BEN‐HAYYIM, G., Department of Biochemistry, George S. Wise Center for Life Sciences, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel
SCHEJTER, A., Department of Biochemistry, George S. Wise Center for Life Sciences, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel

Oxidized Euglena cytochrome f has an absorbance band at 695 nm which disappears upon reduction. Various treatments that cause known changes of conformation of cytochrome c appear to have similar effects on cytochromef. Heating cytochrome f results in the gradual disappearance of the 695‐nm band; the thermodynamic parameters for this change of the tertiary structure at 26 °C and ionic strength 0.01 are: ΔF=+2.67 kcal/mole, ΔH=+ 33 kcal/mol and ΔS=+102cal × K−1× mol−1. The pK values obtained by equilibrium titrations of the heme‐linked acid and alkaline ionizations are roughly 2.1 and 9.4. Equilibrium studies of the alkaline pK reveal that more than one hemelinked proton is dissociated in this pH range. The carboxymethylated product of the cyanide complex of oxidized cytochrome f has the same properties as cytochrome c carboxymethylated under the same conditions. The results suggest that a methionine sulfur is involved in iron coordination in oxidized Euglena cytochrome f. Copyright © 1974, Wiley Blackwell. All rights reserved