Triacylglycerol lipases

FIGURE 25.17 Hormonal signals regulate fatty acid synthesis, primarily through actions on acetyl-CoA carboxylase. Availability of fatty acids also depends upon hormonal activation of triacylglycerol lipase. 

Triacylglycerol lipase Lipase, triglyceride lipase Triglycerides... 

L. Brady, A. M. Brzozowski, Z. S. Derewenda, E. Dodson, G. Dodson, S. Tolley, J. P. Turkenburg, L. Christiansen, B. Huge-Jensen, L. Norskov, L. Thim, U. Menge, A Serine Protease Triad Forms the Catalytical Centre of a Triacylglycerol Lipase , Nature 1990, 343,161-110. 

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). 

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 hypothesis that is formulated on the basis of these properties is as follows. The signal for increased thermogenesis, when the body temperature falls, is an increase in the level of catecholamines, probably the local concentration of noradrenaline, which will be increased via stimulation of sympathetic nervous system. The catecholamine increases the activity of triacylglycerol lipase within the brown adipose tissue, by a similar mechanism to that which occurs in white adipose tissue (Chapter 7), i.e. by an... 

Figure 9.29 Control of heat production in brown adipose tissue. Catecholamines increase cyclic AMP concentrab on which stimulates triacylglycerol lipase which increases the long-chain fatty acid level, which increases the fluxes through P-oxidation and the Krebs cycle, and the activity of the uncoupling protein. Uncoupling decreases the ATP concentration which further increases the activity of the uncoupling.

Lipases are enzymes that catalyze the in vivo hydrolysis of lipids such as triacylglycerols. Lipases are not used in biological systems for ester synthesis, presumably because the large amounts of water present preclude ester formation due to the law of mass action which favors hydrolysis. A different pathway (using the coenzyme A thioester of a carboxylic acid and the enzyme synthase [Blei and Odian, 2000]) is present in biological systems for ester formation. However, lipases do catalyze the in vitro esterification reaction and have been used to synthesize polyesters. The reaction between alcohols and carboxylic acids occurs in organic solvents where the absence of water favors esterification. However, water is a by-product and must be removed efficiently to maximize conversions and molecular weights. 

Chylomicrons deliver tiiacylglycerols to tissues, where lipoprotein lipase releases free fatty acids for entry into cells. Triacylglycerols stored in adipose tissue are mobilized by a hormone-sensitive triacylglycerol lipase. The released fatty acids bind to serum albumin and are carried in the blood to the heart, skeletal muscle, and other tissues that use fatty acids for fuel. 

T Glycogen phosphorylase I Glycogen synthase I PFK-1 T FBPase-2 i Pyruvate kinase T PEP carboxykinase T Triacylglycerol lipase Perilipin phosphorylation T Acetyl-CoA carboxylase... 

Adipokinetic hormones control metabolism of insects during long-distance flight.359 363 In the migratory locust these hormones consist of a pair of related octapeptides and a decapeptide (Table 30-5). The hormones stimulate triacylglycerol lipase in the insects fat bodies, induce release of carbohydrates from body stores, and affect many other aspects of metabolism.363 Insects also have hormones of the insulin family, proteins consisting of disulfide-linked A and B chains as in insulin. The silkworm Bombyx mori has 38 genes for the insulinlike bombyxins, which are synthesized in the brain.364... 

Hydroxymethylglutaryl-CoA reductase Acetyl-CoA carboxylase Triacylglycerol lipase... 

When certain hormones (e.g., epinephrine) bind to their receptors in adipose tissue, adenylate cyclase is activated. The cAMP that is formed activates protein kinase A, which phosphorylates triacylglycerol lipase. The phosphorylated form of this enzyme is the active species, and triacylglycerols are degraded to fatty acids. 

Carboxylesterase Arylesterase Triacylglycerol lipase Phospholipase A2 Lysophospholipase Acetylesterase Acetylcholinesterase Cholinesterase... 

