What ion does the hemoglobin molecule need..?

Adult hemoglobin is a [a(2):b(2)] tetrameric hemeprotein found in erythrocytes where it is responsible for binding oxygen in the lung and transporting the bound oxygen throughout the body where it is used in aerobic metabolic pathways. For a description of the different types of hemoglobin tetramers see the section below on Hemoglobin Genes. Each subunit of a hemoglobin tetramer has a heme prosthetic group identical to that described for myoglobin. The common peptide subunits are designated a, b, g and d which are arranged into the most commonly occurring functional hemoglobins