Transferase fatty acid acyl

Carnitine fatty acid acyl transferase Chap. 12... 

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. 

The steps in the subsequent utilization of muscle LCFAs may be summarized as follows. The free fatty acids, liberated from triglycerides by a neutral triglyceride lipase, are activated to form acyl CoAs by the mediation of LCFAcyl-CoA synthetase which is situated on the outer mitochondrial membrane. The next step involves carnitine palmitoyl transferase I (CPT I, see Figure 9) which is also located on the outer mitochondrial membrane and catalyzes the transfer of LCFAcyl residues from CoA to carnitine (y-trimethyl-amino-P-hydroxybutyrate). LCFAcyl... 

Fatty-acid synthase (acyl-CoA malonyl-CoA C-acyltransferase, EC 2.3.1.85) is a multifunctional transferase that also has the capacity to hydrolyze thiolesters. The role of its thiolesterase domain is to terminate the growth of fatty acids by hydrolyzing acyl-CoA intermediates [131]. 

Figure 7.11 Mechanism of transport of long-chain fatty adds across the inner mitochondrial membrane as fatty acyl carnitine. CRT is the abbreviation for carnitine palmitoyl transferase. CPT-I resides on the outer surface of the inner membrane, whereas CPT-II resides on the inner side of the inner membrane of the mitochondria. Transport across the inner membrane is achieved by a carrier protein known as a translocase. FACN - fatty acyl carnitine, CN - carnitine. Despite the name, CRT reacts with long-chain fatty acids other than palmitate. CN is transported out of the mitochondria by the same translocase.

The initial acylation at the 1-position of glycerol 3-phosphate is catalysed by glycerol 3-phosphate acyl-transferase-1, abbreviated to GPAT-1. This enzyme is specific for a saturated fatty acid (in the acyl form). 

The third acylation is catalysed by diacylglycerol acyl-transferase (DGAT), which is less specific, so that an unsaturated or saturated fatty acid can be incorporated. The final product is as shown in Figure 11.7. 

Incorporation of different fatty acids into lipids depends on the relative abundance of their CoA derivatives and their acyl-transferase )< , values. The synthetic enzymes which form membrane phospholipids may select the acid by molecular features not in accord with the optimal physiological properties of the products (110), resulting in the formation of membranes which do not function adequately. 

Figure 3.8 One complete cycle and the first step in the next cycle of the events during the synthesis of fatty acids. ACP = acyl carrier protein, a complex of six enzymes i.e. acetyl CoA-ACP transacetylase (AT) malonyl CoA-ACP transferase (MT) /3-keto-ACP synthase (KS) /J-ketoacyl-ACP reductase (KR) / - hydroxyacyl-ACP-dehydrase (HD) enoyl-ACP reductase (ER).

Much of the cholesterol synthesis in vertebrates takes place in the liver. A small fraction of the cholesterol made there is incorporated into the membranes of he-patocytes, but most of it is exported in one of three forms biliary cholesterol, bile acids, or cholesteryl esters. Bile acids and their salts are relatively hydrophilic cholesterol derivatives that are synthesized in the liver and aid in lipid digestion (see Fig. 17-1). Cholesteryl esters are formed in the liver through the action of acyl-CoA-cholesterol acyl transferase (ACAT). This enzyme catalyzes the transfer of a fatty acid from coenzyme A to the hydroxyl group of cholesterol (Fig. 21-38), converting the cholesterol to a more hydrophobic form. Cholesteryl esters are transported in secreted lipoprotein particles to other tissues that use cholesterol, or they are stored in the liver. 

One enzyme regulated by AMPK is acetyl-CoA carboxylase, which produces malonyl-CoA, the first intermediate committed to fatty acid synthesis. Malonyl-CoA is a powerful inhibitor of the enzyme carnitine acyl-transferase I, which starts the process of ]3 oxidation by transporting fatty acids into the mitochondrion (see Fig. 17-6). By phosphorylating and inactivating acetyl-CoA carboxylase, AMPK inhibits fatty acid synthesis while relieving the inhibition (by malonyl-CoA) of )3 oxidation (Fig. 23-37). 

