Degradation of tyrosine to fumarate and acetoacetate starts with transamination of tyrosine (Figs. 4.2 and 4.3) to para-hydroxyphenylpyruvic acid, while a-ketoglutarate is the cofactor. The change of the lateral chain to the ortho position, oxidative decarboxylation and hydroxylation results in the formation of homogentisic acid. The next step of tyrosine metabolism is degradation of homogentisic acid.