View source for Succinyl coenzyme A synthetase

{{Short description|Class of enzymes}}
{{infobox enzyme
| Name       = Succinate—CoA ligase (GDP-forming)
| EC_number  = 6.2.1.4
| CAS_number = 9014-36-2
| GO_code    = 0045244
| image      = 2fp4.png
| width      =
| caption    = Pig GTP-specific succinyl-CoA synthetase with GTP. PDB {{PDBe|2fp4}}<ref>{{cite journal | vauthors = Fraser ME, Hayakawa K, Hume MS, Ryan DG, Brownie ER | title = Interactions of GTP with the ATP-grasp domain of GTP-specific succinyl-CoA synthetase | journal = The Journal of Biological Chemistry | volume = 281 | issue = 16 | pages = 11058–65 | date = Apr 2006 | pmid = 16481318 | doi = 10.1074/jbc.M511785200 | doi-access = free }}</ref> 
}}{{infobox enzyme
| Name       = Succinate—CoA ligase (ADP-forming)
| EC_number  = 6.2.1.5
| CAS_number = 9080-33-5
| GO_code    = 0042709
| image      = 2scu.png
| width      =
| caption    = Succinyl-COA synthetase from ''Escherichia coli''. PDB {{PDBe|2scu}}<ref>{{cite journal | vauthors = Fraser ME, James MN, Bridger WA, Wolodko WT | title = A detailed structural description of Escherichia coli succinyl-CoA synthetase | journal = Journal of Molecular Biology | volume = 285 | issue = 4 | pages = 1633–53 | date = Jan 1999 | pmid = 9917402 | doi = 10.1006/jmbi.1998.2324 }}</ref> 
}}

'''Succinyl coenzyme A synthetase''' ('''SCS''', also known as '''succinyl-CoA synthetase''' or '''succinate thiokinase''' or '''succinate-CoA ligase''') is an enzyme that [[catalysis|catalyzes]] the reversible reaction of [[succinyl-CoA]] to [[succinate]].<ref name="Voet">{{Cite book  | last1 = Voet | first1 = Donald J. | title = Biochemistry / Donald J. Voet ; Judith G. Voet | year = 2011 | publisher = Wiley, J | location = New York, NY | isbn = 978-0-470-57095-1 }}</ref>  The enzyme facilitates the coupling of this reaction to the formation of a [[nucleoside triphosphate]] molecule (either [[guanosine triphosphate|GTP]] or [[adenosine triphosphate|ATP]]) from an [[inorganic phosphate]] molecule and a nucleoside diphosphate molecule (either [[guanosine diphosphate|GDP]] or [[adenosine diphosphate|ADP]]).  It plays a key role as one of the catalysts involved in the [[citric acid cycle]], a central pathway in [[metabolism|cellular metabolism]], and it is located within the [[mitochondrial matrix]] of a cell.<ref name="Berg">{{Cite book | last1 = Berg | first1 = Jeremy M. (Jeremy M.) | last2 = Tymoczko | first2 = John L. | last3 = Stryer | first3 = Lubert. | last4 = Stryer | first4 = Lubert. Biochemistry. | title = Biochemistr | year = 2002 | publisher = W.H. Freeman | location = New York | isbn = 0-7167-3051-0 | pages = [https://archive.org/details/biochemistrychap00jere/page/475 475]–477 | url-access = registration | url = https://archive.org/details/biochemistrychap00jere }}</ref>

==Chemical reaction and enzyme mechanism==
Succinyl CoA synthetase catalyzes the following [[reversible reaction]]:

