Mobashsher U. Khan

Differential thermal effects on the energy distribution between photosystem II and photosystem I in thylakoid membranes of a psychrophilic and a mesophilic alga.

Sensitivity of the photosynthetic thylakoid membranes to thermal stress was investigated in the psychrophilic Antarctic alga Chlamydomonas subcaudata. C. subcaudata thylakoids exhibited an elevated heat sensitivity as indicated by a temperature-induced rise in F(o) fluorescence in comparison with the mesophilic species, Chlamydomonas reinhardtii. This was accompanied by a loss of structural stability of the photosystem (PS) II core complex and functional changes at the level of PSI in C. reinhardtii, but not in C. subcaudata. Lastly, C. subcaudata exhibited an increase in unsaturated fatty acid content of membrane lipids in combination with unique fatty acid species. The relationship between lipid unsaturation and the functioning of the photosynthetic apparatus under elevated temperatures is discussed.

The role of phosphatidylcholine in fatty acid exchange and desaturation in Brassica napus L. leaves.

The role of phosphatidylcholine (PC) in fatty acid exchange and desaturation was examined and compared with that of monogalactosyldiacylglycerol (MGDG) in Brassica napus leaves using (14)C-labelling in vivo. Data are presented which indicate that in the chloroplast newly formed saturated (palmitic acid, 16:0) and monounsaturated (oleic acid, 18:1) fatty acid is incorporated into MGDG and desaturated in situ. In the non-plastidic compartments, however, newly formed fatty acid is exchanged with polyunsaturated fatty acid in PC, the probable major site of subsequent desaturation. The unsaturated fatty acid is released to the acyl-CoA pool, which is then used to synthesize diacylglycerol (DAG) containing a high level of unsaturated fatty acid. This highly unsaturated DAG may be the source for the biosynthesis of other cellular glycerolipids. The generally accepted pathway in which PC is synthesized from molecular species of DAG containing 16:0 and 18:1 followed by desaturation of the 18:1 to linoleic (18:2) and linolenic (18:3) acids is questioned.

Microwave-mediated methanolysis of lipids and activation of thin-layer chromatographic plates

A technique is described for the methanolysis of fatty acids from acylglycerols with HCl/CH3OH or NaOH/CH3OH using a microwave oven. The esterification is rapid and complete and does not result in significant degradation of polyunsaturated fatty acids, even in the presence of oxygen. The fatty acid compositions of intact tissues were also determined using this technique. The microwave oven has also been used to condition normal silica gel and argentation thin-layer chromatographic plates in a fraction of the time normally required.

Lipid biosynthesis in Brassica napus leaves: I. 14C-labelling kinetics of the fatty acids of the major glycerolipids

Abstract Data obtained from in vivo studies on the biosynthesis and desaturation of fatty acids in galactolipid biosynthesis of Brassica napus are significantly different from previous results obtained from Vicia faba. The differences are attributed to the difference in fatty acid composition between ‘16:3 plants’ (B. napus) and ‘18:3 plants’ (V. faba). The data suggest that, unlike V. faba, B. napus does not use the ‘phospholipid pathway’ in which phosphatidylcholine may act as a precursor to the diacylglycerols used in galactolipid biosynthesis. The results suggest the use of multiple pathways of biosynthesis of the diacylglycerol precursors of galactolipid which differ in importance from one plant species to another.

Low temperature-induced fatty acid desaturation in Brassica napus : thermal deactivation and reactivation of the process

When Brassica napus plants are grown at low temperatures (e.g., 5 degrees C) the rate of desaturation in leaves of newly formed fatty acids in both chloroplastic (MGDG) and cytosolic (PC) diacylglycerols is higher or more rapid than in plants grown at higher temperatures (e.g., 30 degrees C). This low temperature-induced increase in the rate of desaturation is lost within hours if plants are transferred to higher temperatures. However, if plants are then returned to low temperatures they regain the ability to rapidly desaturate fatty acids. This process is restored relatively slowly (over days) in contrast to the more rapid loss at high temperatures. This has important physiological consequences on the level of unsaturated fatty acids in plant membranes and the process of temperature control of the fatty acid composition of membrane lipids.

Computer Simulation Model for the Biosynthesis of Galactosyldiacylglycerols and Fatty Acid Desaturation in Plants (Determination of Rates of Desaturase Activity in Monogalactosyldiacylglycerol).

