Ferredoxin electron transfer kinetics

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. 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

If the rate constants for parallel reactions are to be resolved, then analysis of the products is essential (Sec. 1.4.2). This is vital for understanding, for example, the various modes of deactivation of the excited state (Sec. 1.4.2), Only careful analysis of the products of the reactions of Co(NH3)jH20 + with SCN, at various times after initiation, has allowed the full characterization of the reaction (1.95) and the detection of linkage isomers. Kinetic analysis by a number of groups failed to show other than a single second-order reaction.As a third instance, the oxidation of 8-Fe ferredoxin with Fe(CN)g produces a 3Fe-cluster, thus casting some doubt on the reaction being a simple electron transfer. 

R Hisada and T Yagi (1977) 1-methyl-5-methylphenazinium methyl sulfate. J Biochem 82 1469-1473 IR Vassiliev, Y-S Jung, F Yang and JH Golbeck (1998) PsaC subunit of photosystem I is oriented with iron-sulfur clusterFb as the immediate electron donor to ferredoxin and flavodoxin. Biophys J 74 2029-2035 39. VP Shinkarev, IR Vassiliev and JH Golbeck (2000) A kinetic assessment of the sequence of electron transfer from Fx to F and further to Fb in photosystem I The value of equilibrium constant between Fx and F. Biophys J 78 363-372... 

Fig. 16. Absorbance changes measured at 480 nm and associated with ferredoxin reduction induced at 660-nm by 1-r s dye laser flashes. (A) In the absence and presence of spinach ferredoxin baseline for each trace is shifted for clarity (B) plot of rate constant vs. concentration of ferredoxins from spinach and the green alga Monoraphidium braunni. Figure source Herv s, Navarro and Toltin (1992) A laser flash spectroscopy study of the kinetics of electron transfer from spinach photosystem I to spinach and algal ferredoxins. Photochem Photobiol 56 321.

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