text: An enzyme dependent on this metallic element as a cofactor is inhibited by hydroxyurea.
full_question: An enzyme dependent on this metallic element as a cofactor is inhibited by hydroxyurea. An ion of this element is found in the +4 oxidation state bound to an oxo ligand in the key mechanistic step of an enzyme that also uses the cofactor tetrahydrobiopterin; that enzyme is phenylalanine hydroxylase. This element is used in the Bechamp reduction. The first known ligands of hapticity 5 were bonded to a central atom of this metal. It's not aluminum, but this element's trichloride is often used as a catalyst in the chlorination of arenes. This element is present both as the counter-ion and in the central complex ion of the dye Prussian blue. An alloy of this element and carbon can undergo martensite or pearlite phase transitions. For 10 points, name this transition metal found in ferrocene.
first_sentence: An enzyme dependent on this metallic element as a cofactor is inhibited by hydroxyurea.
raw_answer: iron [or Fe]