# Succinyl-CoA synthetase
Succinyl-CoA synthetase (SCS), also called succinyl-CoA ligase or succinate thiokinase, is an enzyme that converts succinyl-CoA to succinate while generating a nucleoside triphosphate, either GTP or ATP, from the corresponding diphosphate and inorganic phosphate. It catalyzes the only substrate-level phosphorylation step of the citric acid cycle, and its reaction is reversible, allowing the enzyme to build succinyl-CoA for ketone-body activation and heme synthesis as well.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/)
| Key fact | Detail |
|---|---|
| Reaction | succinyl-CoA + NDP + Pi ⇌ succinate + CoA + NTP (ADP-forming form: EC 6.2.1.5; GTP-forming form: EC 6.2.1.4)[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/) • [2](https://enzyme.expasy.org/EC/6.2.1.5) |
| Place in metabolism | The cycle's only substrate-level phosphorylation[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/) |
| Energetics | ΔG°′ = −3.4 kJ mol−1 (−0.8 kcal mol−1); readily reversible, and one of several near-equilibrium reactions of the cycle under physiological conditions, along with those catalyzed by aconitase, succinate dehydrogenase, fumarase, and malate dehydrogenase[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch19_4.html) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3549575/) • [8](https://ebooks.inflibnet.ac.in/biocp04/chapter/citric-acid-cycle/) |
| Isoforms | Shared SUCLG1 α-subunit plus either SUCLG2 β (GTP-forming) or SUCLA2 β (ATP-forming)[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/) |
| Tissue distribution | ATP-specific form predominates in heart, brain, and skeletal muscle; GTP-specific form in kidney and liver[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/) |
| Catalytic intermediate | Phosphorylated histidine on a mobile loop, shuttling the phosphoryl group about 29–30 Å between two active sites[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/) |
| Disease link | SUCLA2 and SUCLG1 mutations cause succinyl-CoA ligase deficiency with mitochondrial DNA depletion[6](https://www.reactome.org/content/detail/R-HSA-70997.6) |
## What succinyl-CoA synthetase does
The enzyme catalyzes the reversible reaction succinyl-CoA + NDP + Pi ⇌ succinate + CoA + NTP.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/) The ADP-forming enzyme carries EC number 6.2.1.5 and the synonyms succinate thiokinase and succinyl-CoA synthetase (ADP-forming), with the written reaction succinate + ATP + CoA = succinyl-CoA + ADP + phosphate.[2](https://enzyme.expasy.org/EC/6.2.1.5) Within the citric acid cycle this is the only substrate-level phosphorylation step.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/)
Reversibility sets this step apart: like the reactions catalyzed by aconitase, succinate dehydrogenase, fumarase, and malate dehydrogenase, it is near equilibrium under physiological conditions.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3549575/) • [8](https://ebooks.inflibnet.ac.in/biocp04/chapter/citric-acid-cycle/) That property is what allows the same enzyme to serve the forward, energy-conserving direction of the cycle and the reverse direction that supplies succinyl-CoA to other pathways.
## How the mechanism works
The reaction proceeds through a covalent phosphorylated histidine intermediate. The ε-N atom of the catalytic histidine acts as a nucleophile toward the phosphoryl moiety of the nucleotide substrate, forming phosphohistidine; the phosphorylated residue then donates phosphate to succinate, forming a non-covalently bound succinyl phosphate, which is attacked by CoA to yield succinyl-CoA.[7](https://www.ebi.ac.uk/thornton-srv/m-csa/entry/476/) The phosphorylated residue is a histidine on the Nε atom, identified by Kreil and Boyer in 1964.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/)
The histidine sits on a mobile phosphohistidine loop that acts as a moving arm: it detaches the phosphoryl group from succinyl phosphate at the CoA/succinate/phosphate active site, swings across the enzyme, and transfers the phosphoryl group to the nucleotide diphosphate bound at the second active site.[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch19_4.html) Crystal structures support this shuttling mechanism, with the phosphoryl transfer bridging about 29 Å between the phosphohistidine positions in the two sites.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/)
Structurally, mammalian SCS is an αβ-heterodimer, while the E. coli enzyme is an α2β2-heterotetramer; the phosphohistidine resides in the α-subunit.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/) The nucleotide binds in an ATP-grasp fold in the amino-terminal domain of the β-subunit, roughly 30 Å from the transiently phosphorylated histidine.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/) Recent structures also revealed a second succinate-binding site at the α-β subunit interface and an additional magnesium ion interacting with Glu141β and Glu204β.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/)