The level of unsaturation of the constituent fatty acids of many glycerolipids in plant membranes is modified by environmental factors. The measurement of the rate of the desaturation of these fatty acids is essential to an understanding of how plants adapt to changing environments. This is difficult because of the complexity of the system and the problems involved in measuring rates of these enzyme reactions in cell-free preparations. A computer program has been developed that simulates the synthesis of galactosyldiacylglycerols and desaturation of their fatty acids in chloroplasts. The program uses the rate of incorporation and distribution of 14C in fatty acids after 14CO2 feeding to estimate rates of desaturation in the fatty acids of glycerolipids. Data are presented to demonstrate the use of the program in comparing rates of desaturation in the five enzyme reactions associated with monogalactosyldiacylglycerol in the chloroplastic pathway of leaves from Brassica napus. The method represents a quick, reliable, and accurate measure of desaturase activity in vivo and is the only method available to estimate desaturase activity of all five enzymes at the same time.

Growth Temperature and Irradiance Modulate Trans-Δ3-Hexadecenoic Acid Content and Photosynthetic Light-Harvesting Complex Organization

Photosynthetic acclimation is an essential physiological process allowing plants to tolerate environmental stress conditions, such as low temperature [5]. Chloroplast thylakoid membranes represent a unique combination of pigments, proteins, and lipids whose organization allows for light harvesting, electron transport, and ultimately, the fixation of carbon. The major chlorophyll (Chi) a/b-binding protein associated with photosystem II (PSII), light-harvesting complex II (LHCII), is thought to be a trimer composed of Lhcbl and Lhcb2 polypeptides and plays a fundamental role in photosynthetic light-harvesting [6]. In addition, phosphatidylglycerol (PG) is the major phospholipid present in thylakoid membranes and characterized by the presence of a novel fatty acid, trans-Δ3-hexadecenoic acid (trans-16:1)[3]. Previously, it has been established that growth at cold-hardening temperatures (5°C) modulates LHCII organization such that the oligomeric form (LHCII 1) predominates at 20°C and monomeric (LHCII 3) and/or intermediate forms (LHCII 2) predominate at 5°C [3, 4, 7, 8, 9]. However, low temperature also results in a specific decrease in trans-16:1 content associated with PG, both in vivo and in situ,with minimal changes in lipid and pigment profiles [3, 4, 7]. Thus, the modulation of the supramolecular organization of LHCII may be achieved by specifically decreasing the trans-16:1 content associated with thylakoid PG, which has been shown to be important for the stabilization of oligomeric LHCII [3, 4, 7, 8, 9].

Temperature Regulation of Desaturation of Fatty Acids in Cytosolic and Chloroplastic Glycerolipids

Adaptation or acclimation to low temperatures is a phenomenon found in most poikilothermic organisms which must adjust to changing environmental conditions without an internal mechanism of homeostasis. In plants this acclimation normally takes the form of adjustment to high and low temperature by changes in protein and lipid components ofthe cell. These changes not only protect the cell from frost or chilling damage but may allow the cell to acclimate to optimal metabolic conditions. Our studies have focussed on one of the most important changes that take place in cell membranes during acclimation: the changing level of unsaturation of the membrane constituent acylglycerols. This, along with changes in the sterol components, is generally believed to affect the degree of fluidity of the membrane at different temperatures.

MODULATION OF FATTY ACIDS IN THE MEMBRANES OF ANACYSTIS NIDULANS (CYANOBACTERIA): INCORPORATION OF ODD-NUMBERED CARBON FATTY ACIDS1

Anacystis nidulans Richt., a unicellular cyanobacterium, can incorporate exogenously supplied fatty acids, including odd‐numbered carbon fatty acid (OFAs), into the acylglycerols of cell membranes. Data are presented for the uptake of undecanoic acid (11:0) into cells of A. nidulans, the subsequent elongation up to C17, and incorporation of OFA into the four major membrane acylglycerols. The incorporation of OFAs was followed by desaturation of part of the saturated fatty acid to monoenoic fatty acid. Positional analyses of the double bonds of these manoenoic fatty acids suggest that there is one desaturase that inserts a Δ9 bond in both odd‐ and even‐numbered fatty acids of varying chain length. Our data also suggest that there is no positional specificity for chain length on the glycerol backbone by the acyltransferases.