Catalyzes cis-trans isomerization of the C2-C3 double bond in maleate to yield fumarate in the aerobic nicotinate degradation pathway. maleate = fumarate Cofactor degradation; nicotinate degradation. Homodimer. Repressed by NicR in the absence of 6-hydroxynicotinate (6HNA) inducer. In presence of 6HNA, repression is alleviated. Reaction is initiated by nucleophilic attack of cysteine at the double bond, yielding a covalent succinylcysteine-like intermediate. Belongs to the maleate isomerase family.