Catalyzes cis-trans isomerization of the C2-C3 double bond in maleate to yield fumarate. Shows a strict specificity for maleate, with no activity detected toward structurally related substrates including citraconate, mesaconate, dimethylmaleate, and maleamide. maleate = fumarate kcat is 2.8 sec(-1). Optimum pH is 7.5. Homodimer. Reaction is initiated by nucleophilic attack of cysteine at the double bond, yielding a covalent succinylcysteine-like intermediate. Belongs to the maleate isomerase family.