Fumaric acid is used extensively in the food, pharmaceutical, and polymer industries because of its reductive ability and nontoxic nature. It is primarily produced through the isomerization of its geometric isomer, maleic acid, with an annual global production of ~300,000 metric tons. Industrial methods employ catalysts such as aqueous hydrochloric acid and thiourea for their isomerization reaction, but these processes require extensive heating to achieve satisfactory reaction rates. For chemical conversion under milder reaction conditions, natural production of fumaric acid is achieved via fermentation with maleate isomerase (EC 5.2.1.1), also known as maleate cis-trans isomerase. This enzyme, part of the Asp/Glu racemase superfamily discovered in bacteria, catalyzes the industrially useful isomerization reaction under ambient conditions. Structural analysis and mutational studies of the enzymes revealed that thiols play a crucial role in the reaction. As such, thiol-based molecules have been investigated as promoters of cis-trans isomerization of unsaturated hydrocarbons. However, none of them was genuinely catalytic conducive to practical applications (Table ). Indeed, Chatgilialoglu and co-workers found that the combination of 2-mercaptoethanol and Co γ-radiolysis led to catalytic cis-trans isomerization of monounsaturated fatty acids, but the use of organic solvents may hinder efficient conversion. Adducts of thiols to double bonds by nucleophilic addition are frequently encountered as stable products, while the subsequent elimination of thiols following the pseudo-E/Z isomerization is impeded by both kinetics and thermodynamics.