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.

红素是一个由4个吡咯亚基形的环状大分子，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.

红素是个大分子，由四个吡咯亚基形环形，这个结构叫作卟啉。