:Succinyl CoA + Pi + NDP  ↔  Succinate + CoA + NTP

where Pi denotes inorganic phosphate, NDP denotes nucleotide diphosphate (either GDP or ADP), and NTP denotes nucleotide triphosphate (either GTP or ATP).  As mentioned, the enzyme facilitates coupling of the conversion of succinyl CoA to succinate with the formation of NTP from NDP and Pi.  The reaction has a biochemical standard state [[Gibbs free energy|free energy]] change of -3.4 kJ/mol.<ref name="Berg" />  The reaction takes place by a three-step [[reaction mechanism|mechanism]]<ref name="Voet" /> which is depicted in the image below.  The first step involves displacement of [[coenzyme A|CoA]] from succinyl CoA by a [[nucleophile|nucleophilic]] inorganic phosphate molecule to form succinyl phosphate.  The enzyme then utilizes a [[histidine]] residue to remove the phosphate group from succinyl phosphate and generate succinate.  Finally, the phosphorylated histidine transfers the phosphate group to a nucleoside diphosphate, which generates the high-energy carrying nucleoside triphosphate.

[[Image:Succinyl CoA Synthetase Mechanism Revised.png|450px|Mechanism for the reaction catalyzed by Succinyl-CoA Synthetase.]]

== Structure==

=== Subunits ===

Bacterial and mammalian SCSs are made up of α and β [[protein subunit|subunits]].<ref name="Nishimura-1986">{{cite book | vauthors = Nishimura JS | title = Advances in Enzymology and Related Areas of Molecular Biology | chapter = Succinyl-CoA synthetase structure-function relationships and other considerations | volume = 58 | pages = 141–72 | year = 1986 | pmid = 3521216 | doi =  10.1002/9780470123041.ch4| isbn = 9780470123041 }}</ref>  In ''[[Escherichia coli|E. coli]]'' two αβ [[protein dimer|heterodimers]] link together to form an α<sub>2</sub>β<sub>2</sub> [[heterotetramer]]ic structure.  However, mammalian mitochondrial SCSs are active as αβ dimers and do not form a heterotetramer.<ref name="Wolodko-1986">{{cite journal | vauthors = Wolodko WT, Kay CM, Bridger WA | title = Active enzyme sedimentation, sedimentation velocity, and sedimentation equilibrium studies of succinyl-CoA synthetases of porcine heart and Escherichia coli | journal = Biochemistry | volume = 25 | issue = 19 | pages = 5420–5 | date = Sep 1986 | pmid = 3535876 | doi = 10.1021/bi00367a012 }}</ref>  
The ''E. coli'' SCS heterotetramer has been [[protein crystallography|crystallized]] and characterized in great detail.<ref name="Wolodko-1986" /><ref name="Fraser-1999">{{cite journal | vauthors = Fraser ME, James MN, Bridger WA, Wolodko J | title = A detailed structural description of escherichia coli succinly-CoA synthetase | journal = Journal of Molecular Biology | volume = 288 | issue = 3 | pages = 501 | date = May 1999 | pmid = 10329157 | doi = 10.1006/jmbi.1999.2773 | doi-access = free }}</ref>  As can be seen in Image 2, the two α subunits (pink and green) reside on opposite sides of the structure and the two β subunits (yellow and blue) interact in the middle region of the protein.  The two α subunits only interact with a single β unit, whereas the β units interact with a single α unit (to form the αβ dimer) and the β subunit of the other αβ dimer.<ref name="Wolodko-1986" />  A short amino acid chain links the two β subunits which gives rise to the tetrameric structure.

[[Image:SCS heterotetramer with ligands GNU.png|thumb|right|500px|'''Image 2:''' The ''E. coli'' Succinyl-CoA Synthetase Heterotetramer; α subunits: <span style="color:fuchsia;">pink</span> and <span style="color:lime;">green</span>, β subunits: <span style="color:gold;">yellow</span> and <span style="color:aqua;">blue</span>. Pink and yellow form the one dimer and green and blue form the other dimer. PDB ID: 1CQG]]

The [[protein crystallography|crystal structure]] of Succinyl-CoA synthetase alpha subunit (succinyl-CoA-binding isoform) was determined by Joyce et al. to a resolution of 2.10 A, with [[Protein Data Bank|PDB]] code 1CQJ. [http://www.rcsb.org/pdb/explore.do?structureId=1CQJ].<ref name="Joyce-2000">{{cite journal | vauthors = Joyce MA, Fraser ME, James MN, Bridger WA, Wolodko WT | title = ADP-binding site of Escherichia coli succinyl-CoA synthetase revealed by x-ray crystallography | journal = Biochemistry | volume = 39 | issue = 1 | pages = 17–25 | date = Jan 2000 | pmid = 10625475 | doi = 10.1021/bi991696f }}</ref>

