Acyl-CoA esters

FIGURE 25.7 The pathway of palmhate synthesis from acetyl-CoA and malonyl-CoA. Acetyl and malonyl building blocks are introduced as acyl carrier protein conjugates. Decarboxylation drives the /3-ketoacyl-ACP synthase and results in the addition of two-carbon units to the growing chain. Concentrations of free fatty acids are extremely low in most cells, and newly synthesized fatty acids exist primarily as acyl-CoA esters. 

Long-chain fatty acids (e.g., palmitate Cig) diffuse through pores in the outer mitochondrial membrane, and then form long-chain acyl-CoA esters catalyzed reversibly by palmitoyl-CoA synthase (assumed to be on the inner face of the outer membrane). 

Long-chain acyl-CoA esters are then converted to acylcamitine esters by readily reversible reactions with L-camitine catalyzed by carnitine palmitoyltransferase I (CPT I). 

Figure 3. Mitochondrial fatty acid oxidation. Long-chain fatty acids are converted to their CoA-esters as described in the text, and their fatty-acyl-groups transferred to CoA in the matrix by the concerted action of CPT 1, the acylcarnitine/carnitine exchange carrier and CPT (A) as described in the text. Medium-chain and short-chain fatty acids (Cg or less) diffuse directly into the matrix where they are converted to their acyl-CoA esters by a acyl-CoA synthase. The mechanism of p-oxidation is shown below (B). Each cycle of P-oxidation removes -CH2-CH2- as an acetyl unit until the fatty acids are completely converted to acetyl-CoA. The enzymes catalyzing each stage of P-oxidation have different but overlapping specificities. In muscle mitochondria, most acetyl-CoA is oxidized to CO2 and H2O by the citrate cycle (Figure 4) some is converted to acylcamitine by carnitine acetyltransferase (associated with the inner face of the inner membrane) and exported from the matrix. Some acetyl-CoA (if in excess) is hydrolyzed to acetate and CoASH by acetyl-CoA hydrolase in the matrix. Enzymes ...

K. Waku, Origins and Fates of Fatty Acyl-CoA Esters , Biochim. Biophys. Acta 1992, 1124, 101-111. 

The principle of this assay relies on the assumption that skin fibroblasts of patients affected with an FAO deficiency will accumulate certain species of acyl-CoA esters, reflecting the metabolic defect, when the cell medium is supplemented with... 

Fatty acyl-CoA esters formed at the cytosolic side of the outer mitochondrial membrane can be transported into the mitochondrion and oxidized to produce ATP, or they can be used in the cytosol to synthesize... 

Detergents (26, 41, 46, 96, 97) Optimal concentrations of deoxycholate, cholate, Triton X-100, and cetyltri-methylammonium bromide activate, as does urea. Activation of phosphotransferase > that of phosphohydrolase. Supraoptimal levels inhibit, as do all tested concentrations of sodium lauryl sulfate and Tweens 20 and 80. (See also Lysolecithin, Fatty acids, and Long-chain fatty acyl-CoA esters, above)... 

Fatty acyl-CoA esters are similarly capable of participating as starter groups. Fatty acid... 

Knudsen, J., Grunnet, I. 1982. Transacylation as a chain-termination mechanism in fatty acid synthesis by mammalian fatty acid synthetase. Synthesis of medium-chain-length (Cg — C12) acyl-CoA esters by goat mammary gland fatty acid synthetase. Biochem J. 202, 139-143. 

Marshall, M.O., Knudsen, J. 1977. The specificity of 1-acyl-jn-glycerol 3-phosphate acyltrans-ferase in microsomal fractions from lactating cow mammary gland towards short, medium and long chain acyl-CoA esters. Biochim. Biophys. Acta 489, 236-241. 

Rasmussen, J.T., Borchers, T., Knudsen, J. 1990. Comparison of the binding affinities of acyl-CoA-binding protein and fatty-acid-binding protein for long-chain acyl-CoA esters. Biochem. J. 265, 849-855. 

