The pair in which both species have iron is

Cytochromes are conjugated proteins consisting of an apoprotein and a prosthetic group (heme). The heme consists of a porphyrin with a central iron atom. The name haemoglobin is the concatenation of heme and globin, reflecting the fact that each subunit of haemoglobin is a globular protein with an embedded heme (or haem) group; each heme group contains an iron atom, and this is responsible for the binding of oxygen. The most common types of haemoglobin contains four such subunits, each with one heme group.