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Succinate dehydrogenase

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Enzyme

succinate dehydrogenase (succinate-ubiquinone oxidoreductase)
The structure of SQR in a phospholipid membrane.SDHA,SDHB,SDHC andSDHD
Identifiers
EC no.1.3.5.1
CAS no.9028-11-9
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Protein family
Succinate dehydrogenase
Identifiers
SymbolRespiratory complex II
OPM superfamily3
OPM protein1zoy
Membranome656

Succinate dehydrogenase (SDH) orsuccinate-coenzyme Q reductase (SQR) orrespiratory complex II is anenzyme complex, found in manybacterialcells and in theinner mitochondrial membrane ofeukaryotes. It is the only enzyme that participates in both thecitric acid cycle andoxidative phosphorylation.[1] Histochemical analysis showing high succinate dehydrogenase in muscle demonstrates high mitochondrial content and high oxidative potential.[2]

In step 6 of thecitric acid cycle, SQRcatalyzes theoxidation ofsuccinate tofumarate with thereduction ofubiquinone toubiquinol. This occurs in the inner mitochondrialmembrane bycoupling the two reactions together.

Structure

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Image 5: Subunits of succinate dehydrogenase

Subunits

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Mitochondrial and manybacterial SQRs are composed of four structurally differentsubunits: twohydrophilic and twohydrophobic. The first two subunits, aflavoprotein (SDHA) and aniron-sulfur protein (SDHB), form a hydrophilic head where enzymatic activity of the complex takes place. SDHA contains acovalently attachedflavin adenine dinucleotide (FAD)cofactor and thesuccinatebinding site and SDHB contains three iron-sulfur clusters: [2Fe-2S], [4Fe-4S], and [3Fe-4S]. The second two subunits are hydrophobic membrane anchor subunits, SDHC and SDHD. Human mitochondria contain two distinct isoforms of SDHA (Fp subunits type I and type II), these isoforms are also found inAscaris suum andCaenorhabditis elegans.[3] The subunits form a membrane-boundcytochrome b complex with sixtransmembranehelices containing oneheme b group and aubiquinone-binding site. Twophospholipid molecules, onecardiolipin and onephosphatidylethanolamine, are also found in the SDHC and SDHD subunits (not shown in the image). They serve to occupy the hydrophobic space below the heme b. These subunits are displayed in the attached image. SDHA is green, SDHB is teal, SDHC is fuchsia, and SDHD is yellow. Around SDHC and SDHD is aphospholipid membrane with the intermembrane space at the top of the image.[4]

Table of subunit composition[5]

[edit]
No.Subunit nameHuman proteinProtein description fromUniProtPfam family with Human protein
1SDHASDHA_HUMANSuccinate dehydrogenase [ubiquinone] flavoprotein subunit, mitochondrialPfamPF00890,PfamPF02910
2SDHBSDHB_HUMANSuccinate dehydrogenase [ubiquinone] iron-sulfur subunit, mitochondrialPfamPF13085,PfamPF13183
3SDHCC560_HUMANSuccinate dehydrogenase cytochrome b560 subunit, mitochondrialPfamPF01127
4SDHDDHSD_HUMANSuccinate dehydrogenase [ubiquinone] cytochrome b small subunit, mitochondrialPfamPF05328

Ubiquinone binding site

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Two distinctiveubiquinonebinding sites can be recognized on mammalian SDH – matrix-proximal QP and matrix-distal QD. Ubiquinone binding site Qp, which shows higher affinity to ubiquinone, is located in a gap composed of SDHB, SDHC, and SDHD.Ubiquinone is stabilized by theside chains of His207 of subunit B, Ser27 and Arg31 of subunit C, and Tyr83 of subunit D. The quinone ring is surrounded by Ile28 of subunit C and Pro160 of subunit B. Theseresidues, along with Il209, Trp163, and Trp164 of subunit B, and Ser27 (C atom) of subunit C, form thehydrophobic environment of thequinone-binding pocket Qp.[6] In contrast, ubiquinone binding site QD, which lies closer to inter-membrane space, is composed of SDHD only and has lower affinity to ubiquinone.[7]

Succinate binding site

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SDHA provides thebinding site for theoxidation ofsuccinate. Theside chains Thr254, His354, and Arg399 of subunit A stabilize themolecule whileFADoxidizes and carries theelectrons to the first of theiron-sulfur clusters, [2Fe-2S].[8] This can be seen in image 5.

