A possible new control mechanism suggested by resonance Raman spectra from a deep ocean fish hemoglobin

The rattail fish, Coryphaenoides armatus, lives at ocean depths of 3000 m. As an adaptation for pumping oxygen into the swim bladder against the extreme pressures at the ocean bottom, the hemoglobin from this fish at low pH exhibits an extraordinarily low affinity for ligands. In this study, continuous wave and time-resolved Raman techniques are used to probe the binding site in this hemoglobin. The findings show an association between the low-affinity material and a highly strained heme-proximal histidine linkage. The transient Raman studies reveal differences in the protein structural dynamics at pH 6 and 8. The emerging picture derived from both this and earlier studies is that in vertebrate hemoglobins the heme-proximal histidine linkage represents a key channel through which species- and solution-dependent variations in the globin are communicated both statically and dynamically to the heme to produce an extensive range of ligand binding properties. Also presented is a new model that relates both intensity and frequency of the resonance Raman band involving the iron-proximal histidine stretching mode to specific protein controlled structural degrees of freedom. There emerges from this model a mechanism whereby modifications in the proximal heme pocket can further reduce the affinity of an already highly strained T state structure of hemoglobin.