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===Enzymatic activity=== Lipoic acid is a [[Cofactor (biochemistry)|cofactor]] for at least five [[enzyme]] systems.<ref name=lpi/> Two of these are in the [[citric acid cycle]] through which many organisms turn nutrients into energy. Lipoylated [[enzymes]] have lipoic acid attached to them covalently. The lipoyl group transfers [[acyl]] groups in [[2-oxoacid dehydrogenase]] complexes, and [[methylamine]] group in the [[glycine cleavage complex]] or [[glycine dehydrogenase]].<ref name=lpi/> Lipoic acid is the cofactor of the following enzymes in humans:<ref>{{Cite journal |last1=Mayr |first1=Johannes A. |last2=Feichtinger |first2=RenΓ© G. |last3=Tort |first3=Frederic |last4=Ribes |first4=Antonia |last5=Sperl |first5=Wolfgang |date=2014 |title=Lipoic acid biosynthesis defects |url=https://onlinelibrary.wiley.com/doi/10.1007/s10545-014-9705-8 |journal=Journal of Inherited Metabolic Disease |language=en |volume=37 |issue=4 |pages=553β563 |doi=10.1007/s10545-014-9705-8 |pmid=24777537 |s2cid=27408101 |issn=0141-8955}}</ref><ref>{{Cite journal |last1=Solmonson |first1=Ashley |last2=DeBerardinis |first2=Ralph J. |date=2018 |title=Lipoic acid metabolism and mitochondrial redox regulation |journal=Journal of Biological Chemistry |language=en |volume=293 |issue=20 |pages=7522β7530 |doi=10.1074/jbc.TM117.000259 |doi-access=free |pmc=5961061 |pmid=29191830}}</ref><ref>{{Cite journal |last1=Nemeria |first1=Natalia S. |last2=Nagy |first2=Balint |last3=Sanchez |first3=Roberto |last4=Zhang |first4=Xu |last5=Leandro |first5=JoΓ£o |last6=Ambrus |first6=Attila |last7=Houten |first7=Sander M. |last8=Jordan |first8=Frank |date=2022-07-26 |title=Functional Versatility of the Human 2-Oxoadipate Dehydrogenase in the L-Lysine Degradation Pathway toward Its Non-Cognate Substrate 2-Oxopimelic Acid |journal=International Journal of Molecular Sciences |language=en |volume=23 |issue=15 |pages=8213 |doi=10.3390/ijms23158213 |doi-access=free |issn=1422-0067 |pmc=9367764 |pmid=35897808}}</ref> {| class="wikitable" |+ !EC-number !Enzyme !Gene !Multienzyme complex !Role of the complex |- |[[Enzyme Commission number|EC]] [https://enzyme.expasy.org/EC/2.3.1.12 2.3.1.12] |[[dihydrolipoyl transacetylase]] (E2) |[[DLAT]] |[[pyruvate dehydrogenase complex]] (PDC) |Connection of [[glycolysis]] with the [[citric acid cycle]] |- | rowspan="2" |EC [https://enzyme.expasy.org/EC/2.3.1.61 2.3.1.61] | rowspan="2" |[[Dihydrolipoyllysine-residue succinyltransferase|dihydrolipoyl succinyltransferase]] (E2) | rowspan="2" |[[DLST]] |[[oxoglutarate dehydrogenase complex]] (OGDC) |[[Citric acid cycle]] enzyme |- |[[2-oxoadipate dehydrogenase complex]] (OADHC) |[[Lysine]] degradation |- |EC [https://enzyme.expasy.org/EC/2.3.1.168 2.3.1.168] |[[dihydrolipoyl transacylase]] (E2) |[[DBT (gene)|DBT]] |[[Branched-chain alpha-keto acid dehydrogenase complex|branched-chain Ξ±-ketoacid dehydrogenase complex]] (BCKDC) |[[Leucine]], [[isoleucine]] and [[valine]] degradation |- | |[[GCSH|H-protein]] |[[GCSH]] |[[glycine cleavage system]] (GCS) |[[Glycine]] and [[serine]] metabolism, [[folate]] metabolism |} The most-studied of these is the pyruvate dehydrogenase complex.<ref name=lpi/> These complexes have three central subunits: E1-3, which are the decarboxylase, lipoyl transferase, and [[dihydrolipoamide dehydrogenase]], respectively. These complexes have a central E2 core and the other subunits surround this core to form the complex. In the gap between these two subunits, the lipoyl domain ferries intermediates between the active sites.<ref name=lpi/> The lipoyl domain itself is attached by a flexible linker to the E2 core and the number of lipoyl domains varies from one to three for a given organism. The number of domains has been experimentally varied and seems to have little effect on growth until over nine are added, although more than three decreased activity of the complex.<ref>{{cite journal |last1= Machado |first1= RS |last2= Clark |first2= DP |last3= Guest |first3= JR |title= Construction and properties of pyruvate dehydrogenase complexes with up to nine lipoyl domains per lipoate acetyltransferase chain |journal= FEMS Microbiology Letters |year= 1992 |pages= 243β8 |volume= 79 |issue= 1β3 |doi= 10.1111/j.1574-6968.1992.tb14047.x |pmid= 1478460|doi-access= free }}</ref> Lipoic acid serves as co-factor to the [[acetoin dehydrogenase]] complex catalyzing the conversion of [[acetoin]] (3-hydroxy-2-butanone) to acetaldehyde and [[acetyl coenzyme A]].<ref name=lpi/> The [[glycine cleavage system]] differs from the other complexes, and has a different nomenclature.<ref name=lpi/> In this system, the H protein is a free lipoyl domain with additional helices, the L protein is a dihydrolipoamide dehydrogenase, the P protein is the decarboxylase, and the T protein transfers the [[methylamine]] from lipoate to [[tetrahydrofolate]] (THF) yielding methylene-THF and ammonia. Methylene-THF is then used by serine hydroxymethyltransferase to synthesize [[serine]] from [[glycine]]. This system is part of plant [[photorespiration]].<ref>{{cite journal |last1= Douce |first1= R |last2= Bourguignon |first2= J |last3= Neuburger |first3= M |last4= Rebeille |first4= F |title= The glycine decarboxylase system: A fascinating complex |journal= [[Trends (journals)|Trends in Plant Science]] |year= 2001 |pages= 167β76 |volume= 6 |issue= 4 |doi= 10.1016/S1360-1385(01)01892-1 |pmid= 11286922|bibcode= 2001TPS.....6..167D }}</ref>
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