Phosphoenolpyruvate carboxylase
Encyclopedia
Phosphoenolpyruvate carboxylase (also known as PEP carboxylase, PEPCase, or PEPC; EC 4.1.1.31) is an enzyme
in the family of carboxy-lyases
that catalyzes the addition of bicarbonate
to phosphoenolpyruvate
(PEP) to form the four-carbon compound oxaloacetate:
This reaction is used for carbon fixation
in so-called "CAM"
and "C4" plants
where it plays a key role in photosynthesis
. The enzyme is also found in some bacteria, but not in animals or fungi.
to bicarbonate by carbonic anhydrase
, PEP carboxylase assimilates the available bicarbonate into a four-carbon compound (oxaloacetate, which is further converted to malate
) that can be stored or shuttled between plant cells. This allows for a separation of initial carbon fixation by contact with air and secondary carbon fixation into sugars by RuBisCO
during the light-independent reaction
s of photosynthesis.
In succulent CAM plants adapted for growth in very dry conditions, PEP carboxylase fixes bicarbonate during the night when the plant opens its stoma
ta to allow for gas exchange. During the day time, the plant closes the stomata to preserve water and releases CO2 inside the leaf from the storage compounds produced during the night. This allows the plants to thrive in dry climates by conducting photosynthesis without losing water through open stomata during the day.
In C4 plants, for example maize, PEP carboxylase fixes bicarbonate in the mesophyll
cells of the leaf and the resulting four-carbon compound, malate
, is shuttled into the bundle sheath cells where it releases CO2 for fixation by RuBisCO. Thus, the two processes are separated spatially, allowing for RuBisCO to operate in a low-oxygen environment to circumvent photorespiration
. Photorespiration occurs due to the inherent oxygenase
activity of RuBisCO in which the enzyme uses oxygen instead of carbon dioxide without incorporating carbon into sugars or generating ATP
. As such, it is a wasteful reaction for the plant. By comparison, C4 carbon fixation via PEP carboxylase is more efficient
.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
in the family of carboxy-lyases
Carboxy-lyases
Carboxy-lyases, also known as decarboxylases, are carbon–carbon lyases that add or remove a carboxyl group from organic compounds.These enzymes catalyze the decarboxylation of amino acids, beta-keto acids and alpha-keto acids....
that catalyzes the addition of bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
to phosphoenolpyruvate
Phosphoenolpyruvate
Phosphoenolpyruvic acid , or phosphoenolpyruvate as the anion, is an important chemical compound in biochemistry. It has the high-energy phosphate bond found in living organisms, and is involved in glycolysis and gluconeogenesis...
(PEP) to form the four-carbon compound oxaloacetate:
- PEP + HCO3- → oxaloacetate + Pi
This reaction is used for carbon fixation
Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...
in so-called "CAM"
Crassulacean acid metabolism
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. The stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide...
and "C4" plants
C4 carbon fixation
C4 carbon fixation is one of three biochemical mechanisms, along with and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon molecule present in the first product of carbon fixation in these plants, in contrast to the 3-carbon molecule products in plants. fixation is an...
where it plays a key role in photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. The enzyme is also found in some bacteria, but not in animals or fungi.
PEP carboxylase in photosynthesis
After conversion of CO2Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
to bicarbonate by carbonic anhydrase
Carbonic anhydrase
The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...
, PEP carboxylase assimilates the available bicarbonate into a four-carbon compound (oxaloacetate, which is further converted to malate
Malate
Malate is the ionized form of malic acid. It is an important chemical compound in biochemistry. In the C4 carbon fixation process, malate is a source of CO2 in the Calvin cycle....
) that can be stored or shuttled between plant cells. This allows for a separation of initial carbon fixation by contact with air and secondary carbon fixation into sugars by RuBisCO
RuBisCO
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...
during the light-independent reaction
Light-independent reaction
The light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of the thylakoid membranes. These reactions take the light-dependent reactions...
s of photosynthesis.
In succulent CAM plants adapted for growth in very dry conditions, PEP carboxylase fixes bicarbonate during the night when the plant opens its stoma
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...
ta to allow for gas exchange. During the day time, the plant closes the stomata to preserve water and releases CO2 inside the leaf from the storage compounds produced during the night. This allows the plants to thrive in dry climates by conducting photosynthesis without losing water through open stomata during the day.
In C4 plants, for example maize, PEP carboxylase fixes bicarbonate in the mesophyll
Mesophyll
Mesophyll can refer to:* Mesophyll tissue, in plant anatomy, photosynthetic parenchyma cells that lie between the upper and lower epidermis layers of a leaf...
cells of the leaf and the resulting four-carbon compound, malate
Malate
Malate is the ionized form of malic acid. It is an important chemical compound in biochemistry. In the C4 carbon fixation process, malate is a source of CO2 in the Calvin cycle....
, is shuttled into the bundle sheath cells where it releases CO2 for fixation by RuBisCO. Thus, the two processes are separated spatially, allowing for RuBisCO to operate in a low-oxygen environment to circumvent photorespiration
Photorespiration
Photorespiration, or "'photo-respiration'", is a process in plant metabolism by which RuBP has oxygen added to it by the enzyme , instead of carbon dioxide during normal photosynthesis. This is the beginning step of the Calvin-Benson cycle...
. Photorespiration occurs due to the inherent oxygenase
Oxygenase
An oxygenase is any enzyme that oxidizes a substrate by transferring the oxygen from molecular oxygen O2 to it. The oxygenases form a class of oxidoreductases; their EC number is EC 1.13 or EC 1.14....
activity of RuBisCO in which the enzyme uses oxygen instead of carbon dioxide without incorporating carbon into sugars or generating ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
. As such, it is a wasteful reaction for the plant. By comparison, C4 carbon fixation via PEP carboxylase is more efficient
Photosynthetic efficiency
The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in plants and algae. Photosynthesis can be described by the simplified chemical reactionwhere CH2O represents carbohydrates such as sugars, cellulose, and lignin.The value of the...
.