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4吡咯亚基形的环状大分子，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.

血红素是大分子，四吡咯亚基形环形，构叫作卟啉。