Commonly abbreviated as Hb, Hemoglobin, or Heamoglobin, is the iron containing oxygen in our red blood cells which transport metalloproteins. All mammals on earth have hemoglobin, as it is a necessary function in the blood. It contains globin, apoprotien, and four heme groups (organic molecules with one atom of iron attached to each).The gene for the hemoglobin protien can sometimes mutate. This occurence results in one or more of many diseases, but most commonly turns into Thalassemia or Sickle-cell disease. Heme groups are located in each sub-unit of a hemoglobin molecule. A heme group consists of a single iron atom, held in a heterocyclic ring, commonly known as a "porphyrin". Oxygen binding takes place in this iron atom. The one iron atom binds itself equally to all four nitrogens in the center of the heterocyclic ring, which lies on one plane. In addition, two bonds perpendicular to the plane on each side, are sometimes formed with the iron to produce the fifth and sixth positions. The name hemoglobin comes from "heme" and "globin". Globin is a generic term used for a globular protein. Since any single subunit of hemoglobin is made of a heme imbedded in a globular protein, the name makes perfect sense. There are many heme containing hemoglobins and proteins. Hemoglobin A is the most commonly known. In adults, the most common hemoglobin is a tetramer (hemoglobin containing 4 subunit proteins) called hemoglobin A. The subunits are similar in structure, and approximatly the same size. Each subunits molecular weight is about 16,000 daltons, for a total combined molecular weight in the tetramer of approximatly 64,000 daltons. A single heme is contained in each subunit of hemoglobin, so that the overall binding capacity of human adults hemoglobin for oxygen is four oxygen molecules.