Lipids (Introduction)

LIPIDS
INTRODUCTION
These are a chemically diverse group of compounds, the common and defining feature of which is their insolubility in water. The biological functions of the lipids are as diverse as their chemistry. Energy is stored in many organisms in the form of fats and oils. Phospholipids and sterols are major structural elements of biological
membranes. Other lipids, although present in relatively small quantities, play crucial roles as enzyme cofactors, electron carriers, light-absorbing pigments, hydrophobic anchors for proteins, “chaperones” to help membrane proteins fold, and emulsifying agents in the digestive tract, hormones, and intracellular messengers.

TYPES OF LIPIDS AND THEIR USES
Storage Lipids
➢ The fats and oils used almost universally as stored forms of energy in living organisms are derivatives of fatty acids.
➢ The fatty acids are hydrocarbon derivatives, at about the same low oxidation state (that is, as highly reduced) as the hydrocarbons in fossil fuels.
➢ The cellular oxidation of fatty acids (to CO2 and H2O), like the controlled, rapid burning of fossil fuels in internal combustion engines, is highly exergonic.
➢ We introduce here the structures and nomenclature of the fatty acids most commonly found in living organisms.
➢ Two types of fatty acid—containing compounds, triacylglycerol and waxes, are described to illustrate the diversity of structures and physical properties in this
family of compounds.
Fatty Acids Are Hydrocarbon Derivatives
➢ Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons long (C4 to C36). In some fatty acids, this chain is unbranched and fully
saturated (contains no double bonds); in others, the chain contains one or more double bonds. A few contain three-carbon rings, hydroxyl groups, or methylgroup branches.
➢ All acids are shown in their non-ionized form. At pH 7, all free fatty acids have an ionized carboxylate. Note that numbering of carbon atoms begins at the carboxyl carbon.( b)The prefix n- indicates the “normal” unbranched structure. For instance, “dodecanoic” simply indicates 12 carbon atoms, which could be arranged in a variety of branched forms; “n-dodecanic” specifies the linear, unbranched form. For unsaturated fatty acids, the configuration of each double bond is indicated; in biological fatty acids the configuration is almost always cis.

The most commonly occurring fatty acids have even numbers of carbon atoms in an unbranched chain of 12 to 24 carbons (in Table 1-1) the even number of carbons results from the mode ofsynthesis of these compounds, which involves successive condensations of two-carbon (acetate) units.

➢ There is also a common pattern in the location of double bonds; in most monounsaturated fatty acids the double bond is between C-9 and C-10 (Δ9), and the other double bonds of polyunsaturated fatty acids are generally Δ12 and Δ15. (Arachidonic acid is an exception to this generalization)
➢ The double bonds of polyunsaturated fatty acids are almost never conjugated (alternating single and double bonds, as in —CH=CH—CH=CH —), but are separated by a methylene group: —CH=CH—CH2—CH=CH—. In nearly all naturally occurring unsaturated fatty acids, the double bonds are in the cis configuration.
➢ Trans fatty acids are produced by fermentation in the rumen of dairy animals and are obtained from dairy products and meat.