=== Catalytic residues ===

Crystal structures for the ''E. coli'' SCS provide evidence that the coenzyme A [[molecular binding|binds]] within each α-subunit (within a [[Rossmann fold]]) in close proximity to a histidine residue (His246α).<ref name="Fraser-1999" />  This histidine residue becomes phosphorylated during the succinate forming step in the reaction mechanism.  The exact binding location of succinate is not well-defined.<ref name="Fraser-2002">{{cite journal | vauthors = Fraser ME, Joyce MA, Ryan DG, Wolodko WT | title = Two glutamate residues, Glu 208 alpha and Glu 197 beta, are crucial for phosphorylation and dephosphorylation of the active-site histidine residue in succinyl-CoA synthetase | journal = Biochemistry | volume = 41 | issue = 2 | pages = 537–46 | date = Jan 2002 | pmid = 11781092 | doi = 10.1021/bi011518y }}</ref>  The formation of the nucleotide triphosphate occurs in an [[ATP-grasp|ATP grasp]] domain, which is located near the [[N-terminus]] of the each β subunit.  However, this grasp domain is located about 35 Å away from the phosphorylated histidine residue.<ref name="Joyce-2000" />  This leads researchers to believe that the enzyme must undergo a major change in [[protein conformation|conformation]] to bring the histidine to the grasp domain and facilitate the formation of the nucleoside triphosphate.  [[Mutagenesis]] experiments have determined that two [[glutamate]] residues (one near the catalytic histidine, Glu208α and one near the ATP grasp domain, Glu197β) play a role in the phosphorylation and dephosphorylation of the histidine, but the exact mechanism by which the enzyme changes conformation is not fully understood.<ref name="Fraser-2002" />

=== Isoforms ===

Johnson et al. describe two isoforms of succinyl-CoA synthetase in [[amniote]]s, one that specifies synthesis of ATP, and one that synthesises GTP.<ref name="Johnson-1998">{{cite journal | vauthors = Johnson JD, Mehus JG, Tews K, Milavetz BI, Lambeth DO | title = Genetic evidence for the expression of ATP- and GTP-specific succinyl-CoA synthetases in multicellular eucaryotes | journal = The Journal of Biological Chemistry | volume = 273 | issue = 42 | pages = 27580–6 | date = Oct 1998 | pmid = 9765291 | doi = 10.1074/jbc.273.42.27580 | doi-access = free }}</ref>

* {{EC number|6.2.1.5}} - ATP-forming - [[SUCLA2]]
* {{EC number|6.2.1.4}} - GTP-forming - [[SUCLG2]]

In amniotes, the enzyme is a [[heterodimer]] of an α- and a β-subunit. The specificity for either [[adenosine]] or [[guanosine]] phosphates is defined by the β-subunit,<ref name="Johnson-1998" /> which is encoded by 2 genes. SUCLG2 is GTP-specific and SUCLA2 is ATP-specific, while [[SUCLG1]] encodes the common α-subunit. β variants are produced at different amounts in different tissues,<ref name="Johnson-1998" /> causing [[Guanosine triphosphate|GTP]] or [[Adenosine triphosphate|ATP]] substrate requirements.

Mostly consuming tissues such as heart and brain have more ATP-specific succinyl-CoA synthetase (ATPSCS), while synthetic tissues such as kidney and liver have the more GTP-specific form (GTPSCS).<ref name="Lambeth-2004"/> Kinetics analysis of ATPSCS from the breast muscle of pigeons and GTPSCS from pigeon liver showed that their apparent [[Michaelis constant]]s were similar for CoA, but different for the nucleotides, phosphate, and succinate. The largest difference was for succinate: K<sub>m</sub>app of ATPSCS = 5mM versus that of GTPSCS = 0.5mM.<ref name="Johnson-1998" />

