Succinic acid

Succinic acid is a dicarboxylic acid with the chemical formula ₂₂. In living organisms, succinic acid takes the form of an anion, succinate, which has multiple biological roles as a metabolic intermediate being converted into fumarate by the enzyme succinate dehydrogenase in complex 2 of the electron transport chain which is involved in making ATP, and as a signaling molecule reflecting the cellular metabolic state.
Succinate is generated in mitochondria via the tricarboxylic acid cycle. Succinate can exit the mitochondrial matrix and function in the cytoplasm as well as the extracellular space, changing gene expression patterns, modulating epigenetic landscape or demonstrating hormone-like signaling. As such, succinate links cellular metabolism, especially ATP formation, to the regulation of cellular function.
Dysregulation of succinate synthesis, and therefore ATP synthesis, happens in some genetic mitochondrial diseases, such as Leigh syndrome, and Melas syndrome, and degradation can lead to pathological conditions, such as malignant transformation, inflammation and tissue injury.
Succinic acid is marketed as food additive E363. The name derives from Latin, meaning amber.

Physical properties

Succinic acid is a white, odorless solid with a highly acidic taste. In an aqueous solution, succinic acid readily ionizes to form its conjugate base, succinate. As a diprotic acid, succinic acid undergoes two successive deprotonation reactions:
The pK_a of these processes are 4.3 and 5.6, respectively. Both anions are colorless and can be isolated as the salts, e.g., Na₂ and Na₂₂₂. In living organisms, primarily succinate, not succinic acid, is found.
As a radical group it is called a succinyl group.
Like most simple mono- and dicarboxylic acids, it is not harmful but can be an irritant to skin and eyes.

Commercial production

Historically, succinic acid was obtained from amber by distillation and has thus been known as spirit of amber. Common industrial routes include hydrogenation of maleic acid, oxidation of 1,4-butanediol, and carbonylation of ethylene glycol. Succinate is also produced from butane via maleic anhydride. Global production is estimated at 16,000 to 30,000 tons a year, with an annual growth rate of 10%.
Genetically engineered Escherichia coli and Saccharomyces cerevisiae are proposed for the commercial production via fermentation of glucose.

Chemical reactions

Succinic acid can be dehydrogenated to fumaric acid or be converted to diesters, such as diethylsuccinate ₂. This diethyl ester is a substrate in the Stobbe condensation. Dehydration of succinic acid gives succinic anhydride. Succinate can be used to derive 1,4-butanediol, maleic anhydride, succinimide, 2-pyrrolidinone and tetrahydrofuran.

Applications

In 2004, succinate was placed on the US Department of Energy's list of top 12 platform chemicals from biomass.

Precursor to polymers, resins, and solvents

Succinic acid is a precursor to some polyesters and a component of some alkyd resins. 1,4-Butanediol can be synthesized using succinic acid as a precursor. The automotive and electronics industries heavily rely on BDO to produce connectors, insulators, wheel covers, gearshift knobs and reinforcing beams. Succinic acid also serves as the bases of certain biodegradable polymers, which are of interest in tissue engineering applications.
Acylation with succinic acid is called succination. Oversuccination occurs when more than one succinate adds to a substrate.

Food and dietary supplement

As a food additive and dietary supplement, succinic acid is generally recognized as safe by the U.S. Food and Drug Administration. Succinic acid is used primarily as an acidity regulator in the food and beverage industry. It is also available as a flavoring agent, contributing a somewhat sour and astringent component to umami taste. As an excipient in pharmaceutical products, it is also used to control acidity or as a counter ion. Drugs involving succinate include metoprolol succinate, sumatriptan succinate, doxylamine succinate or solifenacin succinate.

Biosynthesis

Tricarboxylic acid (TCA) cycle

Succinate is a key intermediate in the tricarboxylic acid cycle, a primary metabolic pathway used to produce chemical energy in the presence of O₂. Succinate is generated from succinyl-CoA by the enzyme succinyl-CoA synthetase in a GTP/ATP-producing step:
Succinyl-CoA + NDP + Pi → Succinate + CoA + NTP
Catalyzed by the enzyme succinate dehydrogenase, succinate is subsequently oxidized to fumarate:
Succinate + FAD → Fumarate + FADH₂
SDH also participates in the mitochondrial electron transport chain, where it is known as respiratory complex II. This enzyme complex is a 4 subunit membrane-bound lipoprotein which couples the oxidation of succinate to the reduction of ubiquinone via the intermediate electron carriers FAD and three 2Fe-2S clusters. Succinate thus serves as a direct electron donor to the electron transport chain, and itself is converted into fumarate.

