C4 Photosynthesis. This mechanism of photosynthesis occurs in two adjoining types of cells, the mesophyll and bundle sheath cells in plant species called C4 plants.
Both C3 and C4 cycles operate in the non-light-requiring or Dark Reactions of photosynthesis but spatially, that is, in different cells: C4 in the mesophyll cells immediately followed by C3 cycle in the bundle sheath cells.
CO2 first enters the leaf and into the mesophyll cell.
It is then hydrated to produce bicarbonate ion (HCO3-) in the cytoplasm with carbonic anhydrase (CA) as catalyst.
This is the first step in C4 photosynthesis, followed by carboxylation reaction utilizing HCO3- instead of CO2 as the inorganic carbon substrate, Hatch and Burnell (1990) emphasized.
HCO3- reacts with the three-carbon acid phosphoenolpyruvate (PEP or PEPA, C3H5O6P) to form oxaloacetate (OAA, oxaloacetic acid= C4H4O5).
The reaction is catalyzed by the carboxylating enzyme phosphoenolpyruvate carboxylase (PEPcase, PEPC, or PEPCO).
OAA is a four-carbon product, hence the term C4 photosynthesis.
(1) Hydration of CO2 (catalyzing enzyme is carbonic anhydrase):
CO2 + H2O ————> H2CO3 ———-> HCO3- + H+
(2) Carboxylation of HCO3- (catalyzing enzyme is PEPcase):
HCO3- + PEP ———->OAA
The summary reaction is commonly written as shown below in which the hydration reactions leading to the formation of HCO3- and its carboxylation are skipped :
CO2 + PEP ————————————–> OAA
OAA is then reduced to malate (malic acid= C4H6O5) or transaminated to aspartate (aspartic acid= C4H7NO4) and transported to the adjacent bundle-sheath cells.
As to malate, it is utilized in two ways: for the regeneration of PEP, and for the supply of CO2 for the succeeding C3 cycle.
First, malate is decarboxylated in which CO2 is removed and pyruvate (pyruvic acid= C3H4O3) is formed.
Pyruvate goes back to the mesophyll cell where it is phosphorylated to PEP, the CO2 acceptor in the C4 cycle.
The freed CO2 enters the C3 cycle within the bundle sheath cell.
As in C3 photosynthesis, the product of the biochemical reactions in the bundle sheath cells is the three-carbon sugar glyceraldehyde-3-phosphate (G3P, C3H7O6P), also called triose phosphate and phosphoglyceraldehyde (PGAL).
Similarly, some molecules of G3P undergo reactions to regenerate RuBP, the CO2 acceptor in the C3 cycle.
Other molecules of G3P leave the cycle and proceed with the formation of glucose and other organic compounds that plants need.
In contrast to C3 photosynthesis, the C4 photosynthetic pathway is more efficient based on resistance to photorespiration which is a wasteful process.
Unlike in C3 photosynthesis, the initial CO2-fixing enzyme PEPcase in the C4 cycle does not act as oxygenase and therefore it does not fix O2 even when it is in high concentration within the cell.
This enzyme initially fixes atmospheric CO2 in the mesophyll cells which is then delivered to the bundle sheath cells in the form of organic acids.
The C4 cycle in C4 photosynthesis, therefore, serves as a CO2-concentrating mechanism for the bundle sheath cells.
The high concentration of CO2 favors the fixing of CO2, instead of O2, by rubisco. Photorespiration is thus suppressed.
However, the C4 pathway of CO2 reduction expends more energy (5 ATP and 2 NADPH) than the C3 pathway (3 ATP and 2 NADPH) (Hopkins 1999).
Nevertheless, the former is efficient under conditions of high light intensity, high temperature, and limited water.