Redox centers

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Thesuccinate-binding site andubiquinone-binding site are connected by a chain of redox centers includingFAD and theiron-sulfur clusters. This chain extends over 40 Å through theenzymemonomer. All edge-to-edge distances between the centers are less than the suggested 14 Å limit forphysiologicalelectron transfer.[4] Thiselectron transfer is demonstrated in image 8.

Assembly and maturation

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All subunits of human mitochondrial SDH are nuclear encoded. After translation,SDHA subunit is translocated asapoprotein into the mitochondrial matrix. Subsequently, one of the first steps is covalent attachment of theFADcofactor (covalent flavinylation). This process is enhanced bysuccinate dehydrogenase assembly factor 2 (SDHAF2;[9] also called SDH5 in yeast and SDHE in bacteria) and by some of the Krebs cycle intermediates. Fumarate most strongly stimulates covalent flavinylation of SDHA.[10] Through studies of the bacterial system, the mechanism of FAD attachment has been shown to involve a quinone:methide intermediate.[11] In mitochondrial, but not bacterial, assembly, SDHA interacts with a second assembly factor called succinate dehydrogenase assembly factor 4 (SDHAF4; called SDH8 in yeast) before it is inserted into the final complex.[7]

Fe-Sprosthetic groups of the subunitSDHB are being preformed in the mitochondrial matrix by protein complex ISU. The complex is also thought to be capable of inserting the iron-sulphur clusters in SDHB during its maturation. The studies suggest that Fe-S cluster insertion precedes SDHA-SDHB dimer forming. Such incorporation requires reduction ofcysteine residues within active site of SDHB. Both reduced cysteine residues and already incorporated Fe-S clusters are highly susceptible toROS damage. Two more SDH assembly factors,SDHAF1 (SDH6) and SDHAF3 (SDH7 in yeast), seem to be involved in SDHB maturation in way of protecting the subunit or dimer SDHA-SDHB from Fe-S cluster damage caused by ROS.[7]

Assembly of the hydrophobic anchor consisting of subunitsSDHC andSDHD remains unclear. Especially in case ofheme b insertion and even its function. Heme b prosthetic group does not appear to be part of electron transporting pathway within the complex II.[5] The cofactor rather maintains the anchor stability.

Mechanism

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Image 6:E2 Succinate oxidation mechanism.
Image 7:E1cb Succinate oxidation mechanism.

Succinate oxidation

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Much is known about thesuccinateoxidationmechanism, which involves the transfer of a proton and a hydride. A combination of mutagenesis and structural analysis identifies Arg-286 of the SDHA subunit (E. coli numbering) as the proton shuttle.Crystal structures of the enzymes from multiple organisms shows that this is well poised for the proton transfer step. Thereafter, there are two possible elimination mechanisms: E2 or E1cb. In the E2 elimination, the mechanism is concerted. The basicresidue orcofactor deprotonates thealpha carbon, and FAD accepts thehydride from thebeta carbon,oxidizing the boundsuccinate tofumarate—refer to image 6. In E1cb, anenolate intermediate is formed, shown in image 7, beforeFAD accepts thehydride. Further research is required to determine which elimination mechanism succinate undergoes in Succinate Dehydrogenase.Oxidizedfumarate, now loosely bound to theactive site, is free to exit theprotein.

Electron tunneling

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After theelectrons are derived fromsuccinateoxidation viaFAD, they tunnel along the [Fe-S] relay until they reach the [3Fe-4S] cluster. Theseelectrons are subsequently transferred to an awaitingubiquinonemolecule within theactive site. TheIron-Sulfurelectron tunneling system is shown in image 9.

Ubiquinone reduction

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Image 8:Ubiquinone reduction mechanism.
Image 9:Electron carriers of the SQR complex. FADH2, iron-sulfur centers, heme b, and ubiquinone.

The O1carbonyloxygen ofubiquinone is oriented at the active site (image 4) byhydrogen bond interactions with Tyr83 of subunit D. The presence ofelectrons in the [3Fe-4S] iron sulphur cluster induces the movement ofubiquinone into a second orientation. This facilitates a secondhydrogen bond interaction between the O4carbonyl group ofubiquinone and Ser27 of subunit C. Following the first singleelectronreduction step, asemiquinone radical species is formed. The secondelectron arrives from the [3Fe-4S] cluster to provide full reduction of theubiquinone toubiquinol. This mechanism of theubiquinone reduction is shown in image 8.

