Length of the fatty acid chains

The formula for a triglyceride may vary because (a) the length of the fatty acid chains may vary from 14 to 24 carbon atoms (b) a triglyceride may contain as many as three different fatty acids and (c) the bonding between adjacent carbon atoms may consist of combinations of single and/or double covalent bonds. 

Cells regulate the lipid compositions of their plasma membrane so that a reasonable membrane fluidity is main-tained. They do this by controlling fatty acid biosynthesis so as to vary the lengths of the fatty acid chains and the ratio of unsaturated to saturated fatty acids (see chapter 19). If cells are grown at low temperatures, their phospholipids contain more unsaturated fatty acids or fatty acids with shorter chains or both. These adjustments shift the Tm to lower temperatures, with the result that (to the extent that the melting transition is sharp enough to be measureable) the Tm re-... 

Note that even solid fats contain some unsaturated fatty acids, and oils contain saturated fatty acids as well. Besides the degree of unsaturation, the length of the fatty acid chain also influences whether a fat is solid or liquid. Short chain fatty acids, such as found in coconut oil, convey liquid consistency in spite of the low unsaturated fatty acid content. Two of the unsaturated fatty acids, linoleic and linolenic acids, are essential fatty acids because the body cannot synthesize them from precursors they must be included in the diet. 

Because mammals lack the enzymes to introduce double bonds at carbon atoms beyond C-9 but can increase the length of the fatty acid chain at the carboxyl end, the easiest way to determine which unsaturated fatty acid is the precursor is to note the number of carbons from the co end (CH3 end) to the nearest double bond. Thus, in (a) this number is seven carbons hence, palmitoleate is the precursor. In (b) it is six carbon atoms hence, linoleate. In (e) it is nine carbon atoms hence, oleate etc. 

Few comparative studies have been made of the rates of lipase hydrolysis of esters of different alcohols. In 1938 Balls and Madack (4, 5) investigated the rates of hydrolysis of esters of ethyl and benzyl alcohols by pancreatic lipase. They concluded that the rate of hydrolysis depended on the length of the fatty acid chain, and that only esters of primary alcohol groups were split. 

The number of milligrams of KOH needed to saponify completely (hydrolyse the triglyceride ester into fatty acids and glycerol) 1 g of vegetable oil is the saponification value of the oil. It is an indication of the average length of the fatty acid chains of triglycerides. The oil is saponified with an excess of alcoholic KOH for a specified time and the excess is back titrated with the standard HCl. This is expressed as follows ... 

Oral detection thresholds were also obtained by an ascending, three-alternative, forced-choice procedure for three saturated fatty acids that varied in chain length, and one long-chain cA-unsaturated fatty acid [2], Although textural cues were not eliminated, stearic acid emulsions in the oral cavity yielded a mean detection threshold near 0.03% w/v [2], Similar threshold results were obtained with and without nasal airflow. In addition, the mean detection threshold for stearic acid in this population was statistically similar to thresholds for caproic, lauric, and linoleic acids. These results suggest that humans can perceive short-, medium- and long-chain fatty acids in the oral cavity with detection thresholds that do not appear to correlate with the length of the fatty acid chain. 

The length of the fatty acid chain may vary from 4 to 20 carbons, but the number of carbon atoms in the chain is nearly always even. 

The common structure of vegetable oils discussed here is that of aliphatic triglycerides (Scheme 1.1), in which the fatty acid chains Ri, Ri and R3 are most often identical, but can also vary, within a given molecule. The length of the fatty-acid chain is 14-22 carbon atoms, but most members bear 16 or 18 units. The other important feature of these linear aliphatic motifs is the possible presence of C=C unsaturations, which range from 0 to 3. More than 1,000 fatty acids have been identified, but only 20 are present in appreciable quantities in vegetable oils [2, 3]. 

Points worth noting are (1) All the reactants are acyl derivatives of CoA. (2) All the enzymes are localized within the mitochondria with those of the citrate cycle and electron-transport chain. This ensures efficient utilization of the acetyl-CoA released by the fatty acid oxidation. (3) Only one activation step is necessary, regardless of the length of the fatty acid chain. This uses two high-energy bonds, since ATP is broken down to AMP and pyrophosphate during the thiokinase reaction. 

As early as 1915, Hurtley recognized that an amplifjdng, if not an additional, mechanism must be postulated to account for the ketone bodies formed from fat in diabetic patients. He therefore proposed that the whole length of the fatty acid chain is /3-oxidized (i.e., oxidized at alternate carbons) with subsequent splitting of the chain into blocks containing 4 carbon atoms, the acetoacetic acid precursors. Accordingly, palmitic acid could yield 4 moles of acetoacetic acid ... 

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