This family consists of fumarylacetoacetate (FAA) hydrolase, or fumarylacetoacetate hydrolase (FAH) and it also includes HHDD isomerase/OPET decarboxylase from E. coli strain W. FAA is the last enzyme in the tyrosine catabolic pathway, it hydrolyses fumarylacetoacetate into fumarate and acetoacetate which then join the citric acid cycle [1]. Mutations in FAA cause type I tyrosinemia in humans this is an inherited disorder mainly affecting the liver leading to liver cirrhosis, hepatocellular carcinoma, renal tubular damages and neurologic crises amongst other symptoms [1]. The enzymatic defect causes the toxic accumulation of phenylalanine/tyrosine catabolites [3]. The E. coli W enzyme HHDD isomerase/OPET decarboxylase contains two copies of this domain and functions in fourth and fifth steps of the homoprotocatechuate pathway; here it decarboxylates OPET to HHDD and isomerises this to OHED. The final products of this pathway are pyruvic acid and succinic semialdehyde. This family also includes various hydratases and 4-oxalocrotonate decarboxylases which are involved in the bacterial meta-cleavage pathways for degradation of aromatic compounds. 2-hydroxypentadienoic acid hydratase encoded by mhpD in E. coli Swiss:P77608 is involved in the phenylpropionic acid pathway of E. coli and catalyses the conversion of 2-hydroxy pentadienoate to 4-hydroxy-2-keto-pentanoate and uses a Mn2+ co-factor [5]. OHED hydratase encoded by hpcG in E. coli Swiss:P42270 is involved in the homoprotocatechuic acid (HPC) catabolism [6]. XylI in P. putida Swiss:P49155 is a 4-Oxalocrotonate decarboxylase [7].

1: Mutations in the fumarylacetoacetate hydrolase gene causing hereditary tyrosinemia type I: overview. St-Louis M, Tanguay RM; Hum Mutat 1997;9:291-299. PMID:9101289

2: Molecular characterization of the 4-hydroxyphenylacetate catabolic pathway of Escherichia coli W: engineering a mobile aromatic degradative cluster. Prieto MA, Diaz E, Garcia JL; J Bacteriol 1996;178:111-120. PMID:8550403

3: Fungal metabolic model for human type I hereditary tyrosinaemia. Fernandez-Canon JM, Penalva MA; Proc Natl Acad Sci U S A 1995;92:9132-9136. PMID:7568087

4: Purification, nucleotide sequence and some properties of a bifunctional isomerase/decarboxylase from the homoprotocatechuate degradative pathway of Escherichia coli C. Roper DI, Cooper RA; Eur J Biochem 1993;217:575-580. PMID:8223600

5: Purification, characterisation and reaction mechanism of monofunctional 2-hydroxypentadienoic acid hydratase from Escherichia coli. Pollard JR, Bugg TD; Eur J Biochem 1998;251:98-106. PMID:9492273

6: Sequence of the hpcC and hpcG genes of the meta-fission homoprotocatechuic acid pathway of Escherichia coli C: nearly 40% amino- acid identity with the analogous enzymes of the catechol pathway. Roper DI, Stringfellow JM, Cooper RA; Gene 1995;156:47-51. PMID:7737515

7: Comparison of the nucleotide sequences of the meta-cleavage pathway genes of TOL plasmid pWW0 from Pseudomonas putida with other meta- cleavage genes suggests that both single and multiple nucleotide substitutions contribute to enzyme evolution. Harayama S, Rekik M; Mol Gen Genet 1993;239:81-89. PMID:8510667