The reaction of ferric cytochrome <i>c</i> with dithionite at very low ionic strength

Goldkorn, Tzipora; Schejter, Abel

doi:10.1016/0014-5793(77)80605-4

Cited by 11 publications

(1 citation statement)

References 7 publications

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“…Similar biphasic kinetics at low ionic strength has also been observed for the reduction of c-type cytochromes with flavodoxin [20,21] and for the ascorbateand dithionite-mediated reduction of horse heart cytochrome c [22,23]' at pH 7.0. It was described to be due to the existence of two conformers of cytochrome c. The rate constants for the fast phase increased proportionally with the concentrations of MDH until a maximum value was reached.…”

Section: Stopped-flow Kineticssupporting

confidence: 73%

Studies on electron transfer from methanol dehydrogenase to cytochrome cL, both purified from Hyphomicrobium X

Dijkstra

Frank

Duine

1989

Biochemical Journal

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Ferricytochrome cL isolated from Hyphomicrobium X is an electron acceptor in assays for homologous methanol dehydrogenase (MDH), albeit a poor one compared with artificial dyes. The intermediates of MDH seen during the reaction are identical with those observed with Wurster's Blue as electron acceptor, indicating that the reaction cycles are similar. The assay showed a pH optimum of approx. 7.0 and scarcely any stimulation by NH4Cl, this being in contrast with assays with artificial dyes, where strong activation by NH4Cl and much higher pH optima have been reported. From the results obtained with stopped-flow as well as steady-state kinetics, combined with the isotope effects found for C2H3OH, it appeared that the dissimilarities between the electron acceptors can be explained from different rate-limiting steps in the reaction cycles. Ferricytochrome cL is an excellent oxidant of the reduced MDH forms at pH 7.0, but the substrate oxidation step is very slow and the activation by NH4Cl is very poor at this pH. At pH 9.0 the reverse situation exists: ferricytochrome cL is a poor oxidant of the reduced forms of MDH at this pH. No C2H3OH isotope effect was observed under these conditions, indicating that substrate oxidation is not rate-limiting, so that activation by NH4Cl cannot be found. Since just the opposite holds for assays with artificial dyes, the poor electron-acceptor capability and the different pH optimum of ferricytochrome cL as well as the insignificant activating effect of NH4Cl (all compared with artificial assays) can be explained. Although different views have been reported on the rate-limiting steps in the systems from Methylophilus methylotrophus and Methylobacterium sp. strain AM1, these are most probably incorrect, as rate-limiting electron transfer between ferrocytochrome cL and horse heart ferricytochrome c can occur. Therefore the conclusions derived for the Hyphomicrobium X system might also apply to the systems from other methylotrophic bacteria. Comparison of the assays performed in vitro (at pH 7.0) having ferricytochrome cL and Wurster's Blue as electron acceptor with methanol oxidation by whole cells shows that the former has similarity whereas the latter has not, this being although ferricytochrome cL is a poor electron acceptor in the assay performed in vitro. The reason for this is the absence of a (natural) activator able to activate the (rate-limiting) substrate oxidation step at physiological pH values.

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Section: Stopped-flow Kineticssupporting

confidence: 73%

Studies on electron transfer from methanol dehydrogenase to cytochrome cL, both purified from Hyphomicrobium X

Dijkstra

Frank

Duine

1989

Biochemical Journal

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Calorimetric determination of the enthalpy of ionization of oxidized and reduced horse heart cytochrome c

1980

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SynopsisThe heats of ionization of protons, AH,, of oxidized and reduced horse heart cytochrome c in 0.15M KC1 a t 20°C were determined using a titration calorimeter which simultaneously afforded the potentiometric titration curve. Reproducibility of the thermal titrations is within 2%, and evaluation of the heats observed after the heat loss corrections is estimated to be within 5%. A single titration of oxidized cytochrome c from pH 11 to 3 is in excellent agreement with the thermal titration of this protein obtained with flow calorimetry. The thermal titration, however, is not reversible, due in part to the loss of titratable group(s) in this pH region and to the heat contribution of the acid and alkaline conformational changes which occur. Although of lesser magnitude, the reduced form also indicates similar thermal transitions. These differences are due solely to conformational contributions to the thermal process, since the potentiometric curves are reversible. The nature of the irreversibility for oxidized cytochrome c appears to involve the loss of a group with pK' 8.9 and the shift of two groups from pK' 5.6 to 4.8. Thermal difference curves for this process indicate that heats of -7.8 and -24.1 kcal/mol are liberated which are centered at pH 9.3 and 3.9, respectively.

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