== Function ==

=== Generation of nucleotide triphosphates ===

SCS is the only enzyme in the citric acid cycle that catalyzes a reaction in which a nucleotide triphosphate (GTP or ATP) is formed by [[substrate-level phosphorylation]].<ref name="Berg" />  Research studies have shown that ''E. coli'' SCSs can catalyze either GTP or ATP formation.<ref name="Fraser-1999" />  However, mammals possess different types of SCSs that are specific for either GTP (G-SCS) or ATP (A-SCS) and are native to different types of tissue within the organism.  An interesting study using [[pigeon]] cells showed that GTP specific SCSs were located in pigeon liver cells, and ATP specific SCSs were located in the pigeon breast muscle cells.<ref name="JohnsonPigeon-1998">{{cite journal | vauthors = Johnson JD, Muhonen WW, Lambeth DO | title = Characterization of the ATP- and GTP-specific succinyl-CoA synthetases in pigeon. The enzymes incorporate the same alpha-subunit | journal = The Journal of Biological Chemistry | volume = 273 | issue = 42 | pages = 27573–9 | date = Oct 1998 | pmid = 9765290 | doi = 10.1074/jbc.273.42.27573 | doi-access = free }}</ref>  Further research revealed a similar phenomenon of GTP and ATP specific SCSs in rat, mouse, and human tissue.  It appears that tissue typically involved in [[anabolism|anabolic metabolism]] (like the liver and kidneys) express G-SCS, whereas tissue involved in [[catabolism|catabolic metabolism]] (like the brain, the heart, and muscular tissue) express A-SCS.<ref name="Lambeth-2004">{{cite journal | vauthors = Lambeth DO, Tews KN, Adkins S, Frohlich D, Milavetz BI | title = Expression of two succinyl-CoA synthetases with different nucleotide specificities in mammalian tissues | journal = The Journal of Biological Chemistry | volume = 279 | issue = 35 | pages = 36621–4 | date = Aug 2004 | pmid = 15234968 | doi = 10.1074/jbc.M406884200 | doi-access = free }}</ref>

=== Formation of metabolic intermediates ===

SCS facilitates the [[metabolic flux|flux]] of molecules into other [[metabolic pathways]] by controlling the interconversion between succinyl CoA and succinate.<ref name="Labbe-1965">{{cite journal | vauthors = Labbe RF, Kurumada T, Onisawa J | title = The role of succinyl-CoA synthetase in the control of heme biosynthesis | journal = Biochimica et Biophysica Acta (BBA) - General Subjects | volume = 111 | issue = 2 | pages = 403–15 | date = Dec 1965 | pmid = 5879477 | doi = 10.1016/0304-4165(65)90050-4 }}</ref>  This is important because succinyl CoA is an intermediate necessary for [[porphyrin]], [[heme]],<ref name="Ottaway-1981">{{cite journal | vauthors = Ottaway JH, McClellan JA, Saunderson CL | title = Succinic thiokinase and metabolic control | journal = The International Journal of Biochemistry | volume = 13 | issue = 4 | pages = 401–10 | year = 1981 | pmid = 6263728 | doi = 10.1016/0020-711x(81)90111-7 }}</ref> and [[ketone body]] [[biosynthesis]].<ref name="Jenkins-1986">{{cite journal | vauthors = Jenkins TM, Weitzman PD | title = Distinct physiological roles of animal succinate thiokinases. Association of guanine nucleotide-linked succinate thiokinase with ketone body utilization | journal = FEBS Letters | volume = 205 | issue = 2 | pages = 215–8 | date = Sep 1986 | pmid = 2943604 | doi = 10.1016/0014-5793(86)80900-0 | s2cid = 23667115 | doi-access = free | bibcode = 1986FEBSL.205..215J }}</ref>

