Hemoglobin, a tetrameric protein found in high concentrations in red blood cells, is responsible for binding and transporting oxygen in the body. Each hemoglobin protein is made up of four subunits - two alpha subunits and two beta subunits - and each subunit is capable of binding to an oxygen molecule via its heme group. This animated pathway shows conformational changes upon oxygen binding, focusing first on a heme group and then zooming out to see the structure of the tetramer and then shows conformation changes from T to R states, and finally allosteric regulation by BPG.