Heme prosthetic group

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Although the functionality of theheme in succinate dehydrogenase is still being researched, some studies[by whom?] have asserted that the firstelectron delivered toubiquinone via [3Fe-4S] may tunnel back and forth between theheme and theubiquinone intermediate. In this way, thehemecofactor acts as anelectron sink. Its role is to prevent the interaction of the intermediate withmolecular oxygen to producereactive oxygen species (ROS). Theheme group, relative toubiquinone, is shown in image 4.

It has also been proposed that a gatingmechanism may be in place to prevent theelectrons from tunneling directly to theheme from the [3Fe-4S] cluster. A potential candidate isresidue His207, which lies directly between the cluster and theheme. His207 of subunit B is in direct proximity to the [3Fe-4S] cluster, the boundubiquinone, and theheme; and could modulateelectron flow between these redox centers.[12]

Proton transfer

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To fully reduce thequinone in SQR, twoelectrons as well as twoprotons are needed. It has been argued that awater molecule (HOH39) arrives at theactive site and is coordinated by His207 of subunit B, Arg31 of subunit C, and Asp82 of subunit D. Thesemiquinone species isprotonated byprotons delivered from HOH39, completing theubiquinone reduction toubiquinol. His207 and Asp82 most likely facilitate this process. Other studies claim that Tyr83 of subunit D is coordinated to a nearbyhistidine as well as the O1carbonyloxygen ofubiquinone. Thehistidineresidue decreases thepKa oftyrosine, making it more suitable to donate itsproton to the reducedubiquinone intermediate.

Inhibitors

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There are two distinct classes of inhibitors (SDHIs) of complex II: those that bind in the succinate pocket and those that bind in the ubiquinone pocket. Ubiquinone type inhibitors includecarboxin andthenoyltrifluoroacetone. Succinate-analogue inhibitors include the synthetic compoundmalonate as well as the TCA cycle intermediates,malate andoxaloacetate. Indeed, oxaloacetate is one of the most potent inhibitors of Complex II. Why a common TCA cycle intermediate would inhibit Complex II is not entirely understood, though it may exert a protective role in minimizing reverse-electron transfer mediated production of superoxide by Complex I.[13] Atpenin 5a are highly potent Complex II inhibitors mimicking ubiquinone binding.

Ubiquinone type inhibitors have been used asfungicides in agriculture since the 1960s. Carboxin was mainly used to control disease caused bybasidiomycetes such asstem rusts andRhizoctonia diseases. In the 1980s simplebenzanilides were found to have comparable activity to carboxin and a number of these were marketed, includingbenodanil,flutolanil andmepronil.[14] More recently, other compounds with a broader spectrum against a range of plant pathogens have been developed includingboscalid,fluopyram,fluxapyroxad,pydiflumetofen andsedaxane.[15][14] Some agriculturally important fungi are not sensitive towards members of the new generation of ubiquinone type inhibitors.[16]FRAC has a working group[17] for SDHIs and recommendsresistance management practices.[18]

Complex II inhibitors are also used asinsecticides andacaricides, inIRAC group 25.[19]

Role in disease

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The fundamental role of succinate-coenzyme Q reductase in bothoxidative phosphorylation and thecitric acid cycle makes it vital in all eukaryotic organisms. Loss of function of SDH via mutations or through toxins can cause a wide range of disease.

When SDH is dysfunctional in the citric acid cycle, it can lead to a buildup of the oncometabolite succinate, which can lead to tumorogenesis. This is well-known to occur inchromaffin cells, causing neuroendocrine tumors such asparaganglioma,renal carcinoma, andGastrointestinal stromal tumor (GISTs).[20] The penetrance data for SDH mutations causing tumorigenesis is lacking, and international guidelines suggest thorough screening for any carriers.[21] The penetrance of paraganglioma in loss of function mutations of SDH is incomplete and varies by subunit. SDHB mutations have a penetrance between 8% and 37%, SDHD mutations have a penetrance between 38% and 64% with some maternal imprinting effects, and the penetrance for both SDHA and SDHC mutations are poorly studied, but likely between 1% and 30%.[22][23] Mammalian SDH functions not only in energy generation, but also has a role inoxygen sensing. Buildup of succinate due to defective SDH can cause a pseudo-hypoxia and angiogenesis, both of which contribute to the distinctly vascular and characteristic "salt and pepper" appearance of paraganglioma on imaging.[24]

