Pick from:
-- DNA ring;
-- petrarchan ring;
-- reagent ring;
-- porphyrin ring;

Answer the following question given this paragraph: The yellow subunits are identical in structure, as are the gray subunits. A smaller organic structure called a porphyrin ring is attached to each protein subunit and contains the iron ion used to bind oxygen. Q: The iron ion used to bind oxygen is on what type of "ring"? A:
Answer:
porphyrin ring