Reductive branch of the TCA cycle

Succinate can alternatively be formed by reverse activity of SDH. Under anaerobic conditions certain bacteria such as Actinobacillus succinogenes, A. succiniciproducens and M. succiniciproducens, run the TCA cycle in reverse and convert glucose to succinate through the intermediates of oxaloacetate, malate and fumarate. This pathway is exploited in metabolic engineering to net generate succinate for human use. Additionally, succinic acid produced during the fermentation of sugar provides a combination of saltiness, bitterness and acidity to fermented alcohols.
Accumulation of fumarate can drive the reverse activity of SDH, thus enhancing succinate generation. Under pathological and physiological conditions, the malate-aspartate shuttle or the purine nucleotide shuttle can increase mitochondrial fumarate, which is then readily converted to succinate.

Glyoxylate cycle

Succinate is also a product of the glyoxylate cycle, which converts two two-carbon acetyl units into the four-carbon succinate. The glyoxylate cycle is utilized by many bacteria, plants and fungi and allows these organisms to subsist on acetate or acetyl CoA yielding compounds. The pathway avoids the decarboxylation steps of the TCA cycle via the enzyme isocitrate lyase which cleaves isocitrate into succinate and glyoxylate. Generated succinate is then available for either energy production or biosynthesis.

GABA shunt

Succinate is the re-entry point for the gamma-aminobutyric acid shunt into the TCA cycle, a closed cycle which synthesizes and recycles GABA. The GABA shunt serves as an alternate route to convert alpha-ketoglutarate into succinate, bypassing the TCA cycle intermediate succinyl-CoA and instead producing the intermediate GABA. Transamination and subsequent decarboxylation of alpha-ketoglutarate leads to the formation of GABA. GABA is then metabolized by GABA transaminase to succinic semialdehyde. Finally, succinic semialdehyde is oxidized by succinic semialdehyde dehydrogenase to form succinate, re-entering the TCA cycle and closing the loop. Enzymes required for the GABA shunt are expressed in neurons, glial cells, macrophages and pancreatic cells.

Cellular metabolism

Metabolic intermediate

Succinate is produced and concentrated in the mitochondria and its primary biological function is that of a metabolic intermediate. All metabolic pathways that are interlinked with the TCA cycle, including the metabolism of carbohydrates, amino acids, fatty acids, cholesterol, and heme, rely on the temporary formation of succinate. The intermediate is made available for biosynthetic processes through multiple pathways, including the reductive branch of the TCA cycle or the glyoxylate cycle, which are able to drive net production of succinate. In rodents, mitochondrial concentrations are approximately ~0.5 mM while plasma concentration are only 2–20 μM.

ROS production

The activity of succinate dehydrogenase, which interconverts succinate into fumarate participates in mitochondrial reactive oxygen species production by directing electron flow in the electron transport chain. Under conditions of succinate accumulation, rapid oxidation of succinate by SDH can drive reverse electron transport. If mitochondrial respiratory complex III is unable to accommodate excess electrons supplied by succinate oxidation, it forces electrons to flow backwards along the electron transport chain. RET at mitochondrial respiratory complex 1, the complex normally preceding SDH in the electron transport chain, leads to ROS production and creates a pro-oxidant microenvironment.

Additional biologic functions

In addition to its metabolic roles, succinate serves as an intracellular and extracellular signaling molecule. Extra-mitochondrial succinate alters the epigenetic landscape by inhibiting the family of 2-oxogluterate-dependent dioxygenases. Alternative, succinate can be released into the extracellular milieu and the blood stream where it is recognized by target receptors. In general, leakage from the mitochondria requires succinate overproduction or underconsumption and occurs due to reduced, reverse or completely absent activity of SDH or alternative changes in metabolic state. Mutations in SDH, hypoxia or energetic misbalance are all linked to an alteration of flux through the TCA cycle and succinate accumulation. Upon exiting the mitochondria, succinate serves as a signal of metabolic state, communicating to neighboring cells how metabolically active the originating cell population is. As such, succinate links TCA cycle dysfunction or metabolic changes to cell-cell communication and to oxidative stress-related responses.