Phosphoenolpyruvate carboxykinase
5625565
210148702
2008-05-04T18:20:55Z
Arcadian
104523
/* Classification */ wik
:''Not to be confused with [[Phosphoenolpyruvate carboxylase]], PEPC''
'''Phosphoenolpyruvate carboxykinase''' (or '''PEPCK''') is an [[enzyme]] used in the natural process of [[gluconeogenesis]]. It converts [[oxaloacetate]] into [[phosphoenolpyruvate]] and [[carbon dioxide]].
==Reversibility==
Whereas most reactions of [[gluconeogenesis]] can 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.
For example, its structure and its specificity differ in humans, ''Escherichia coli'' (''[[E. coli]]''), and the parasite ''[[Trypanosoma cruzi]]''.<ref name="Trapani">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. </ref>
In mammals, it is most abundant in the liver, kidney, and adipose tissue.<ref name="Chakravarty">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.</ref>
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]]''.
In an effort to explore the role of PEPCK, researchers caused the overexpression of PEPCK in ''E. coli'' bacteria via [[recombinant DNA]].<ref name="Aich">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.</ref>
==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.<ref name="Vanderbilt ">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</ref>
A recent study suggests that the role that PEPCK plays in gluconeogenesis may be mediated by the [[citric acid cycle]], the activity of which was found to be directly related to PEPCK abundance.<ref name="Burgess">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. </ref>
PEPCK levels alone were not found to be highly correlated with gluconeogenesis in the mouse liver, as previous studies have suggested.<ref name="Burgess"> </ref> Therefore, the role of PEPCK in gluconeogenesis may be more complex and involve more factors than was previously believed.
PEPCK of ''[[Mycobacterium tuberculosis]]'' has been shown to trigger the immune system in mice by increasing [[cytokine]] activity.<ref name="Liu">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. </ref>
As a result, it has been found that PEPCK may be an appropriate ingredient in the development of an effective subunit vaccination for [[tuberculosis]].<ref name="Liu"> </ref>
== PEPCK in plants and bacteria ==
PEPCK acts in plants that undergo [[C4 carbon fixation]], where its action has been localized to the [[cytosol]], in contrast to mammals, where it has been found that PEPCK works in [[mitochondria]].<ref name="Voznesenskaya">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. </ref>
Although it is found in many different parts of plants, it has been seen only in specific cell types, including the areas of the [[phloem]].<ref name="Chen">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. </ref>
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.<ref name="Chen"> </ref>
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.
In leaves, 5% [[CO2]] content in the atmosphere lead to higher PEPCK abundance.<ref name="Chen"> </ref>
== Structure ==
The structures formed when PEPCK complexes with other substances provide insight into the structure and also the mechanism of PEPCK enzymatic activity.<ref name="Holyoak">Holyoak, T., Sullivan, S.M., & Nowak, T. (2006). Structural insights into the mechanism of PEPCK catalysis. Biochemistry, 45(27), 8254-8263.</ref>
The three mitochondrial isoforms of PEPCK complex with Mn<sup>2+</sup>, Mn<small>2+</small>-[[phosphoenolpyruvate]] (PEP), and Mn<sup>2+</sup>-[[malonate]]- Mn<sup>2+</sup> + GDP to give more information about its structure and how this enzyme catalyzes reactions.<ref name="Holyoak"> </ref>
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.<ref name="Delbaere">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. </ref>
Phosphoryl groups are transferred during PEPCK action, which is likely facilitated by the [[eclipsed conformation]] of the phosphoryl groups when ATP is bound to PEPCK.<ref name="Delbaere"> </ref>
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. <ref name="Delbaere"> </ref>
==Reaction Pathway==
PEPCase converts [[oxaloacetate]] into [[phosphoenolpyruvate]] and [[carbon dioxide]].
<gallery>
Image:Oxaloacetic acid.png|[[oxaloacetate]]
Image:Phosphoenolpyruvic acid.svg|[[phosphoenolpyruvate]]
</gallery>
As PEPCK acts at the junction between [[glycolysis]] and the Krebs cycle, it causes [[decarboxylation]] of a C4 molecule, creating a C3 molecule. As the first committed step in gluconeogenesis, PEPCK decarboxylates, and [[phosphorylate]]s [[oxaloacetate]] (OAA) for its conversion to PEP, when GTP is present. As a phosphate is transferred, the reaction results in a GDP molecule.<ref name="Holyoak "> </ref> It is interesting to note that, 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.<ref name="Zamboni">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 Engineering, 6(4), 277-284. </ref> 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.<ref name="Zamboni"> </ref>
In [[Fermentation (biochemistry)|fermentation]], PEPCK catalyzes the reaction of PEP and carbon dioxide to OAA and ADP is therefore converted to ATP with the addition of a phosphate group.<ref name="Chao ">Chao YP, Liao JC. (1994). Metabolic responses to substrate futile cycling in Escherichia coli. Journal of Biological Chemistry,269(7): 5122-6.</ref>
==Regulation==
===In humans===
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]].<ref name="Obrien">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.</ref> 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.<ref name="Obrien"> </ref> PEPCK activity is also inhibited by [[hydrazine]] sulfate, and the inhibition therefore decreases the rate of gluconeogenesis.<ref name="Mazzio">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. </ref>
In prolonged [[acidosis]], PEPCK is upregulated in [[renal proximal tubule brush border cell]]s, in order to secrete more [[ammonia|NH<sub>3</sub>]] and thus to produce more [[bicarbonate|HCO<sub>3</sub><sup>-</sup>]].<ref name=boron858> {{cite book |author=Walter F., PhD. Boron |title=Medical Physiology: A Cellular And Molecular Approaoch |publisher=Elsevier/Saunders |location= |year= |pages= |isbn=1-4160-2328-3 |oclc= |doi=}} Page 858 </ref>
The GTP-specific activity of PEPCK is highest when Mn2+ and Mg2+ are available.<ref name="Aich"> </ref> In addition, hyper-reactive [[cysteine]] (C307) is involved in the binding of Mn2+ to the active site.<ref name="Holyoak"> </ref>
===Plants===
As discussed previously, PEPCK abundance increased when plants were watered with low-pH ammonium chloride, though high pH did not have this effect.<ref name="Chen"> </ref>
==Classification==
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:
* 4.1.1.32 - [[Guanosine triphosphate|GTP]] ([[PCK1]], [[PCK2]])
* 4.1.1.38 - [[diphosphate]]
* 4.1.1.49 - [[Adenosine triphosphate|ATP]]
== References ==
{{Reflist|2}}
==External links==
* {{MeshName|Phosphoenolpyruvate+Carboxykinase+(ATP)}}
* {{MeshName|Phosphoenolpyruvate+Carboxykinase+(GTP)}}
* {{GPnotebook|-872021957}}
* "mighty mice" (PEPCK-Cmus mice) http://blog.case.edu/case-news/2007/11/02/mightymouse
{{Carbon-carbon lyases}}
{{Glycolysis enzymes}}
[[Category:EC 4.1.1]]