| Abstract
| - On the basis of static and time-resolved resonance Raman spectroscopy of HbA and of a mutant,HbK (Dα99N), a specific reaction coordinate is proposed for the allosteric transition in human hemoglobin.The heme is held between proximal (F) and distal (E) helices, whose orientation is responsive to forcesgenerated by ligation and deligation. The E and F helices are in turn tethered via H-bonds to the A andH helices. These outer helices follow the E−F motion, thereby repositioning the N- and C-termini, whichform the intersubunit salt bridges in the T quaternary structure. When the T state interface is weakenedby Asp → Asn substitution at a quaternary H-bond (HbK), the Fe−His bond is relaxed and becomesresponsive to allosteric effectors. The same E−F motion is observed in HbK, but the A−H followingmotion is delayed, relative to HbA, as is the Asn H-bond formation.
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