Phosphoenolpyruvate carboxylase
5625361
205592116
2008-04-14T17:31:53Z
Cambrasa
5079429
Reverted edits by [[Special:Contributions/130.64.34.214|130.64.34.214]] to last version by ClueBot
'''Phosphoenolpyruvate carboxylase''' (also known as '''PEP carboxylase''', '''PEPCase''', or '''PEPC'''; [http://www.genome.jp/dbget-bin/www_bget?enzyme+4.1.1.31 EC 4.1.1.31]) is an [[enzyme]] in the family of [[carboxy-lyases]] that catalyzes the addition of [[Carbon dioxide|CO<sub>2</sub>]] to [[phosphoenolpyruvate]] (PEP) to form the four-carbon compound [[oxaloacetate]]:
:PEP + CO<sub>2</sub> → oxaloacetate + Pi
This reaction is used for [[carbon fixation]] in so-called [[Crassulacean acid metabolism|"CAM"]] and [[C4 carbon fixation|"C4" plants]] where it plays a key role in [[photosynthesis]]. Besides plants, the enzyme is also found in some bacteria, but not in animals or fungi.<ref>[http://www.ncbi.nlm.nih.gov/sutils/blink.cgi?pid=79319816 NCBI BLINK entry for ''Arabidopsis'' PEP carboxylase]</ref>
==PEP carboxylase in photosynthesis==
Carbon fixation via PEP carboxylase assimilates the available CO<sub>2</sub> 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 CO<sub>2</sub> fixation by contact with air and secondary CO<sub>2</sub> 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 CO<sub>2</sub> during the night when the plant opens its [[stoma|stomata]] to allow for gas exchange. During the day time, the plant closes the stomata to preserve water and releases CO<sub>2</sub> 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 CO<sub>2</sub> in the [[mesophyll]] cells of the leaf and the resulting four-carbon compound shuttles into the [[bundle sheath]] cells where it releases CO<sub>2</sub> 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 [[adenosine triphosphate|ATP]]. As such, it is a wasteful reaction for the plant. By comparison, C4 carbon fixation via PEP carboxylase is more efficient.
==Notes==
{{reflist}}
==External links==
* {{MeshName|Phosphoenolpyruvate+Carboxylase}}
* [http://pfam.wustl.edu/cgi-bin/getdesc?name=PEPcase Overview at wustl.edu]
* [http://www.gla.ac.uk/ibls/BMB/hgn/resint.htm Diagram of mechanism at gla.ac.uk]
* [http://3e.plantphys.net/printer.php?ch=8&id=123 Diagram of reaction at plantphys.net]
{{enzyme-stub}}
{{Carbon-carbon lyases}}
[[Category:Enzymes]]
[[Category:Photosynthesis]]