Fatty acid metabolism triacylglycerol hydrolysis

The metabolic breakdown of triacylglycerols begins with their hydrolysis to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase, whose mechanism of action is shown in Figure 29.2. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps. Hydrolysis is accomplished by two sequential nucleophilic acyl substitution reactions, one that covalently binds an acyl group to the side chain -OH of a serine residue on the enzyme and a second that frees the fatty acid from the enzyme. 

The fatty acids released on triacylglycerol hydrolysis are transported to mitochondria and degraded to acetyl CoA, while the glycerol is carried to the liver for further metabolism. In the liver, glycerol is first phosphorylated by reaction with ATP. Oxidation by NAD+ then yields dihydroxyacetone phosphate (DHAP), which enters the carbohydrate metabolic pathway. We ll discuss this carbohydrate pathway in more detail in Section 29.5. 

The physiological pathway for oxidation of fatty acids in organs or tissues starts with the enzyme triacylglycerol lipase within adipose tissue, that is, the hormone-sensitive lipase. This enzyme, plus the other two lipases, results in complete hydrolysis of the triacylglycerol to fatty acids, which are transported to various tissues that take them up and oxidise them by P-oxidation to acetyl-CoA. This provides a further example of a metabolic pathway that spans more than one tissue (Figure 7.13) (Box 7.1). 

Substrate cycling (e.g. the Cori cycle and the intra- and inter-cellular triacylglycerol/fatty acid cycles (Chapter 3)) in which there is no net metabolic change so that the energy from ATP hydrolysis is released as heat. 

Animals store energy in the form of triacylglycerols, kept in a layer of fat cells below the surface of the skin. This fat serves to insulate the organism, as well as provide energy for its metabolic needs for long periods. The first step in the metabolism of a triacylglycerol is hydrolysis of the ester bonds to form glycerol and three fatty acids. This reaction is simply ester hydrolysis. In cells, this reaction is carried out with enzymes called lipases. 

Hydrolysis of a triacylglycerol with water in the presence of either acid, base, or an enzyme yields glycerol and three fatty acids. This cleavage reaction follows the same mechanism as any other ester hydrolysis (Section 22.11). This reaction is the first step in triacylglycerol metabolism. 

The metabolic breakdown of triacylglycerols begins with their hydrolysis in the stomach and small intestine to yield glycerol plus fatty acids. Glycerol is first phosphorylated by reaction with ATP and is then oxidized to yield glyceraldehyde 3-phosphate, which enters the carbohydrate catabolic pathway. [Pg.1218]

These compounds are members of a broader group of chemical substances called lipids, which has been classified by the National Research Council into (1) nonpolar lipids— including esters of fatty acids (triacylglycerols and cholesteryl esters) that are virtually insoluble in water but soluble in most organic solvents, and enter metabolic pathways only after hydrolysis and (2) polar or amphi-pathic lipids—including fatty acids, cholesterol, sphingolipids, and glycerophospholipids (mainly lecithins). The term phospholipids... 

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