Hemoglobin: Studying the T to R Transition
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.
Structural studies have shown that hemoglobin exists in one of two conformations, known as T (taut) and R (relaxed). Deoxygenated hemoglobin (blue) is found in the T state, and oxygen binding (red) triggers the transition to the R state. The animation on the right shows a close-up view of the heme group (white, in ball and stick representation) in one of the hemoglobin subunits. In the deoxygenated (T) state, the iron atom is non-planar with the rest of the heme group due to its association with a histidine side chain. Oxygen binding causes the iron atom in the heme to move such that it becomes planar with the rest of the heme group, which then pulls the histidine, causing a larger scale structural change in the protein.
Hemoglobin can be thought of as a tetramer made up of two alpha-beta dimers. The conformational change that occurs during the T to R transition takes place primarily in the positions of these two dimers relative to one another (rather than between the alpha and beta subunits within the same dimer). This is illustrated in the last (black and white) segment of the animation on the right.
The T to R transition requires that at least two of the hemoglobin subunits be bound by oxygen. Since hemoglobin in the T state only has a low affinity for oxygen, the conformational change can only occur under relatively high oxygen concentrations (such as in the lung capillaries). In the R state, hemoglobin binds to oxygen with much greater affinity, leading to any remaining deoxygenated subunits quickly binding to oxygen. This concept is shown in the center animation on the right.
Oxygen-rich red blood cells in the lungs must circulate throughout the body to provide tissues with oxygen for metabolic processes. There are several key molecules that contribute to the ability of hemoglobin to unload oxygen into oxygen-hungry tissues.
Protons are important allosteric effectors of hemoglobin. At relatively low pH (such as in respiring muscle tissues), hemoglobin has a lower affinity for oxygen than it does at higher pHs (such as in the lung tissue).
Another allosteric regulator of the T to R transition is 2,3-DPG. As shown in the animation on the right, 2,3-DPG can bind in the central pocket of hemoglobin when hemoglobin is in the T state. Binding of 2,3-DPG is mediated by a rosette of amino acid side chains from both beta subunits. By this mechanism, 2,3-DPG stabilizes the T state and lowers the affinity of hemoglobin for oxygen. Upregulation of 2,3-DPG increases the delivery of oxygen to tissues in low-oxygen conditions.
Please note that animations and illustrations from this website are licensed under a Creative Commons License, and may be freely downloaded for non-commercial uses with proper attribution. See link at bottom of page for more information.
movie 1: conformational changes upon oxygen binding, focusing first on a heme group and then zooming out to see the structure of the tetramer.
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movie 1 can also be downloaded as separate segments that can be looped:
A. includes the close-up of the heme group
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B. includes the zoomed-out tetramer structure
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C. shows just the dimer-dimer interface
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movie 2: shows requirement for 2 bound oxygens for T to R transition.
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movie 3: shows binding site of 2,3-DPG and dissociation of 2,3-DPG upon T to R transition
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Hemoglobin: Molecule of the Month on the RCSB Protein Data Bank website, with great images by David Goodsell and Shuchismita Dutta.
Wikipedia article on hemoglobin
Hemoglobin morphs on the Database of Macromolecular Movements
Many thanks to Frank Bunn, Anu Seshan and Randy King (Harvard Medical School) for collaborating on this project.This work by Janet Iwasa is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.