Fatty adds glycerol

Glycerol. From these different processes for hydrolyzing fats into soap or fatty adds, glycerol is recovered and refined for the various purposes to which it is now appded. Practicady ad glycerol refining is now accomplished by distillation with steam under diminished pressure. 

Pettitt, T.R., Martin, A., Horton, T., liossis, C, Lord, J.M., and Wakelam M.J.O., 1997, Ehacyl-glycerol and phosphatidate generated by phospholipases C and D, respectively, have distinct fatty add compositions and functions. J. Biol. Chem. 272 17354 17359 Powis G., Seewald, M.J., Gratas, C., Melder, D., Riehow, J., and Modest, E.J., 1992, Selective inhibition of phosphatidyUnositol phosphohpase C by cytotoxic ether hpid analogs. Cancer Res. 52 2835-2840... 

Triglycerides, the storage form of fatty acids, are formed by attaching three fatty adds (as fetty acyl CoA) to glycerol. Triglyceride formation from fatty acids and glycerol 3-phosphate occurs primarily in liver and adipose tissue. 

Table 11.4 Content of some unsaturated fatty adds in triacyl-glycerol in muscle of a fish, a bird and a ruminant and in green...

Figure 16.1 The glucose/fatty add cycle. The dotted Lines represent regulation. Glucose in adipose tissue produces glycerol 3-phosphate which enhances esterification of fatty acids, so that less are available for release. The effect is, therefore, tantamount to inhibition of lipolysis. Fatty acid oxidation inhibits pyruvate dehydrogenase, phosphofructokinase and glucose transport in muscle (Chapters 6 and 7) (Randle et al. 1963).

The naturally occurring fatty acids are carboxylic acids with unbranched hydrocarbon chains of 4-24 carbon atoms. They are present in all organisms as components of fats and membrane lipids, in these compounds, they are esterified with alcohols (glycerol, sphingosine, or cholesterol). However, fatty acids are also found in small amounts in unesterified form. In this case, they are known as free fatty adds (FFAs). As free fatty acids have strongly amphipathic properties (see p. 28), they are usually present in protein-bound forms. 

The mobilization of stored fat requires the hydrolytic release of fatty adds and glycerol from their TAG form. This process is initiated by hormone-sensitive lipase, which removes a fatty acid from carbon 1 and/or carbon 3 of the TAG. Additional lipases specific for diacylglyc-erol or monoacylglycerol remove the remaining fatty acids. 

As fuel for the body, fatty acids are stored in the form of triacylglycerols. Triacylglycerols can be hydrolyzed to form glycerol and the corresponding fatty adds in a process called lipolysis. Notice that this process is simply the reverse of esterification. In the lab triacylglycerols can be cleaved by the addition of NaOH, a process called saponification. Saponification is the production of soap. 

Figure 1 (a) Esters of glycerol with faiy adds can be hydrolyzed by the different types of human lipase with varying sped fid lies, while (b) synthetic esters of polyglycerol with fatty adds are resistant... 

The most abundant fatty add-containing compounds in the human organism are the triglycerides. These are esters of three fatty adds and one glycerol molecule, and they most commonly occur in fat storage areas—the adipose tissue. Triglycerides are nonamphipathic. Because about two-thirds of all fatty adds in... 

Triglycerides are hydrolyzable into their component fatty adds and glycerol. They are espedally susceptible to alkaline hydrolysis. If KOH or NaOH is used, the process is saponification and the products, sodium and potassium salts of fatty adds, are called soaps. In the human organism, triglycerides are hydrolyzed by various esterases called lipases. These enzymes are quite spedfic, and they do not necessarily remove all three fatty add molecules from a triglyceride molecule. Thus, pancreatic lipase, the main lipid digestive enzyme of the small intestine, catalyzes the removal of fatty acids from positions 1 and 3 only. 

