Elucidation of the true picture of the hemoglobin allosteric equilibrium

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The molecular mechanisms of protein allostery have been attributed to the two-state transition between T (tense) and R (relaxed) states, based largely on the knowledge about hemoglobin (Hb). However, an equilibrium ensemble for solution Hb cannot be adequately described by just these two states. Here we report an unprecedented crystal form in which Hb is free to adopt any allosteric structure, depending on the conditions, and at the same time allows oxygen binding to be monitored directly in the crystal. We identify nine conformations including a new intermediate between T and R. Our findings give a comprehensive picture of the equilibrium conformers and transition pathway for Hb.