Open AccessThe Properties of Cytochrome f and P700 in Chloroplasts Suspended in Fluid Media at Sub‐zero Temperatures
Author(s) -
COX Raymond P.
Publication year - 1975
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1975.tb02200.x
Subject(s) - p700 , cytochrome , cytochrome b6f complex , cytochrome c , cytochrome f , chemistry , plastoquinone , photosystem i , photochemistry , redox , cytochrome b , cytochrome c peroxidase , electron transport chain , chloroplast , photosynthesis , inorganic chemistry , biochemistry , photosystem ii , mitochondrion , enzyme , thylakoid , mitochondrial dna , gene
1 The properties of P700 and cytochrome f have been studied at sub‐zero temperatures in chloroplasts suspended in a medium containing 50% (v/v) ethylene glycol. The dark reduction of these components after a period of illumination provided information about the rate‐limiting step of photosynthetic electron transport under these conditions. 2 The oxidation of P700 on illumination in the presence of methyl viologen and its subsequent dark reduction can be observed at −35°C. This cycle of reactions could be repeated many times. The rate of reduction was increased by NH 4 Cl and reduction was inhibited by 3(3,4‐dichloro‐phenyl)‐1,1‐dimethylurea. 3 The oxidation and reduction of cytochrome f could also be observed under similar conditions. The activation energies for the reduction of cytochrome f and P700 are similar (about 75 kJ mol −1 ) and the reduction of cytochrome f is also inhibited by dichlorophenyldimethylurea and stimulated by NH 4 Cl. 4 The reduction of both cytochrome f and P700 seemed to follow first‐order kinetics, but the t 1/2 for the reduction of the cytochrome was at least three times that for the reduction of P700 at the same temperature. It was concluded that the results were only compatible with a model in which the main pathway of electrons from plastoquinone to P700 involved cytochrome f if the equilibrium constant between the cytochrome and P700 was very much less than that expected from their redox potentials.