This page needs JavaScript to draw the hemoglobin model. In words: hemoglobin has four chains, two alpha and two beta, each holding a heme with one iron atom that can carry one oxygen. Without oxygen it sits in the T (tense) state. As oxygen binds, each iron settles into its heme ring and one alpha-beta pair turns about 15 degrees against the other, giving the R (relaxed) state, which holds oxygen more strongly.