Primary carnitine deficiency is caused by a deficiency in the plasma-membrane carnitine transporter. Intracellular carnitine deficiency impairs the entry of long-chain fatty acids into the mitochondrial matrix. Consequently, long-chain fatty acids are not available for p oxidation and energy production, and the production of ketone bodies (which are used by the brain) is also impaired. Regulation of intramitochondrial free CoA is also affected, with accumulation of acyl-CoA esters in the mitochondria. This in turn affects the pathways of intermediary metabolism that require CoA, for example the TCA cycle, pyruvate oxidation, amino acid metabolism, and mitochondrial and peroxisomal -oxidation. Cardiac muscle is affected by progressive cardiomyopathy (the most common form of presentation), the CNS is affected by encephalopathy caused by hypoketotic hypoglycaemia, and skeletal muscle is affected by myopathy. 

Carnitine Carrier The resulting acyl CoA ester is still not permeable to the mitochondrial membrane so a carrier system is needed. In this system the fatty acyl group is transferred from CoA-S to carnitine, diffuses across the membrane, and then transferred back to another CoA-S within the matrix ... 

Gregersen N> Kolvraa S, Mortensen PB. Acyl-CoA glycine N-acyltransferase in vitro studies on the glycine conjugation of straight- and branched-chained acyl-CoA esters in human liver. Biochem MedMetab Biol 1986 35 210-8. 

KEY, Malonyl-CoA and long chain acyl-CoA esters as metabolic coupling factors in nutrient-induced insulin secretion, J. Biol. Chem., 1992, 267, 5802-5810. 

Type III synthases, as a whole, employ a wider spectrum of physiological starter molecules than their type I and II counterparts, including a variety of aromatic and aliphatic CoA esters such as coumaiyl-CoA, methyl-anthraniloyl-CoA, as well as the recently identified medium- and long-chain fiitty acyl-CoA ester starters used by certain bacterial and plant type III enzymes involved in the biosyndiesis of phenolic lipids (22, 24, Cook et al., unpublished results). The most extensively studied type III en mie, chalcone synthase (Fig. 4), uses 4-coumaryl-CoA as the starter unit and catalyzes three successive condensation reactions with malonyi-CoA as the extender. Cyclization and aromatization of the linear tetraketide intermediate is performed within the same active site, yielding the final product 4 ,2 ,4 ,6 -tetrahydroxychalcone. 

The chiral samples of malonyl-CoA were then used to probe the steric course of fatty acid biosynthesis. Again, the experiment was complicated by tritium exchange during the incubation, both before and after the Claisen condensation step, resulting in 51% tritium retention from the S isomer and 23% tritium retention from the R isomer. Fatty acid biosynthesis involves Claisen condensation of malonyl-CoA with an acyl-CoA ester of 2 n carbon chain length to... 

Hiltunen, J.K., Qin, Y.-M. 2000. p-Oxidation — strategies for the metabolism of a wide variety of acyl-CoA esters. Biochim. Biophys. Acta 1484 117-128. 

The terms NEFA, unesterified fatty acids, and free fatty acids have been used synonymously in the literature, although these fatty acids are mainly bound to plasma proteins. Enzymatic assays are convenient with NEFA in the presence of added acyl-coenzyme A (CoA) synthetase forming acyl CoA esters, which are then oxidized by acyl CoA oxidase to yield hydrogen peroxide. The hydrogen peroxide produced is measured by a peroxidase-linked colorimetric detection reaction. Plasma samples should be separated promptly and stored frozen to prevent in vitro lipolysis (Zambon, Hashimoto, and Brunzell 1993). 

Inhibition of adenine nucleotide transport by fatty acyl-CoA esters. 

It has been proposed that fatty acyl-CoA esters, which are potent inhibitors of ADP transport in isolated mitochondria, also control flux through the translocator in vivo and hence the production of ATP (see [4] for literature). However, as pointed out by Stubbs [7], such inhibition makes little physiological sense in situations like starvation or long-term exercise, when fatty acids are an important fuel especially under the latter conditions the translocator must operate at high capacity to provide the cytosol with ATP. Indeed, in isolated hepatocytes incubated with various concentrations of fatty acids no inhibition of ATP transport by fatty acyl-CoA could be observed [71]. Possibly this inhibition is prevented by a low-molecular weight cytosolic protein with a high affinity for fatty acyl-CoA [72]. 

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