 |
Center for Nonlinear Studies |
Hemoglobin is a model system in which to study protein allostery. In addition, functional and structural studies on hemoglobin and myoglobin from different animals and engineered variants have revealed the significance of residues in the distal pocket for determining the physico-chemical properties of the protein. The mutant Hb$\alpha\beta$YQ (LeuB10$\rightarrow$Tyr, HisE7$\rightarrow$Gln) has a low reactivity for NO, O2 and CO and a reduced cooperativity, and is a candidate for the synthesis of a hemoglobin-based oxygen carrier blood substitute. We present an x-ray study of structural changes induced by photodissociation of fully-ligated T-state Hb$\alpha\beta$YQ at cryogenic temperatures. On photodissociation of CO, the ligand moves from the heme to a protein cavity associated with a docking site associated with a novel xenon-binding position and the $\beta$ subunits adopt a deoxy conformation following photolysis. These data shed new light upon the structural changes required for cooperativity and lay the foundation for future time-resolved studies.