- Not to be confused with Phosphoenolpyruvate carboxylase, PEPC
While most reactions of gluconeogenesis simply use the glycolysis enzymes in the opposite direction, the pyruvate kinase enzyme is irreversible. Therefore, the enzymes pyruvate carboxylase and phosphoenolpyruvate carboxykinase are used to provide an alternate path for effectively reversing its actions.
PEPCK in different species
PEPCK gene transcription (genetics) occurs in many species, and the amino acid sequence of PEPCK is distinct for each species.
In mammals, it is most abundant in the liver, kidney, and adipose tissue.
Researchers at Case Western Reserve University have discovered that overexpression of PEPCK in mice causes them to be more active, more aggressive, and have longer lives than normal mice; see metabolic supermice.
Function in gluconeogenesis
It has been shown that PEPCK catalyzes the reversible rate-controlling step of gluconeogenesis, the process whereby glucose is synthesized. The enzyme has therefore been thought to be essential in glucose homeostasis, as evidenced by laboratory mice that contracted diabetes mellitus type 2 as a result of the overexpression of PEPCK.
PEPCK levels alone were not found to be highly correlated with gluconeogenesis in the mouse liver, as previous studies have suggested. Therefore, the role of PEPCK in gluconeogenesis may be more complex and involve more factors than was previously believed.
PEPCK in plants and bacteria
It has also been discovered that in cucumber (Cucumis sativus L.), PEPCK levels are increased by multiple effects that are known to decrease the cellular pH of plants, although these effects are specific to the part of the plant.
PEPCK levels rose in roots and stems when the plants were watered with ammonium chloride at a low pH (but not at high pH), or with butyric acid. However, PEPCK levels did not increase in leaves under these conditions.
The structures formed when PEPCK complexes with other substances provide insight into the structure and also the mechanism of PEPCK enzymatic activity.
The three mitochondrial isoforms of PEPCK complex with Mn2+, Mn2+-phosphoenolpyruvate (PEP), and Mn2+-malonate- Mn2+ + GDP to give more information about its structure and how this enzyme catalyzes reactions.
Delbaere et al. (2004) resolved PEPCK in E. coli and found the active site sitting between a C-terminal domain and an N-terminal domain. The active site was observed to be closed upon rotation of these domains.
Since the eclipsed formation is one that is high in energy, phosphoryl group transfer has a decreased energy of activation, meaning that the groups will transfer more readily. This transfer likely happens via a mechanism similar to SN2 displacement. 
- Phosphoenolpyruvic acid.svg
As PEPCK acts at the junction between glycolysis and the Kreb’s cycle, it causes decarboxylation of a C4 molecule, creating a C3 molecule. As the first committed step in gluconeogenesis, PEPCK decarboxylates and phosphorylates oxaloacetate (OAA) for its conversion to PEP, when GTP is present. As a phosphate is transferred, the reaction results in a GDP molecule. Interestingly, when pyruvate kinase, the enzyme that normally catalyzes the reaction that converts PEP to pyruvate, is knocked out in mutants of Bacillus subtilis, PEPCK participates in one of the replacement anaplerotic reactions, working in the reverse direction of its normal function, converting PEP to OAA. Although this reaction is possible, the kinetics are so unfavorable that the mutants grow at a very slow pace or do not grow at all.
PEPCK is enhanced, both in terms of its production and activation, by many factors. Transcription of the PEPCK gene is stimulated by glucagon, glucocorticoids, retinoic acid, and adenosine 3’,5’-monophosphate (cAMP), while it is inhibited by insulin. Of these factors, insulin, a hormone that is deficient in the case of diabetes, is considered dominant, as it inhibits the transcription of many of the stimulatory elements. PEPCK activity is also inhibited by hydrazine sulfate, and the inhibition therefore decreases the rate of gluconeogenesis.
As discussed previously, PEPCK abundance increased when plants were watered with low pH ammonium chloride, though high pH did not have this effect.
It is classified under EC number 4.1.1. There are three main types, distinguished by the source of the energy to drive the reaction:
- ↑ Trapani, S., Linss, J., Goldenberg, S., Fischer, H., Craievich, A.F., & Oliva, G. (2001). Crystal structure of the dimeric phosphoenolpyruvate carboxykinase (PEPCK) from Trypanosoma cruzi at 2 A resolution. Journal of Molecular Biology, 313(5), 1059-1072.
- ↑ Chakravarty, K., Cassuto, H., Resef, L., & Hanson, R.W. (2005). Factors that control the tissue-specific transcription of the gene for phosphoenolpyruvate carboxykinase-C. Critical Reviews of Biochemistry and Molecular Biology, 40(3), 129-154.
