Fatty Acids and Phospholipids

Chryssostomos ChatgiHaloglu and Carla Ferreri 6.1

Introduction

Lipids are a group of molecules with a wide structural diversity, classified together for their insolubility in water [1]. The primary building blocks of most cell membranes are glycerol-phosphate-containing lipids, generally referred to as phospholipids. The general structure of an i-a-phosphatidylcholine is shown in Scheme 6.1, with two hydrophobic fatty acid chains in the positions sn-1 and sn-2 of i-gly-cerol and the phosphorous-containing polar headgroup in sn-3 position.

Scheme 6.1 Structure of l-a-phosphatidylchol ine (PC). R1 and R2 are fatty acid residues.

The hydrophobic part consists of fatty acid residues that are carboxylic acids with a long hydrocarbon chain (up to 26 carbon atoms), saturated or unsaturated with up to six double bonds. Some of the most common mono- and polyunsaturated fatty acid (MUFA and PUFA) structures are shown in Scheme 6.2, with their common names and the abbreviations describing the position and geometry of the double bonds (e.g. 9-cis), as well as the notation of the carbon chain length and total number of unsaturations (e.g. C18:1). Naturally occurring MUFA and PUFA residues of glycerol-based phospholipids in eukaryotes generally have the cis double bond geometry, and PUFA double bonds have the characteristic methyl-ene-interrupted motif. Being the cis geometry connected with biological activities

then, during MUFA and PUFA biosynthesis this feature is strictly controlled by the regiospecific and stereoselective enzymatic activity of desaturases [2].

5trans,8cis, 11 cis, 14cis-C20:4

Scheme 6.2 Common names and numerical abbreviations of some natural fatty acids and examples of geometrical

11 14

5trans,8cis, 11 cis, 14cis-C20:4

Scheme 6.2 Common names and numerical abbreviations of some natural fatty acids and examples of geometrical

Research on cis-trans isomerization (CTI) of lipid double bonds focused both on the conversion that occurs in some bacteria enzymatically and on trans isomers that are present in mammalian cells after a dietary supplementation of chemically modified fats [3,4]. It is known that cis/trans isomeric mixtures of fats result from vegetable and fish oils manipulated through partial hydrogenation or deodoriza-tion processes that are frequently utilized in the food industry. Nutritional and epidemiological studies revealed some harmful effects of these unnatural lipids for human health. However, it must be pointed out that in the chemical manipulation of oils the structures of trans fatty acid residues consist of geometrical and positional isomers with unshifted and shifted double bonds compared with the natural cis compounds. With the name "trans lipids" we indicate these unnatural geometrical and positional isomers. It has to be mentioned for clarity that there are a few natural lipids that exist only in the trans configuration, such as conjugated linoleic acid isomers, sphingolipids, and isoprene lipids.

A number of studies have confirmed that the broad spectrum of lipid mixtures plays an important role in the adaptability and flexibility of the cell membrane to environment necessities. The cell membrane is also a supporting matrix for proteins involved in many cellular processes, and therefore its physical and chemical properties can directly or indirectly affect cell metabolism. The isomerization of the cis double bond present in MUFA and PUFA residues of membrane phopsho-lipids to the corresponding more thermodynamically stable trans isomer is one of the lipid structural changes that has recently attracted the interest of diverse research areas. Geometric isomerism has become a topic of research involving several disciplines, such as microbiology, chemistry, biochemistry, pharmacology, nutrition, and medicine [3-5]. Recent work showing that the geometrical cis to trans lipid conversion can occur by a free radical process in a biological environment has certainly contributed to the interdisciplinary context of this subject [6,7]. The results of these fields aim at a global understanding of the occurrence of the cis-trans lipid conversion and its role in cell network signaling and membrane functioning.

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