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A 2.16 Å resolution structure of high-salt human carbonmonoxyhemoglobin crystallized at pH 6.4 is reported. The quaternary structure is similar to that of `classic' R-state hemoglobin; however, subtle but significant tertiary structural changes are observed at the α1β2 and symmetrically equivalent α2β1 interfaces – these are the key subunit interfaces that govern the allosteric transition between the R and T states. Specifically, the movement and weakening of two important hydrogen bonds that are diagnostic for R-state structures, β2His97–α1Thr38 and β2Arg40–α1Thr41, have been observed. In addition, a phosphate molecule bound between the two β-­subunits (at the entrance to the central water cavity) has been identified and electron density indicates that this molecule occupies two alternate positions that are related by the dyad axis. Both positions superimpose on the 2,3-­diphosphoglycerate binding site. One phosphate conformer interacts with β2Asn139, β1His143 and β1His146, while the second interacts with symmetry-related counterparts (β1Asn139, β2His143 and β2His146).

Supporting information

PDB reference: COHbA, 1ljw, r1ljwsf


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