The major source of free fatty acids in the blood is from the breakdown of triacylglycerol stores in adipose tissue which is regulated by the action of hormone-sensitive triacylglycerol lipase (see Topic K4). Fatty acid breakdown and fatty acid synthesis are coordinately controlled so as to prevent a futile cycle (see Topic K3). 

Regulation The concentration of free fatty acids in the blood is controlled by the rate at which hormone-sensitive triacylglycerol lipase hydrolyzes the triacylglycerols stored in adipose tissue. Glucagon, epinephrine and norepinephrine cause an increase in the intracellular level of cAMP which allosterically activates cAMP-dependent protein kinase. The kinase in turn phosphorylates hormone-sensitive lipase, activating it, and leading to the release of fatty acids into the blood. Insulin has the opposite effect it decreases the level of cAMP which leads to the dephosphorylation and inactivation of hormone-sensitive lipase. 

The breakdown of fatty acids in (3-oxidation (see Topic K2) is controlled mainly by the concentration of free fatty acids in the blood, which is, in turn, controlled by the hydrolysis rate of triacylglycerols in adipose tissue by hormone-sensitive triacylglycerol lipase. This enzyme is regulated by phosphorylation and dephosphorylation (Fig. 5) in response to hormonally controlled levels of the intracellular second messenger cAMP (see Topic E5). The catabolic hormones glucagon, epinephrine and norepinephrine bind to receptor proteins on the cell surface and increase the levels of cAMP in adipose cells through activation of adenylate cyclase (see Topic E5). The cAMP allosterically activates... 

Fig. 5. Summary of the control of hormone-sensitive triacylglycerol lipase.

Fig. 2. Targeted lipidomics of 2-AG metabolism. Postulated pathways for 2-AG metabolism. Abbreviations PLC, phospholipase C DAG, diacylglycerol DGL, diacylglycerol lipase MGL, monoacylglycerol lipase PLA, phospholipase A AT, acyltransferase TAGL, triacylglycerol lipase PIP2, phosphatidylinositol bisphosphate ABHD-6/12 hydrolase lyso-PL, lysophospholipid lyso-PA, lysophosphatidic acid PA, phosphatidic add P, phosphatase COX, cydooxygen-ase LOX, lipoxygenase CYP450, cytochrome P450 CDP, cytidine diphosphate.

Triacylglycerol lipases (EC 3.1.1.3) are attracting renewed attention since they were demonstrated to be active in organic solvents and to be suitable catalysts in industrial important reactions, such as the synthesis of flavors, emulsifiers, and chiral compounds, and the transesterifi cation of tow-value fats to triacylglycerols of high commercial value. Fbngi are a particularly valuable source of lipases because the enzymes produced by the majority of them are extracellular and readily separable from the mycelia after fermentation. The recent availability of... 

Sugiura, M., Oikawa, T., Hirano, K., Inukai, T. 1977. Purification, crystallization and properties of triacylglycerol lipase from Pseudomonas fluorescens. Biochim. Biophys. Acta 488, 353-358. 

There is little doubt that lipases claimed to be non-regioselective display activity towards the secondary alcoholic mid-position of TAGs, but this may well be different when it comes to re-esterification of that position with fatty acids or their derivatives. Interestingly enough, there is a recent report on conversion of a carboxylesterase into a triacylglycerol lipase by random mutation (Reyes-Duarto et al., 2005). The hydrolytic enzyme was observed to display preference for the sn-2 position of triacylglycerols in an ethanolysis reaction. 

Reyes-Duarto, D., Polaina, J., L6pez-Cortes, N., Alcalde, M., Plou, F. J., Elborough, K., Ballesteros, A., Timmis, K. N., Golyshin, P. N., and Ferrer, M. 2005. Conversion of a carboxylesterase into a triacylglycerol lipase by a random mutation. Angew. Chem., 117,1-5. 

---