Lipid Biochemistry

Fatty acids are long-chain “carboxylic” acids, that is, hydrocarbon (alkyl) chains containing the terminal -COOH chemical group. (Fig. 2)

[ Fatty Acid Structure Image ]

Fatty acids contain from 4 to 22 carbon atoms. They can be saturated, having no double bonds in the carbon chain, mono-unsaturated with one double bond in the chain, or polyunsaturated with several double bonds in the fatty acid.

In nature, triglycerides occur linked (esterified) to glycerol and hence are called glycerides. If solid they are called fats, if liquid they are called oils. One fatty acid combined (esterified) with glycerol is a monoglyceride, two combined is a diglyceride, and three attached to the glycerol backbone (the most possible) is a triglyceride.

Saturated fat (e.g. palmitic and stearic) exclusively fit in the 1 and 3 positions whereas unsaturated fatty acids can distribute randomly among the three positions on the glycerol backbone. A common configuration is thus a saturated fat in positions 1 and 3 and an unsaturated fatty acid in position 2. (Fig. 3)

[ Structure of a Triglyceride Image ]

Fatty acids containing fewer than 16 carbon atoms, and saturated fatty acids, are largely oxidized to provide energy. Those containing 16 to 22 carbon atoms can also be oxidized for energy, but in addition can be incorporated into cell membranes, regulate metabolism after conversion to eicosanoids (prostaglandins, thromboxanes, leukotrienes, lipoxins, and various other hydroxy analogs discussed later) changed to other fatty acids, or stored in fat (adipose) tissues.

Abbreviated notations simplify fatty acid nomenclature. In the case of linoleic acid (abbreviated LA; notation 18:2w6). The 18 means the molecule has 18 carbon atoms, the 2 means that there are two double bonds in the molecule and the w6 means the first double bond begins with the sixth carbon atom counting from the methyl (CH3), omega (w) end of the carbon chain. (In many publications omega is designated as a small case “n” instead of” w.”) The other end of the chain, the carboxylic acid end, is termed the delta (?) end. (Fig. 4)

[ Linoleic and Linolenic Acid Structure and Nomenclature Image ]

The double bonds in the nutritionally important fatty acids are separated by methylene groups (CH2). Hence, the second double bond in LA must begin with the ninth carbon atom. Common fatty acids are detailed in Figure 5.

[Nomenclature and Structure of Common Fatty Acids Image ]

Fatty acids with 16 and l8-carbon chains can participate in the manufacture of phospholipids which are the main structural components of cell membranes. Phospholipids are similar to triglycerides in that fatty acid molecules are attached to a glycerol molecule, a three carbon alcohol (or, less commonly, sphingosine, a more complex amino alcohol). In triglycerides, all three esterifiable positions on a glycerol molecule are occupied by a fatty acid. In a phospholipids, only two are so occupied and the third is esterified to phosphoric acid which may have in turn other compounds attached to it such as choline, serine, glycerol, inositol or ethanolamine. Lecithin, the best known phospholipids, has choline attached to the phosphate and is thus termed phosphatidylcholine. If phosphoric acid alone is attached, the compound is called a phosphotidate. Many molecular variations are also possible by mixing various fatty acids on the glycerol backbone. (Fig. 6)

[Structure of Phospholipids Image ]

Figure 7 demonstrates the biochemically important cis- and trans- forms of fatty acids. Notice in the cis- form the hydrogen atoms on the carbons adjacent to the double bond are on the same side of the molecule.
The repulsive forces between these “crowded” hydrogen atoms cause unsaturated fatty acids to assume particular non-linear shapes which play an important role in lipid membrane configuration, fluidity, and in biochemical reactions involving enzymes.

