Myoglobin recombination kinetics

A method which can be used to give complementary information about conformations and kinetics is low temperature spectroscopy described by Austin and Shyamsunder [7]. Experimental methods for measuring absorption and fluorescence, theoretical principles involved in their interpretation, and recombination kinetics of carbon monoxide and other ligands with the heme iron in myoglobin are discussed. 

The vast majority of fast ligand substitutions studied to date are in the s, ms, and fjLS time scales, which encompass a vast range of reactions, and for which fast reaction techniques have become both refined and readily commercially available. Not surprisingly the majority of reactions reported in this chapter fall into these time scales, and have been studied predominantly by stopped-flow, temperature-jump, or nuclear magnetic resonance fast reaction techniques, which are referred to by the initials SF, TJ, and NMR hereafter. However, substantial rewards await those who venture into the ns and ps time scale as shown by a study of the recombination kinetics of small ligands at the Fe(II) center of sperm whale and elephant myoglobins in which laser pulses of 1 ps and 4 ns facilitated the determination of rate constants in the range 3 x 10 to 5 x s and 10 to 10 ... 

An example of the process described by Eq. (56) is given by the low-temperature (below 200 K) recombination kinetics CO to myoglobin (Mb, a heme group with a central iron atom) after photolysis of carboxymyoglobin MbCO [40,41], A short account of the kinetics which have been described by Eq. (56) is given in Ref. 37. 

Effects of solvent mixtures can be seen in biochemical systems. Ligand binding to myoglobin in aqueous solution involves two kinetic components, one extramolecular and one intramolecular, which have been interpreted in terms of two sequential kinetic barriers. In mixed solvents and subzero temperatures, the outer barrier increases and the inner barrier splits into several components, giving rise to fast intramolecular recombination. Measurements of the corresponding solvent structural relaxation rates by frequency resolved calorimetry allows the discrimination between solvent composition and viscosity-related effects. The inner barrier and its coupling to structural relaxation appear to be independent of viscosity but change with solvent composition (Kleinert et al., 1998). 

We think that the classic example of what can be done with heme protein kinetics is the series of papers by Frauenfelder et al. (1975,1976a,b, 1978, 1980) on kinetics of recombination of carbon monoxide (CO) to myoglobin, hemoglobin, and protoheme over the temperature range from 350 to 4 K ... 

Figure 6. Kinetics of the recombination of carbon monoxide with myoglobin over the three main temperature regimes. In part (a) only the low-temperature geminate, power-law kinetics are seen. In part (b) the lower "blip shows the beginning of bimolecular kinetics, whereas in (c) the kinetics are totally bimolecular.

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