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. 2017 Jul 14;7:5504. doi: 10.1038/s41598-017-04554-3

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Illustration of six T state stabilizing salt bridges in Hb T-R transition. The Hb T-R transition at the early oxygenation stages and the status of six T state stabilizing salt bridges, as proposed by Perutz are shown. All six T state stabilizing salt bridges, including α₁Arg141–α₂Asp126, α₁Arg141–α₂Lys127, β₁His146–α₂Lys40 and their symmetrical counterparts, α₁Asp126–α₂Arg141, α₁Lys127–α₂Arg141 and β₂His146–α₁Lys40 are highlighted to illustrate the status of each T state stabilizing salt bridge during progressive oxygenation. (a) DeoxyHb in the T state with all six T state stabilizing salt bridges remained intact, (b) oxygenation occurs on α₁ subunit, with the α₁Arg141–α₂Asp126, α₁Arg141–α₂Lys127 salt bridges ruptured, (c) oxygenation occurs on α₂ subunit, with α₂Arg141–α₁Asp126, α₂Arg141–α₁Lys127 salt bridges ruptured. Hb’s α₁/α₂ interface becomes destabilized. (d) The last two T state stabilizing salt bridges are ruptured and 2,3-BPG is repelled out of the β₁/β₂ cavity. The role of αArg141 in constituting the α₁/α₂ interface and in sustaining the low-affinity T state can be visualized.