[ Isomers Image ]

[ Fatty Acid Configurations Image ]

In the trans- form of fatty acids, the hydrogen atoms are on opposite sides of the molecule and their repulsive forces cancel each other, and thus the molecule is not bent. Although the trans- form is more stable, its chemical properties and biological functions are altered. (Figs. 7 and 8)

A biological (plasma) membrane surrounds all cells within tissue as well as the organelles lying within the cytoplasm. To help visualize size, if we were the size of bacteria, a cell would be the size of a large auditorium. (In true size, one billion cells fit in one cubic inch.) This cell “auditorium” is housed by a skin only two molecules thick. The various drawings of biochemicals depicted in these pages would therefore be as they would appear from our size as a bacteria, or as a bacteria sized person would see them through a magnifying glass.

The membrane is not a static sac, but rather a complex of chemicals with gates and pumps to control chemical and ionic balances, receptors for stimuli and signal generators. It is made up primarily of phospholipids, protein, glycolipids and cholesterol, all of which of course come directly from food or are synthesized in situ by components of food after they have been broken down by digestion.

Membrane lipids are amphipathic in that they contain both a hydrophilic polar end and a hydrophobic non-polar end. Phospholipids orient themselves into a bilayer sheet in membranes with the hydrophilic ends pointed to the outside and the hydrophobic hydrocarbon tails pointing to the inside. These properties make salts of fatty acids an important functional component of soaps since their fat soluble hydrophobic ends attract “fatty dirt” and their water soluble hydrophilic ends can attract “watery dirt.”

The neck of a fatty acid is located next to the delta carboxyl end and is stiff. The tail portion next to the omega end, if containing cis- double bonds, is highly active, oscillating at a million vibrations per second. (Figs. 8 and 9)

[ Membrane Image ]

[Triglyceride Fluidity ]

Lipids, and some proteins within membranes, are also in constant lateral motion. In a bacterium, a single phospholipid will travel from one end to the other in one second. Thus membranes are in effect two-dimensional solutions of an array of oriented molecules.
Although membranes are considered lipid bilayers, almost 50% of the membrane is composed of protein which serves many functional roles. The sugar residues of glycolipids (sphingosine + fatty acid + sugar, such as sphingomyelin) and glycoproteins (sugars attached to membrane proteins) are found protruding on the outer surface of membranes. Cholesterol, as well as the length of fatty acid tails and their degree of saturation, affects membrane fluidity. Cholesterol sandwiched between membrane fatty acids prevents their crystallization. (Fig. 10)

[ Bilipid Cell Membrane Image ]

The specific spatial configuration and electronegative discontinuity of essential fatty acids pennit linkage with sulfhydral protein groups in membranes to form pi electron quantum mechanical membrane potentials that affect the transport of oxygen into tissues. Also, it is on the lipid membranes of mitochondria that cellular respiration occurs and energy is packaged for use throughout the body. Thus dietary fatty acids, which ultimately build all membranes, affect the burning of nutrient fuels — the most fundamental of life’s energetic properties.

Classic artistic renderings of biological membranes are overly simplistic and create an impression of static barriers. Similarly a photograph of a rocket ship streaking toward space says nothing about its actual movement, speed or the hubbub of activity occurring inside of it.

The real biological membrane, containing millions of fatty acid tails vibrating at millions of times per second, with deletions and substitutions in constant progress and biochemical doors opening and closing selectively permitting the passage of food and waste, is a dynamic, an action more than a structure and literally beyond comprehension. It can be described with words, like infinity can be, but not rationally fully grasped.

When one considers that fatty acids comprise the membrane structure of all cells and their enclosed organelles, the breadth of their importance begins to emerge. Membrane fatty acids are indeed the gatekeepers of life.

The dynamic and complex: aspects of fatty acid chemistry help us to understand nutrition at a more meaningful level. Percent fat on a food label is valueless in determining the healthiness of a product. Are the fats saturated or unsaturated? If unsaturated, what are the ratios of omega 9’s to 6’s to 3’s? Have the fats been hydrogenated? If so, what are the levels of potentially toxic trans-isomers? Are the lipids oxidized or complexed with other nutrients such as protein? Does the product contain the nutrients which were associated with the lipid in its natural context, such as antioxidants, certain vitamins, and minerals? What is the stability when subject to time, heat, light and air?

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