 is first captured by a non-Rubisco enzyme in mesophyll cells and then released in bundle-sheath cells, where Rubisco operates. This keeps CO_2 concentrated around Rubisco and suppresses photorespiration. The first carboxylation step is catalysed by phosphoenolpyruvate carboxylase (PEPC), which combines phosphoenolpyruvate with HCO_3^- to give oxaloacetate and Pi. This reaction takes place in the cytosol of mesophyll cells, not in the chloroplast stroma.

Key terms:

• PEP carboxylase (PEPC) — the initial CO_2-fixing enzyme of C4 plants; it does not catalyse the carboxylation of ribulose 1,5-bisphosphate.
• Pyruvate orthophosphate dikinase — regenerates PEP from pyruvate using ATP, giving AMP and pyruvate phosphate.
• NADP-malic enzyme — decarboxylates malate in bundle-sheath chloroplasts to release CO_2 for the Calvin cycle.

Energy accounting is central to the NADP-malic enzyme subtype. Regenerating PEP from pyruvate hydrolyses ATP to AMP and pyruvate phosphate, which costs two high-energy phosphate bonds per CO_2 fixed. Adding the three ATP used by the Calvin cycle, the complete pathway requires 5 ATP equivalents per CO_2 assimilated.

PEPC is regulated by reversible phosphorylation. In C4 plants, a light-dependent kinase phosphorylates PEPC during the day, which keeps it active and less sensitive to inhibition by malate. At night the enzyme is dephosphorylated and inactivated.

Some numerical items mix in unrelated constants. The golden angle of spiral phyllotaxis, 360^\circ(1-1/\varphi)\approx137.5^\circ where \varphi is the golden ratio, describes leaf arrangement and has no role in carbon fixation.

C3, C4 and CAM carbon fixation pathways

Plants are classified by where and when CO_2 is first fixed into a stable product. In C3 plants, Rubisco carboxylates ribulose 1,5-bisphosphate, and the reaction RuBP + CO_2 + H_2O gives two molecules of 3-phosphoglycerate, a three-carbon acid. 3-Phosphoglycerate is therefore the first stable product, as in wheat.

In C4 plants, CO_2 is fixed by PEP carboxylase into oxaloacetate, a four-carbon acid, which is the first stable product. Sugarcane is a typical C4 plant, and the separation of fixation and Calvin cycle is spatial, between mesophyll and bundle-sheath cells.

In CAM plants the same enzyme works at night. PEP and HCO_3^- give oxaloacetate and Pi, which is reduced to malate and stored in the vacuole. The malate is decarboxylated during the day, so the separation is temporal rather than spatial. Pineapple is a classic CAM example.

The photorespiratory C2 cycle begins in peroxisomes, where glycolate oxidase converts two glycolate and two O_2 into two glyoxylate and two H_2O_2. The conversion of 2-phosphoglycolate to glycolate and Pi is a C2 step and must not be labelled as a C4 reaction.