text: Succinate is formed in E. coli in several steps.
  Phosphoenolpyruvate (PEP), a glycolysis pathway intermediate, is carboxylated by the enzyme PEP carboxylase to form oxaloacetate.   
  This is followed by the conversion of oxaloacetate to malate by the enzyme malate dehydrogenase. Fumarate hydratase then catalyses the dehydration of malate to produce fumarate.
        phosphoenolpyruvate + HCO → oxaloacetate + phosphate
        oxaloacetate + NADH + H → malate + NAD
        malate → fumarate + HO
  The final reaction in the formation of succinate is the reduction of fumarate. It is catalysed by the enzyme fumarate reductase.
        fumarate + NADH + H → succinate + NAD
  This reduction is an anaerobic respiration reaction in E. coli, as it uses electrons associated with NADH dehydrogenase and the electron transport chain. ATP is generated by using an electrochemical gradient and ATP synthase. This is the only case in the mixed acid fermentation pathway where ATP is not produced via substrate-level phosphorylation.
  Vitamin K, also known as menaquinone, is very important for electron transport to fumarate in E. coli.