Editing Succinic acid (section)

=== Reductive branch of the TCA cycle ===
{{See also|Succinic acid fermentation}}
Succinate can alternatively be formed by reverse activity of SDH. Under anaerobic conditions certain bacteria such as ''A. succinogenes'', ''A. succiniciproducens'' and ''M. succiniciproducens'', run the TCA cycle in reverse and convert glucose to succinate through the intermediates of [[Oxaloacetic acid|oxaloacetate]], [[Malic acid|malate]] and [[Fumaric acid|fumarate]].<ref name=Cheng2013rev>{{Cite journal|last1=Cheng|first1=Ke-Ke|last2=Wang|first2=Gen-Yu|last3=Zeng|first3=Jing|last4=Zhang|first4=Jian-An|date=2013-04-18|title=Improved Succinate Production by Metabolic Engineering|journal=BioMed Research International|volume=2013|pages=538790|doi=10.1155/2013/538790|issn=2314-6133|pmc=3652112|pmid=23691505|doi-access=free}}</ref> This pathway is exploited in metabolic engineering to net generate succinate for human use.<ref name=Cheng2013rev/> Additionally, succinic acid produced during the fermentation of sugar provides a combination of saltiness, bitterness and acidity to fermented alcohols.<ref>{{Cite book|title=Knowing and Making Wine|last=Peynaud|first=Emile|date=1984}}</ref>

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 cycle|purine nucleotide shuttle]] can increase mitochondrial fumarate, which is then readily converted to succinate.<ref name=Haas2016rev/>