== Regulation and inhibition ==

In some bacteria, the enzyme is [[transcriptional regulation|regulated]] at the transcriptional level.<ref name="Park-1997">{{cite journal | vauthors = Kruspl W, Streitmann B | title = [Knotty reticulosis with keloid formation] | journal = Zeitschrift für Hautkrankheiten | volume = 50 | issue = 3 | pages = 117–25 | date = Feb 1975 | pmid=179232}}</ref>  It has been demonstrated that the gene for SCS (sucCD) is [[Transcription (genetics)|transcribed]] along with the gene for [[alpha-ketoglutarate dehydrogenase|α-ketoglutarate dehydrogenase]] (sucAB) under the control of a [[gene promoter|promoter]] called sdhC, which is part of the [[succinate dehydrogenase]] [[operon]].  This operon is up-regulated by the presence of oxygen and responds to a variety of carbon sources. [[Antibacterial|Antibacterial drugs]] that prevent phosphorylation of histidine, like the molecule LY26650, are potent [[enzyme inhibitors|inhibitors]] of bacterial SCSs.<ref name="Hunger-Glaser-1999">{{cite journal | vauthors = Hunger-Glaser I, Brun R, Linder M, Seebeck T | title = Inhibition of succinyl CoA synthetase histidine-phosphorylation in Trypanosoma brucei by an inhibitor of bacterial two-component systems | journal = Molecular and Biochemical Parasitology | volume = 100 | issue = 1 | pages = 53–9 | date = May 1999 | pmid = 10376993 | doi = 10.1016/s0166-6851(99)00032-8 }}</ref>

== Optimal activity ==

Measurements (performed using a soy bean SCS) indicate an optimal temperature of 37&nbsp;°C and an optimal pH of 7.0-8.0.<ref name="Wider de Xifra-1978">{{cite journal | last1 = Wider de Xifra | first1 = EA. | last2 = del C Batlle | first2 = AM | name-list-style = vanc | title = Porphyrin biosynthesis: immobilized enzymes and ligands. VI. Studies on succinyl CoA synthetase from cultured soya bean cells | journal = Biochimica et Biophysica Acta | volume = 523 | issue = 1 | pages = 245–9 | date = Mar 1978 | pmid = 564714 | doi = 10.1016/0005-2744(78)90027-x }}</ref>

== Role in disease ==

'''Fatal infantile lactic acidosis:''' Defective SCS has been implicated as a cause of [[lactic acidosis|fatal infantile lactic acidosis]], which is a disease in infants that is characterized by the build-up of toxic levels of lactic acid. The condition (when it is most severe) results in death usually within 2–4 days after birth.<ref name="Ostergaard-2007">{{cite journal | vauthors = Ostergaard E, Christensen E, Kristensen E, Mogensen B, Duno M, Shoubridge EA, Wibrand F | title = Deficiency of the alpha subunit of succinate-coenzyme A ligase causes fatal infantile lactic acidosis with mitochondrial DNA depletion | journal = American Journal of Human Genetics | volume = 81 | issue = 2 | pages = 383–7 | date = Aug 2007 | pmid = 17668387 | pmc = 1950792 | doi = 10.1086/519222 }}</ref>  It has been determined that patients with the condition display a two [[base pair]] [[deletion mutation|deletion]] within the gene known as [[SUCLG1]] that encodes the α subunit of SCS.<ref name="Ostergaard-2007" />  As a result, functional SCS is absent in metabolism causing a major imbalance in flux between [[glycolysis]] and the citric acid cycle.  Since the cells do not have a functional citric acid cycle, acidosis results because cells are forced to choose [[lactic acid fermentation|lactic acid production]] as the primary means of producing ATP.

== See also ==
{{Portal|Biology}}

*[[Citric acid cycle]]
*[[Succinate dehydrogenase]]
*[[Succinate—CoA ligase (ADP-forming)]]
*[[Succinate—CoA ligase (GDP-forming)]]

== References ==
{{reflist}}

== External links ==
* {{MeSH name|Succinyl+Coenzyme+A+Synthetases}}

{{Citric acid cycle enzymes and ETC}}
{{Mitochondrial proteins}}
{{Ligases CO CS and CN}}
{{Enzymes}}

[[Category:Metabolism]]
[[Category:EC 6.2.1]]