Myoglobin electron transfer kinetics

An alternative application of flash photolysis to study myoglobin electron transfer kinetics has been employed by Hofifinan and co-workers 156). In this approach, the photoactive zinc-substituted derivative of Mb is mixed with an equivalent amoimt of ferricytochrome bs to form an electrostatically stabilized binary complex. Upon transient irradiation, the strongly reducing Zn-Mb intermediate is formed, and the kinetics of ferricytochrome reduction within the preformed complex can be monitored spectrophotometrically. The resulting kinetics represents a mixed-order process consistent with electron transfer both within the electrostatically stabilized complex and between the dissociated components of the complex. 

The single potential step chronoabsorptometry technique has been employed to determine the heterogeneous electron transfer kinetic parameters of myoglobin [36], horse heart cytochrome c [37] and soluble spinach ferredoxin [38]. In every case, the chronoabsorptometric data were analysed according to the irreversible model (the reverse reaction is ignored). The error associated with the use of this model for the kinetic analysis of these systems is most pronounced at low overpotentials, long transient times, and large reaction rates. 

Edmund E. Bowden conducted potential step chronoabsorptometry experiments on the reaction of myoglobin at modified gold minigrid electrodes [40]. Although these experiments were very reproducible, the heterogeneous electron transfer kinetic parameters raised concerns, namely, the rate constant was very low (k° = 3.9 X 10-11 cm/s) and alpha was high (a = 0.88). These issues became muted as the work progressed, as will be discussed below. 

Kinetics and thermodynamics of long-distance electron transfer in Ru(NH3)5 (His-48) myoglobin (Fig. 1, where selected parts of the molecular skeleton of sperm whale myoglobin with Ru(NH3)5 bonded imidazole of His-48 are illustrated) have also been determined 52). The electron transfer observed is represented by... 

Myoglobin is a classic example of a protein with a single Fe " /Fe redox centre that exhibits a reversible Nernstian response. The kinetics of homogeneous electron transfer are reasonably rapid in a myoglobin system despite the tertiary globin structure surrounding the heme iron. Additionally, the porphyrin... 

The kinetics of protein folding triggered by electron transfer in myoglobin and cytochrome b is much faster than in in cytochrome c. Upon reduction of the central iron ion, a-helices cluster around the heme forming, for example, a four-helix bundle. It appears that highly helical proteins have favorable energy landscapes for folding (Wolynes, 1996 Telford et al, 1998). 

It has been considered that protein dynamics are too slow to affect electron transfer. However, the different kinetic constants measured for the electron transfer for proteins indicates that the electron transfer can occur within the range of the protein dynamics timescale. For example, the highest kinetic constant for the electron transfer between Zn porphyrin to Ruthenium both bound to myoglobin is found equal to 7.2 x 10 s (Casimiro et al. 1993.) This means a time equal to 14 ns. This time is in the same range of the protein rotation and even slower from the time of the local rotation which averages 1 ns or less. 

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