- ↑ 3.0 3.1 Aich, S., Imabayashi, F., & Delbaere, L.T. (2003). Expression, purification, and characterization of a bacterial GTP-dependent PEP carboxykinase. Protein Expression and Purification, 31(2), 298-304.
- ↑ Vanderbilt Medical Center. “Granner Lab, PEPCK Research.” 2001. Online. Internet. Accessed 10:46PM, 4/13/07. www.mc.vanderbilt.edu/root/vumc.php?site=granner&doc=119
- ↑ 5.0 5.1 Burgess, S.C., He, T., Yan, Z., Lindner, J., Sherry, A.D., Malloy, C.R., Browning, J.D., & Magnuson, M.A. (2007). Cytosolic phosphoenolpyruvate carboxykinase does not solely control the rate of hepatic gluconeogenesis in the intact mouse liver. Cell Metabolism, 5(4), 313-320.
- ↑ 6.0 6.1 Liu, K., Ba, X., Yu, J., Li, J., Wei, Q., Han, G., Li, G., Cui, Y. (2006). The phosphoenolpyruvate carboxykinase of Mycobacterium tuberculosis induces strong cell-mediated immune responses in mice. Molecular and Cellular Biochemistry, 288(1-2), 65-71.
- ↑ Voznesenskaya, E.V., Franceschi, V.R., Chuong, S.D., & Edwards, G.E. (2006) Functional characterization of phosphoenolpyruvate carboxykinase-type C4 leaf anatomy: immuno-cytochemical and ultrastructural analyses. Annals of Botany, 98(1), 77-91.
- ↑ 8.0 8.1 8.2 8.3 Chen, Z.H., Walker, R.P., Tecsi, L.I., Lea, P.J., & Leegood, R.C. (2004). Phosphoenolpyruvate carboxykinase in cucumber plants is increased both by ammonium and by acidification, and is present in the phloem. Planta, 219(1), 48-58.
- ↑ 9.0 9.1 9.2 9.3 Holyoak, T., Sullivan, S.M., & Nowak, T. (2006). Structural insights into the mechanism of PEPCK catalysis. Biochemistry, 45(27), 8254-8263.
- ↑ 10.0 10.1 10.2 Delbaere, L.T., Sudom, A.M., Prasad, L., Leduc, Y., & Goldie, H. (2004). Structure/function studies of phosphoryl transfer by phosphoenolpyruvate carboxykinase. Biochemical and Biophysical Acta, 1697(1-2), 271-278.
- ↑ 11.0 11.1 Zamboni, N., Maaheimo, H., Szyperski, T., Hohmann, H.P., & Saber, U. (2004). The phosphoenolpyruvate carboxykinase also catalyzes C3 carboxylation at the interface of glycolysis and the TCA cycle of Bacillus subtilis. Metabolic Engin eering, 6(4), 277-284.
- ↑ Chao YP, Liao JC. (1994). Metabolic responses to substrate futile cycling in Escherichia coli. Journal of Biological Chemistry,269(7): 5122-6.
- ↑ 13.0 13.1 O’Brien, R.M., Lucas, P.C., Forest, C.D., Magnuson, M.A., & Granner, D.K. (1990). Identification of a sequence in the PEPCK gene that mediates a negative effect of insulin on transcription. Science, 249(4968), 533-537.
- ↑ Mazzio, E. & Soliman, K.F. (2003). The role of glycolysis and gluconeogenesis in the cytoprotection of neuroblastoma cells against 1-methyl 4-phenylpyridinium ion toxicity. Neurotoxicology, 24(1), 137-147.
- ↑ Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 858
- MeSH Phosphoenolpyruvate+Carboxykinase+(ATP)
- MeSH Phosphoenolpyruvate+Carboxykinase+(GTP)
- -872021957 at GPnotebook
- "mighty mice" (PEPCK-Cmus mice) http://blog.case.edu/case-news/2007/11/02/mightymouse
Carbohydrate metabolism: glycolysis/gluconeogenesis enzymes
|Glycolysis||Glucokinase/Hexokinase/Glucose 6-phosphatase - Glucose isomerase - Phosphofructokinase 1/Fructose 1,6-bisphosphatase - Aldolase - Triosephosphate isomerase - Glyceraldehyde 3-phosphate dehydrogenase - Phosphoglycerate kinase - Phosphoglycerate mutase - Enolase - Pyruvate kinase|
|Gluconeogenesis only||Pyruvate carboxylase - Phosphoenolpyruvate carboxykinase - from lactate (Cori cycle): Lactate dehydrogenase - from alanine (Alanine cycle): Alanine transaminase|
|Regulatory||Phosphofructokinase 2/Fructose 2,6-bisphosphatase - Bisphosphoglycerate mutase|
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