Tetrapyrrole

A tetrapyrrole is a type of organic compound with four ("tetra") pyrrole rings linked together by chemical bonds to make a chain or a bigger ring.

Some of the most important chemicals in biology are tetrapyrrole coordination complexes. In these organometallic compounds, some of the hydrogen atoms are removed from the middle of a tetrapyrrole and replaced with a metal cation. Examples include heme, an iron complex; chlorophyll, a magnesium complex; and vitamin B12, a cobalt complex.