chemical coordination complex of an iron ion chelated to a porphyrin

A haem (British English) (or heme (American English) is a type of co-factor. It is an iron ion in the center of a large organic ring called a porphyrin.

Ball and Stick model of Heme B

Not all porphyrins contain iron, but many porphyrin-containing metalloproteins have haem as their co-factor; these are known as haemoproteins. Haems are components of haemoglobin, the red pigment in blood. They are also components of a number of other haemoproteins.