Association constants for metal hexacyanide binding to cytochrome c

Heme crevice disorder after sixth ligand displacement in the cytochrome c-551 family

1H NMR and visible absorption spectroscopy were used to monitor sixth ligand methionine displacement reactions in four members of the ferricytochrome c-551 family from Pseudomonas aeruginosa, Pseudomonas stutzeri, Pseudomonas stutzeri substrain ZoBell, and Nitrosomonas europae. Potassium cyanide displaces the methionine ligand with very modest changes in the visible spectra, but profound changes in the NMR spectra. The initial product formed kinetically, designated complex I, changes with time and/or heating to a more thermodynamically favored product termed complex II. Spectra indicate that both I and II are actually a family of closely related conformational isomers. Low temperature NMR spectra of complex II indicate that some of the isomers are in chemical exchange on the NMR time scale. High pH also displaces the methionine ligand in a manner similar to the well-known alkaline transition of mitochondrial cytochrome c. However, the reaction occurs at higher pH values and over a narrower pH range for the c-551 family, and the transition pH range is different for the different proteins studied. The final alkaline forms also show peak widths and a number of peaks indicative of multiple conformational isomers.

Electron transfer between cytochrome c and metal hexacyanide complexes. Effect of thermodynamic driving force on the electron transfer rate

The electron transfer reactions between ferrocytochrome c and three isomorphic hexacyanide complexes, [Fe(CN)6]3-, [Os(CN)6]3- and [Ru(CN)6]3-, have been studied using the method of photoexcitation. The transfer rates for [Os(CN)6]3- and [Ru(CN)6]3- are, respectively, about 45- and 200-times higher than that of [Fe(CN)6]3-. A reorganization energy of approx. 0.8 eV was found for the cytochrome c-hexacyanide system when the data were analyzed according to the theory of Marcus.

Kinetics of electron transfer between cytochrome c and iron hexacyanides. Evidence for two electron-transfer sites

The electron-transfer reaction between ferrocytochrome c and ferricyanide has been studied by the method of photoexcitation. The observed transfer rate shows saturation behaviour at high ferricyanide concentration. Data analysis indicates that there are two binding sites of vastly different affinities at which electron transfer occurs. The binding constant for the strong binding site decreases from 1600 M-1 to 80 M-1 as the ionic strength increases from 15 mM to 140 mM. At 20 degrees C, the intramolecular electron-transfer rate for this site is 4.65 X 10(4) s-1, which gives an electron-transfer distance of approx. 9.7 A according to Hopfield's model.

Kinetics of electron transfer between mitochondrial cytochrome c and iron hexacyanides

The reduction of horse and Candida krusei cytochromes c by ferrocyanide has been studied by 1H NMR spectroscopy and the reaction found to involve a precursor complex of ferrocyanide bound to ferricytochrome c (pH* 7.4, 2H2O, I = 0.12, and 25 degrees C). The electron transfer rate constants for the reduction of the two ferricytochromes by associated ferrocyanide were found to be the same at 780 +/- 80 sec-1 but the association constants for binding of ferrocyanide to ferricytochrome c were significantly different: horse, 90 +/- 20 M-1 and Candida, 285 +/- 30 M-1. The different association constants partly accounts for the previously observed reactivity difference between horse and Candida cytochromes c. Comparison of the NMR data with data obtained by other kinetic methods has allowed the electron transfer rate constant for the oxidation of ferrocytochrome c by associated ferricyanide to be determined. This was found to be 4.6 +/- 1 X 10(4) sec-1.