4-Maleylacetoacetate (4-maleylacetoacetatic acid) is an intermediate in the metabolism of tyrosine. It is converted to fumarylacetoacetic acid by the enzyme 4-maleylacetoacetate cis-trans-isomerase.
The amino acid tyrosine is metabolised in a series of reactions, one of which is catalysed by homogentisate 1,2-dioxygenase. This converts the intermediate homogentisic acid into 4-maleylacetoacetic acid.
The enzyme maleylacetoacetate isomerase converts 4-maleylacetoacetic acid to its geometric isomer, fumarylacetoacetic acid.