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.

血红素是一个由4个吡形的分子，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.

血红素是个分子，由四个吡形形，这个结构叫作卟啉。