_ of hemoglobin is also a porphyrin ring.

Explanation

The correct answer is the heam group. Hemoglobin is composed of protein chains and a non-protein component, the haem group, which contains a porphyrin ring. This ring is central to the function of hemoglobin, as it binds iron ions that facilitate oxygen transport. The α and β chains are protein subunits and do not contain the porphyrin ring. The term 'protein' is too broad and does not specifically identify the part of hemoglobin that includes the porphyrin structure.