## GTP versus ATP: the isoforms
Mammals make two isoforms that share one α-subunit but differ in the β-subunit, which determines nucleotide specificity. The ATP-specific form is EC 6.2.1.5 and the GTP-specific form is EC 6.2.1.4; the three genes are SUCLG1 (α-subunit), SUCLG2 (β-subunit of the GTP-forming enzyme), and SUCLA2 (β-subunit of the ATP-forming enzyme).[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/) In vertebrates the α:βA heterodimer couples succinyl-CoA conversion to ATP synthesis from ADP, while the α:βG heterodimer couples it to GTP synthesis from GDP.[6](https://www.reactome.org/content/detail/R-HSA-70997.6)
Expression is tissue-specific: the ATP-specific enzyme is more highly expressed in heart, brain, and skeletal muscle, while the GTP-specific enzyme predominates in kidney and liver.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/) Why this matters mechanistically, beyond the expression pattern itself, is not settled in the sources reviewed here. Even the functional question of how much carbon each isoform handles is open: their relative contributions to the flux of carbon through the TCA cycle are unknown.[6](https://www.reactome.org/content/detail/R-HSA-70997.6)
## By the numbers
The quantitative record for this enzyme is thin but well defined where it exists. The standard free energy of the reaction is ΔG°′ = −3.4 kJ mol−1 (−0.8 kcal mol−1), a small value consistent with ready reversibility.[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch19_4.html) The crystal structure of pig GTP-specific SCS with CoA bound to the amino-terminal domain of the α-subunit was determined at 2.1 Å resolution.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/) The catalytic loop must span roughly 29–30 Å between the two active sites, the distance the phosphoryl group travels during each catalytic cycle.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/)
## Links to heme and ketone-body metabolism
Running in reverse, the enzyme consumes a nucleoside triphosphate to make succinyl-CoA from succinate, CoA, and NDP. That succinyl-CoA can be used for activating ketone bodies and for haem synthesis.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/) Beyond the TCA cycle, the SCS reaction is thus part of ketone-body metabolism and haem biosynthesis, and abnormal SCS activities have been associated with diabetes and neurodegenerative diseases.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3549575/)
Which isoform performs the reverse reaction is reported differently by credible sources. A biochemistry textbook account states that in tissues performing many anabolic reactions, such as the liver, the GDP-requiring enzyme is common and is believed to work in reverse, using GTP to power synthesis of succinyl-CoA as a heme precursor.[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch19_4.html) [Reactome](https://www.edgepedia.org/reactome), citing Furuyama and Sassa (2000), instead reports that genetic and biochemical data suggest the α:βA (ADP-forming) isoform may be required to catalyze the reverse reaction generating succinyl-CoA for heme biosynthesis.[6](https://www.reactome.org/content/detail/R-HSA-70997.6) These accounts have not been reconciled in the sources reviewed here.
## When it fails: SUCL-related disorders
A mutation in SUCLA2 leading to encephalomyopathy and mitochondrial DNA depletion was first identified in 2005 by Elpeleg and colleagues; patients homozygous for the mutant allele are deficient in succinyl-CoA ligase activity.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/) • [6](https://www.reactome.org/content/detail/R-HSA-70997.6) Mutations in SUCLG1, the gene for the shared α-subunit, cause mitochondrial DNA depletion syndrome 9 (MTDPS9; MIM:245400), an infantile metabolic disease.[6](https://www.reactome.org/content/detail/R-HSA-70997.6) Separately, abnormal SCS activity has been associated with diabetes and neurodegenerative diseases.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC3549575/)
## References
1. [Second distinct conformation of the phosphohistidine loop in succinyl-CoA synthetase](https://pmc.ncbi.nlm.nih.gov/articles/PMC7919408/)
2. [ENZYME 6.2.1.5 succinate--CoA ligase (ADP-forming)](https://enzyme.expasy.org/EC/6.2.1.5)
3. [Tymoczko Biochemistry 3e, Chapter 19: Succinyl-CoA synthetase](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch19_4.html)
4. [Identification of the kinetic mechanism of succinyl-CoA synthetase](https://pmc.ncbi.nlm.nih.gov/articles/PMC3549575/)
5. [Structure of GTP-specific succinyl-CoA synthetase in complex with CoA](https://pmc.ncbi.nlm.nih.gov/articles/PMC4528943/)
6. [Reactome: SUCLG1/A2 cleaves succinyl-CoA](https://www.reactome.org/content/detail/R-HSA-70997.6)
7. [M-CSA Mechanism and Catalytic Site Atlas entry 476](https://www.ebi.ac.uk/thornton-srv/m-csa/entry/476/)
8. [Citric acid cycle, Metabolism of carbohydrates](https://ebooks.inflibnet.ac.in/biocp04/chapter/citric-acid-cycle/)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolism and metabolic pathways › Carbohydrate and energy metabolism › Citric acid cycle › Succinyl-CoA to succinate: substrate-level phosphorylation*
*Initially written Sep 17, 2026 · Edited: Oct 11, 2026; Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license