Fatty acids and acyl-CoAs

Although it was initially felt that the free fatty acids acted simply as uncouplers on the mitochondria, this is today considered to be much too simple an explanation. A sketch of the possible interactions of free fatty acids and their derivatives with the mitochondria is shown in Fig. 10.13. Although a series of derivatives are at hand, only extramitochondrial free fatty acids and acyl-CoAs have been more seriously investigated as candidates for the mediator. 

Deutsch J, Rapoport SI. Purdon AD. Relation between free fatty acid and acyl-CoA concentrations in rat brain following decapitation. Neurochem Res 1997 22 759-765. 

Wojtczak, L. (1974). Effect of fatty acids and acyl-CoA on the permeability of mitochondrial membranes to monovalent cations. FEBS letters, 44(1), 25-30. 

Wojtczak, L. (1976). Effect of long-chain fatty acids and acyl-CoA on mitochondrial permeability, transport, and energy-coupling processes. J Bioenerg Biomembr, 5(6), 293-311. 

It appears that dc novo synthesis of saturated fatty acids in the endoplasmic reticulum is not strongly inhibited by fatty acids and acyl CoA s whereas the formation of these acids in the mitochondria, probably by elongation, is completely inhibited under these conditions. Thus the appearance of elongation products in the mitochondria in a mixed incubation may very well involve a transfer process. In order to test this probability, microsomes and mitochondria were incubated separately under optimal conditions with radioactive malonyl or acetyl CoA and in the presence or absence of linoleate. They were then washed free of precursors, and the unincubated particles were added, followed by incubation, separation, and fatty acid and lipid analysis. It can be seen in Tables 3 and 4 that there is very rapid exchange of elongation products in both directions despite the absence of the cytosol which presumably contains... 

Both non-esterified fatty acids and acyl-CoAs are capable of binding to distinct cytosolic proteins known as fatty acid binding proteins (FABP). The best known of these is the Z-protein of liver. These small molecular weight (about 14kDa) FABPs have been suggested to function for intracellular transport or to provide a temporary binding site for potentially damaging compounds such as acyl-CoAs (section 5.3.3). 

Other fatty acid binding proteins, such as the Z-protein, have been described in sections 3.3.1 and 5.3.3. Their function is thought to be to transport fatty acids and acyl-CoAs within cells rather than in the blood. This is important, not only because of the problem of water insolubility but because of the disruptive detergent effects of these substances in the free state. 

One ATP is used in the formation of glycerol 3-phosphate and three ATPs are used to convert three fatty acids to acyl CoAs. The three PPi formed during fatty acid activation are converted to Pj, hence, the total of seven Pj in the equation above. 

Most intracellular routes of fatty acid metabolism require prior activation of these substrates. Activation commonly proceeds according to the reaction ATP + fatty acid + CoASH = acyl-CoA + AMP + PPj. Several acyl-CoA synthetases, differing on the basis of substrate specificity, distribution in tissues, and intracellular localization are known (for reviews, see Londesbo-... 

Thiokinase catalyzes a reaction between the carboxyl group of the fatty acid and reduced CoA to yield acyl-CoA. ATP and magnesium are required in the reaction, and AMP and pyrophosphate are formed. The details of the mechanism of the reaction are not known, but Ingraham and Green have postulated the existence of a number of intermediates in the formation of which magnesium is assumed to play an essential role. 

Khan A A, Kolattukudy P E 1975 Solubilization of fatty acid synthetase, acyl-CoA reductase, and fatty acyl-CoA alcohol transacylase from the microsomes of Euglena gracilis. Arch Biochem Biophys 170 400-408... 

Fatty acids oxidised during the process of -oxidation have to be activated by coenz)me A with the formation a fatty acyl-CoA thioester (short and medium length fatty acids undergo this reaction in the mitochondria, while long chain fatty acids form acyl-CoAs at the outer mitochondrial membrane and are carried across the inner mitochondrial membrane by carnitine). The saturated fatty acid molecule with no double bond is then degraded to acetyl-CoA, its chain is shortened by two carbon atoms and the shortened fatty... 

The desert shrub jojoba Simmondsia chinensis, Link) appears to be the only plant system that synthesizes large quantities of liquid wax. Two enzymes are specific to the wax biosynthetic pathway fatty acyl-CoA reductase, which catalyzes the NADPH-specific reduction of the fatty acyl moiety of acyl-CoA to fatty alcohol, and acyl-CoA fatty alcohol acyltransferase, which catalyzes the coupling of fatty acid to fatty alcohol. 

Carnitine is not involved in the movement of fatty acids into microbodies. Either free fatty acids or acyl-CoAs appear to be able to cross the organelle s membrane. Moreover, because of the absence of an electron transport chain in microbodies there is no internal means of regenerating NAD". The reoxidation of NADH is thought to occur either by a glycerol phosphate shuttle or by movement of NADH to the cytosol and NAD back. 

Phospholipase A2 enzymes also have other important metabolic functions in addition to the overall destruction of phospholipids as catalysed by digestive pancreatic or venom enzymes. An enzyme in mitochondrial membranes seems to be intimately connected with the energy state of this organelle. Thus, the phospholipase is inactive in fully coupled mitochondria and only becomes active when ATP and respiratory control drop to low levels. Also, the widespread distribution of phospholipases A2 allows many tissues to perform retailoring of the molecular species of membrane lipids by the Lands mechanism. In this process, named after Lands, the American biochemist who first described it, cleavage of the acyl group from the sn-2 position yields a lysophospholipid which can be re-acylated with a new fatty acid from acyl-CoA (Figure 7.8). 

Several additional points should be made. First, although oxygen esters usually have lower group-transfer potentials than thiol esters, the O—acyl bonds in acylcarnitines have high group-transfer potentials, and the transesterification reactions mediated by the acyl transferases have equilibrium constants close to 1. Second, note that eukaryotic cells maintain separate pools of CoA in the mitochondria and in the cytosol. The cytosolic pool is utilized principally in fatty acid biosynthesis (Chapter 25), and the mitochondrial pool is important in the oxidation of fatty acids and pyruvate, as well as some amino acids. 

In essence, this series of four reactions has yielded a fatty acid (as a CoA ester) that has been shortened by two carbons, and one molecule of acetyl-CoA. The shortened fatty acyl-CoA can now go through another /3-oxidation cycle, as shown in Figure 24.10. Repetition of this cycle with a fatty acid with an even number of carbons eventually yields two molecules of acetyl-CoA in the final step. As noted in the first reaction in Table 24.2, complete /3-oxidation of palmitic acid yields eight molecules of acetyl-CoA as well as seven molecules of FADHg and seven molecules of NADFI. The acetyl-CoA can be further metabolized in the TCA cycle (as we have already seen). Alternatively, acetyl-CoA can also be used as a substrate in amino acid biosynthesis (Chapter 26). As noted in Chapter 23, however, acetyl-CoA cannot be used as a substrate for gluco-neogenesis. 

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