Bi-Allelic loss of function mutations of SDHA, SDHB, SDHD, and SDHAF1 or monoallelic loss of function mutations of SDHA can causeMitochondrial complex II deficiency. This disruption in oxidative phosphorylation can lead toLeigh syndrome,mitochondrialencephalopathy,optic atrophy,myopathy, and a spectrum of disease. These presentations can range from death within the first year of life or in utero to mild symptoms beginning as an adult.[25]

Reduced levels SDH are observed post mortem in the brains of patients withHuntington's disease, and energy metabolism defects have been identified in both presymptomatic and symptomatic HD patients.[26]

See also

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References

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  1. ^Oyedotun KS, Lemire BD (March 2004)."The quaternary structure of the Saccharomyces cerevisiae succinate dehydrogenase. Homology modeling, cofactor docking, and molecular dynamics simulation studies".The Journal of Biological Chemistry.279 (10):9424–9431.doi:10.1074/jbc.M311876200.PMID 14672929.
  2. ^webmaster (2009-03-04)."Using Histochemistry to Determine Muscle Properties".Succinate Dehydrogenase: Identifying Oxidative Potential.University of California, San Diego. Archived fromthe original on 2018-10-10. Retrieved2017-12-27.
  3. ^Tomitsuka E, Hirawake H, Goto Y, Taniwaki M, Harada S, Kita K (August 2003). "Direct evidence for two distinct forms of the flavoprotein subunit of human mitochondrial complex II (succinate-ubiquinone reductase)".Journal of Biochemistry.134 (2):191–195.doi:10.1093/jb/mvg144.PMID 12966066.
  4. ^abYankovskaya V, Horsefield R, Törnroth S, Luna-Chavez C, Miyoshi H, Léger C, et al. (January 2003). "Architecture of succinate dehydrogenase and reactive oxygen species generation".Science.299 (5607):700–704.Bibcode:2003Sci...299..700Y.doi:10.1126/science.1079605.PMID 12560550.S2CID 29222766.
  5. ^abSun F, Huo X, Zhai Y, Wang A, Xu J, Su D, et al. (July 2005)."Crystal structure of mitochondrial respiratory membrane protein complex II".Cell.121 (7):1043–1057.doi:10.1016/j.cell.2005.05.025.PMID 15989954.
  6. ^Horsefield R, Yankovskaya V, Sexton G, Whittingham W, Shiomi K, Omura S, et al. (March 2006)."Structural and computational analysis of the quinone-binding site of complex II (succinate-ubiquinone oxidoreductase): a mechanism of electron transfer and proton conduction during ubiquinone reduction".The Journal of Biological Chemistry.281 (11):7309–7316.doi:10.1074/jbc.M508173200.PMID 16407191.
  7. ^abcVan Vranken JG, Na U, Winge DR, Rutter J (December 2014)."Protein-mediated assembly of succinate dehydrogenase and its cofactors".Critical Reviews in Biochemistry and Molecular Biology.50 (2):168–180.doi:10.3109/10409238.2014.990556.PMC 4653115.PMID 25488574.
  8. ^Kenney WC (April 1975)."The reaction of N-ethylmaleimide at the active site of succinate dehydrogenase".The Journal of Biological Chemistry.250 (8):3089–3094.doi:10.1016/S0021-9258(19)41598-6.PMID 235539.
  9. ^Sharma P, Maklashina E, Cecchini G, Iverson TM (September 2020)."The roles of SDHAF2 and dicarboxylate in covalent flavinylation of SDHA, the human complex II flavoprotein".Proceedings of the National Academy of Sciences of the United States of America.117 (38):23548–23556.Bibcode:2020PNAS..11723548S.doi:10.1073/pnas.2007391117.PMC 7519310.PMID 32887801.
  10. ^Maklashina E, Iverson TM, Cecchini G (October 2022)."How an assembly factor enhances covalent FAD attachment to the flavoprotein subunit of complex II".The Journal of Biological Chemistry.298 (10): 102472.doi:10.1016/j.jbc.2022.102472.PMC 9557727.PMID 36089066.
  11. ^Sharma P, Maklashina E, Cecchini G, Iverson TM (January 2018)."Crystal structure of an assembly intermediate of respiratory Complex II".Nature Communications.9 (1): 274.Bibcode:2018NatCo...9..274S.doi:10.1038/s41467-017-02713-8.PMC 5773532.PMID 29348404.
