This is the final step of tyrosine degradation. Fumarylacetoacetase catalyzes the hydrolysis of 4-fumarylacetoacetate (4FAA) to form fumarate and acetoacetate. The enzyme is a homodimer, with each monomer binding one Ca(2+) and one Mg(2+) ion as catalytic cofactors, as shown in a mouse structure (Hsiang et al., 1972; Labelle et al. 1993; Bateman et al., 2001). FAH deficiency leads to tyrosinemia type I (TYRSN1, MIM:276700) characterized by 4FAA accumulation with severe metabolic effects (reviewed in Pitkänen et al., 2000; Scott, 2006).