We were also able to use FAB mass spectrometry to determine the amino acid sequence around the active site serine in the acyl transference domain of rabbit mammary fatty acid synthase.6 The synthase was labelled in the acyl transferase domain(s) by the formation of O-ester intermediates after incubation with [" " C]-acetyl- or malonyl-CoA (Fig. 2A). The modified protein was then digested with elastase (Fig. 2B), and radioactive material isolated via successive purification steps on Sephadex G-50 and reverse phase HPLC. The isolated peptides were then sequenced by FAB MS. The data summarized in Table II established the sequences of both the acetyl and malonyl hexapeptides to be N-acyl-Ser-leu-Gly-Glu-Val-Ala. 

Figure 1 Polyketide biosynthesis. Polyketide backbones are formed via condensations from acyl-CoA thioesters of carboxylic acids. The (3-ketone which results from each condensation can undergo a series of reductive steps analogous to fatty acid biosynthesis. However, either none or only some of the reductive activities may occur in a given cycle. This allows PKSs to generate diversity through selection of priming and extender units, variation of the reductive cycle, and stereoselectivity. (ACP, acyl carrier protein AT, acyl transferase KS, ketosynthase DH, dehydratase ER, enoylreductase KR, ketoreductase TE, thioesterase.) The structure depicted in the lower right-hand corner is representative of the possible structural variations that can arise during polyketide biosynthesis.

Fig. 1. Targeted lipidomics of anandamide metabolism. Postulated pathways of anandamide metabolism. Abbreviations PC, phosphatidylcholine PE, phosphatidylethanolamine NAT, JV-acyl transferase LPA, lysophosphatidic acid PA, phosphatidic acid NAPE, jV-acyl-phosphatidylethanolamine Lyso-NAPE, l-lyso,2-acyl-OT-glycero-3-phosphoethanolamine-JV-acyl ABHD-4, a//3 hydrolase-4 GP-anandamide, glycerophospho-anandamide PAEA, phospho-anandamide PLA, phospholipase A NAPE-PLD, NAPE phospholipase D PLC, phospholipase C FAAH, fatty acid amide hydrolase P, phosphatase COX, cyclooxygenase LOX, lipoxygenase CYP450, cytochrome P450 PDE, phosphodiesterase.

Answer The transport of fatty acid molecules into mitochondria requires a shuttle system involving a fatty acyl-carnitine intermediate. Fatty acids are first converted to fatty acyl-CoA molecules in the cytosol (by the action of acyl-CoA synthetases) then, at the outer mitochondrial membrane, the fatty acyl group is transferred to carnitine (by the action of carnitine acyl-transferase I). After transport of fatty acyl-carnitine through the inner membrane, the fatty acyl group is transferred to mitochondrial CoA. The cytosolic and mitochondrial pools of CoA are thus kept separate, and no labeled CoA from the cytosolic pool enters the mitochondrion. 

The animal fatty acid synthase (FAS EC 2.3.1.85) is one of the most complex multifunctional enzymes that have been characterized, as this single polypeptide contains all the catalytic components required for a series of 37 sequential transactions (Smith, 1994). The animal FAS consists of two identical polypeptides of approximately 2500 amino acid residues (MW, ca. 270 kDa), each containing seven catalytic subunits (1) ketoacylsynthase, (2) malonyl/acetyl transferase, (3) dehydrase, (4) enoyl reductase, (5) (3-kcto reductase, (6) acyl carrier protein (ACP), and (7) thioesterase. Although some components of the complex are able to carry out their respective catalytic steps in the monomeric form, only in the FAS dimer do the subunits attain conformations that facilitate coupling of the individual reactions of fatty acid synthesis to occur (Smith et al., 2003). 

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