  12. ^Tran QM, Rothery RA, Maklashina E, Cecchini G, Weiner JH (October 2006)."The quinone binding site in Escherichia coli succinate dehydrogenase is required for electron transfer to the heme b".The Journal of Biological Chemistry.281 (43):32310–32317.doi:10.1074/jbc.M607476200.PMID 16950775.
  13. ^Muller FL, Liu Y, Abdul-Ghani MA, Lustgarten MS, Bhattacharya A, Jang YC, et al. (January 2008). "High rates of superoxide production in skeletal-muscle mitochondria respiring on both complex I- and complex II-linked substrates".The Biochemical Journal.409 (2):491–499.doi:10.1042/BJ20071162.PMID 17916065.
  14. ^abWalter H (2016). "Fungicidal Succinate-Dehydrogenase-Inhibiting Carboxamides".Bioactive Carboxylic Compound Classes: Pharmaceuticals and Agrochemicals. pp. 405–425.doi:10.1002/9783527693931.ch31.ISBN 978-3-527-33947-1.
  15. ^Avenot HF, Michailides TJ (2010). "Progress in understanding molecular mechanisms and evolution of resistance to succinate dehydrogenase inhibiting (SDHI) fungicides in phytopathogenic fungi".Crop Protection.29 (7):643–651.Bibcode:2010CrPro..29..643A.doi:10.1016/j.cropro.2010.02.019.
  16. ^
  17. ^"SDHI Fungicides Working Group".FRAC (Fungicide Resistance Action Committee). 2020-01-31. Retrieved2022-07-05.
  18. ^"Recommendations for SDHI".FRAC. March 2020. Retrieved2022-07-05.
  19. ^Jeschke P, Witschel M, Krämer W, Schirmer U (25 January 2019). "32.3 Inhibitors of Mitochondrial Electron Transport: Acaricides and Insecticides".Modern Crop Protection Compounds (3rd ed.). Wiley-VCH. pp. 1156–1201.doi:10.1002/9783527699261.ISBN 9783527699261.
  20. ^Barletta JA, Hornick JL (July 2012). "Succinate dehydrogenase-deficient tumors: diagnostic advances and clinical implications".Advances in Anatomic Pathology.19 (4):193–203.doi:10.1097/PAP.0b013e31825c6bc6.PMID 22692282.S2CID 32088940.
  21. ^Amar L, Pacak K, Steichen O, Akker SA, Aylwin SJ, Baudin E, et al. (July 2021)."International consensus on initial screening and follow-up of asymptomatic SDHx mutation carriers".Nature Reviews Endocrinology.17 (7):435–444.doi:10.1038/s41574-021-00492-3.PMC 8205850.PMID 34021277.
  22. ^Rijken JA, Niemeijer ND, Jonker MA, Eijkelenkamp K, Jansen JC, van Berkel A, et al. (January 2018)."The penetrance of paraganglioma and pheochromocytoma in SDHB germline mutation carriers"(PDF).Clinical Genetics.93 (1):60–66.doi:10.1111/cge.13055.PMID 28503760.
  23. ^Baysal BE (May 2013). "Mitochondrial complex II and genomic imprinting in inheritance of paraganglioma tumors".Biochimica et Biophysica Acta (BBA) - Bioenergetics.1827 (5):573–577.doi:10.1016/j.bbabio.2012.12.005.PMID 23291190.
  24. ^https://radiopaedia.org/articles/salt-and-pepper-sign-paraganglioma-1?lang=us
  25. ^Fullerton M, McFarland R, Taylor RW, Alston CL (September 2020)."The genetic basis of isolated mitochondrial complex II deficiency".Molecular Genetics and Metabolism.131 (1–2):53–65.doi:10.1016/j.ymgme.2020.09.009.PMC 7758838.PMID 33162331.
  26. ^Skillings EA, Morton AJ (2016)."Delayed Onset and Reduced Cognitive Deficits through Pre-Conditioning with 3-Nitropropionic Acid is Dependent on Sex and CAG Repeat Length in the R6/2 Mouse Model of Huntington's Disease".Journal of Huntington's Disease.5 (1):19–32.doi:10.3233/JHD-160189.PMID 27031731.
Cycle
Anaplerotic
toacetyl-CoA
toα-ketoglutaric acid
tosuccinyl-CoA
tooxaloacetic acid
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electron transport chain/
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Matrix
citric acid cycle
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urea cycle
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1.3.1:NAD/NADP acceptor
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