Oxidation of triacylglycerols

The hydrolysis, hydrogenation, and oxidation of triacylglycerols—reactions originally discussed in Chapters 12, 15, and 22—are summarized here for your reference. 

Fats contain much less oxygen than is contained in carbohydrates or proteins. Therefore, fats are more reduced and yield more energy when oxidized. The complete oxidation of triacylglycerols to CO2 and H2O in the body releases approximately 9 kcal/g, more than twice the energy yield from an equivalent amount of carbohydrate or protein. 

Panek, J., Pokomy, J., and Reblova, Z. 1995. Reaction rates of oxidation of triacylglycerol species in edible oils under storage conditions, in Oils, Fats, Lipids, Vol. 5, Casten-miller, W.A.M., Ed., Bridgewater Barnes, 291-293. 

Calculate the volume of metabolic water available to a camel through fatty acid oxidation if it carries 30 lb of triacylglycerol in its hump. 

Mortensen PB. C6-C10-dicarboxyUc aciduria in starved, fat-fed and diabetic rats receiving decanoic acid or medium-cbain triacylglycerol. An in vivo measure of the rate of beta-oxidation of fatty acids. Biochim BiophysActa, 1981, 664(2), 349-355. 

The regulation of triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine biosynthesis is driven by the availability of free fatty acids. Those that escape oxidation are preferentiaUy converted to phos-phohpids, and when this requirement is satisfied they are used for triacylglycerol synthesis. 

Fatty acids Despite the fact that fatty acids are lipid soluble, so that they will diffuse across membranes without a transporter, one is present in the plasma membrane to speed up entry into the cells, so that it is sufficient to meet the demand for fatty acid oxidation. Triacylglycerol transport into cells also depends on the fatty acid transporter. Since it is too large to be transported per se, it is hydrolysed within the lumen of the capillaries in these tissues and the resultant fatty acids are taken up by the local cells via the fatty acid transporter (Chapter 7). Hence the fatty acid transporter molecule is essential for the uptake of triacylglycerol. 

Figure 7.6 Release of fatty acids from the triacylglycerol in adipose tissue and their utilisation by other tissues. Fatty acids are long-chain fatty acids, abbreviated to FFA (see below). Hydrolysis (lipolysis) of triacylglycerol in adipose tissue produces the long-chain fatty acids that are released from the adipocytes into the blood for oxidation by various tissues by P-oxidation (see below).

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). 

The physiological pathway for oxidation of ketone bodies starts with the hydrolysis of triacylglycerol in adipose tissue, which provides fatty acids that are taken up by the liver, oxidised to acetyl-CoAby P-oxidation and the acetyl-CoA is converted to ketone bodies, via the synthetic part of the pathway. Both hydroxybutyrate and acetoacetate are taken up by the tissues, which can oxidise them to generate ATP (Figure 7.19). 

Figure 7.19 The physiological pathway for ketone body oxidation from triacylglycerol in adipose tissue to their oxidation in a variety of tissues/organs. The pathway spans three tissues/ organs. The flux-generating step is the triacylglycerol lipase and ends with CO2 in one or more of the tissues/organs.

The fatty acids of triacylglycerols furnish a large fraction of the oxidative energy in animals. Dietary triacylglycerols are emulsified in the small intestine by bile salts, hydrolyzed by intestinal lipases, absorbed by intestinal epithelial cells, reconverted into triacylglycerols, then formed into chylomicrons by combination with specific apolipoproteins. 

Oxidation of fatty acids consumes a precious fuel, and it is regulated so as to occur only when the need for energy requires it. In the liver, fatty acyl-CoA formed in the cytosol has two major pathways open to it (1) J3 oxidation by enzymes in mitochondria or (2) conversion into triacylglycerols and phospholipids by enzymes in the cytosol. The pathway taken depends on the rate of... 

Malonyl-CoA, the first intermediate in the cytosolic biosynthesis of long-chain fatty acids from acetyl-CoA (see Fig. 21-1), increases in concentration whenever the animal is well supplied with carbohydrate excess glucose that cannot be oxidized or stored as glycogen is converted in the cytosol into fatty acids for storage as triacylglycerol. The inhibition of carnitine acyltrans-ferase I by malonyl-CoA ensures that the oxidation of... 

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