Open AccessInfluence of Unsaturated Fatty Acids in Chloroplasts
Author(s) -
SIEGENTHALER PaulAndré,
DEPÉRY Françoise
Publication year - 1976
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1976.tb10052.x
Subject(s) - electrochemical gradient , photosystem ii , thylakoid , chemistry , photosystem i , photosystem , ferricyanide , linolenic acid , dcmu , photochemistry , electron transport chain , photosynthesis , chloroplast , fatty acid , biochemistry , biophysics , linoleic acid , biology , membrane , gene
Linolenic acid (C 18:3 ) is the main endogenous unsaturated fatty acid of thylakoid membrane lipids, and seems in its free form to exert significant effects on the structure and function of photosynthetic membranes. In this investigation the effect of linolenic acid was studied at various pH values on the electron flow rate in isolated spinach chloroplasts and related to ΔpH, the proton pump and the pH of the inner thylakoid space (pH i ). The ΔpH and pHi were estimated from the extent of the fluorescence quenching of 9‐aminoacridine. Linolenic acid caused a shift (approximately one unit) of the pH optimum for electron flow toward acidity in the following systems: (a) photosystems II + I (from H 2 O to NADP + or to 2,6‐dichlorophenolindophenol) coupled or non‐coupled; (b) photosystem II (from H 2 0 to 2,6‐dichloro‐phenolindophenol in the presence of dibromothymoquinone). In photosystem I conditions (phenazine methosulphate), the ΔpH of the control increased as a function of external pH o with a maximum around pH 8.8. When linolenic acid was added, the ΔpH dropped, but its optimum was shifted toward more acidic pH o . The same phenomena were also observed in photosystems II + I (from H 2 O to ferricyanide) and in photosystem II conditions (from H 2 O to ferricyanide in the presence of dibromothymoquinone). However, the ΔpH was smaller and the sensitivity of the proton gradient toward linolenic acid was eventually higher than for photosystem I electron flow activity. The proton pump which might be considered as a measure of the internal buffering capacity of thylakoids was optimum at pH o 6.7 in the controls. An addition of linolenic acid diminished the proton pump and shifted its optimum toward higher pH o . As a consequence, pH i increased when pH o was raised. At the optimal pH o 8.6 to 9, pH i were 5 to 5.5. Additions of increasing concentrations of linolenic acid displaced the curves toward higher pH i . A decrease of pH o was therefore required to maintain the pH i in the range of 5 ‐ 5.5 for maximum electron flow. In conclusion, the electron flow activity seems to be delicately controlled by the proton pump (buffer capacity), ΔpH, pH i and pH o . Fatty acids damage the membrane integrity in such a way that the subtle equilibrium between the factors is disturbed.