Interestingly, for maximum TAG synthesis, a preference was shown for palmitoyl-CoA as substrate for the initial acylation of glycerol-3-phosphate (Kinsella and Gross, 1973), apparently accelerating the rate of supply of substrate as acceptor for de novo-synthesized fatty adds, whereas oleic acid reduced total de novo synthesis, apparently by competing with butyryl CoA for the esterification of diacylglycerol (Hansen and Knudsen, 1987). 

The synthesis of triacylglycerol takes place in the endoplasmic reticulum (ER). In liver and adipose tissue, fatty adds in the cytosol obtained from the diet or from de novo synthesis of palmitic add become inserted into the ER membrane. The reactions are shown in Fig. 13-10. Membrane-bound acyl-CoA synthetase activates two fatty acids, and membrane-bound acyl-CoA transferase esterifies them with glycerol 3-phosphate, to form phosphatidic acid. Phosphatidic acid phosphatase releases phosphate, and in the membrane, 1,2-diacylglycerol is esterified with a third molecule of fatty acid. 

Figure 1.4 A triacylglycerol molecule. The glycerol backbone is bonded to three fatty adds (Ri, R2 and R3).

T3i metabcJic breakdown of triacylglyceroJs bt ns with their hydrelyjis ia the stomach and small intestine to yiold glycerol plus fatty adds. Cl>verot is first phoopho-rylated by reaction with ATP and is thm oxidised to yield ycenildeh e 3 phosphate. which enters the carbohydrate catabolic path way. (Well scuss this in detail in Section 2. 3.)... 

Free fatty acids (FFAs) are fatty adds that are not esterified to glycerol or to some other molecule. FFAs are a concern to the food scientist. These fatly acids are not a major source of energy in the diet but arc considered undesirable contaminants in vegetable oils. The term bfrgin oJife oil means that the oil is of high quality and does not contain free fatty acids. The free fatty acids present in the circulatory system are bound to albumin, but not covalently. 

The hydrolysis of TGs to FFAs and glycerol is catalyzed by a lipase in adipocytes called hormone-sensitive lipase. This enzyme catalyzes the hydrolysis of fat deposits in adipose tissue or wherever adipocytes arc found. The rate of liberation of fatty adds and of the byproduct glycerol from the adipose tissue may be greater than the rate at which they can be oxidized by other tissues. Under these conditions, the concentrations of FFAs and glycerol increase in the plasma. The data in Table 4.9 are from a study of human subjects who exercised on a stationaiy bicycle. Blood samples withdrawn during rest and at the indicated times during exercise show that FFA concentrations increased from 0 43 mM at rest to nearly 2.0 mM after 3 hours of work. 

PC can come from the diet or be synthesized in the body. Dietary PC is cleaved by a pancreatic enzyme, phospholipase B, that catalyzes hydrolysis of the fatty add from carbon-2 (C-2) of the glycerol backbone. This hydrolysis produces a free fatty add and l-acyh3-phosphocholine glycerol. The latter compound is called lysolecithin, because its parent compound PC is known as lecithin. Lysoledthin and the free fatty acid are absorbed by the enterocyte, converted back to PC, and then packaged into chylomiorons that are released into the lymphatic system. A small fraction of the choline derived from dietary PC enters the bloodstream as free choline. 

The most common phospholipid, phosphatidylcholine, contains two molecules of fatty add and one molecule of choline phosphate attached to a glycerol backbone (see Figure 1.11). Like all other phospholipids, it is amphipathic. The water-soluble end features a phosphate group, an amino group, and two keto groups. 

Phospholipids may consist of a backbone of glycerol esterified with two fatty adds and one molecule of choline phosphate in the 3 position. In other phospholipids, the 3 position may be occupied by a molecule of ethanolamine phosphate, serine phosphate, or inositol phosphate. A phospholipid containing choline phosphate is called phosphatidylcholine one containing ethanolamine phosphate is called phosphatidylethanolamine. Generally, the fatty acids esterified as triglycerides contain up to one double bond, whereas those esterified as phos-... 

Pancreatic phospholipase A2 Hydrolysis of dietary phospholipids. The enzyme cleaves the fatty add at the 2-position of the glycerol backbone. 

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