The Dynamics of Dioxygen Binding to Hemerythrin
Oxygen carrier heme-proteins have proven to be very convenient tools for investigating internal protein dynamics. When a short pulse of laser light is absorbed by an oxy-protein, the heme-02 bond is immediately disrupted. Since oxy- and deoxy-proteins display different absorption spectra, an optical signal is available for monitoring the sequence of events accompanying the return to the bound state. Because the energetics of rebinding are sensitive to the protein conformation, the reaction constitutes an internal time probe which is modulated by protein movements. Heme-proteins (e.g., Mb or Hbs) have long been investigated in many laboratories. H. Frauenfelder and his co-workers pioneered the field of photolysis in glassy solvents at low temperatures; in other words, at practically infinite viscosity. They showed that proteins exist in a large number of conformational substates with slightly different reaction rates and that O2 binding is governed by several energy barriers in sequence rather than just one. It was later recognized that some of the barriers and associated rate constants were also dependent upon viscosity in agreement with their assumed dynamic nature (1,2).
KeywordsReaction Coordinate Oxygen Carrier Transition State Theory Viscosity Dependence Geminate Recombination
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