This heme is a large molecule that's made up of four pyrrole subunits that form a ring, and this structure is called a porphyrin.

血红素是一由4亚基形的环状，这种结构称之为卟啉。

This heme is a large molecule that's made up of four pyrrole subunits that forms a ring, and this structure is called a porphyrin.

血红素是，由四亚基形环形，这结构叫作卟啉。