Carnitine synthetase

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... 

Long-chain fatty acids must be activated and transported into the mitochondria. Fatty acyl CoA synthetase, on the outer mitochondrial membrane, activates the fatty adds by attaching CoA. The fetty acyl portion is then transferred onto carnitine by carnitine aqdtransferase-I for transport into the mitochondria. The sequence of events is shown in Figure 1-16-2 and indudes the following steps ... 

CARNITINE O-ACETYLTRANSFERASE CARNITINE O-OCTANOYLTRANSFERASE CARNITINE O-PALMITOYLTRANSFERASE CARNOSINASE CARNOSINE SYNTHETASE CARNOT CYCLE... 

After a LCFA enters a cell, it is converted to the CoA derivative by long-chain fatty acyl CoA synthetase (thiokinase) in the cytosol (see p. 174). Because 0-oxidation occurs in the mitochondrial matrix, the fatty acid must be transported across the mitochon drial inner membrane. Therefore, a specialized carrier transports the long-chain acyl group from the cytosol into the mitochondrial matrix. This carrier is carnitine, and the transport process is called the carnitine shuttle (Figure 16.16). 

Enzymes of the upper pathway BcoA 4-Butyrobetainyl-CoA-synthetase BcoB Crotonobetainyl-CoA-synthetase BcoC 4-Butyrobetainyl-CoA-dehydrogenase BcoD Crotonobetainyl-CoA-hydrolase BcoE Carnitine-dehydrogenase Beu Betaine utilization Dcs Dehydrocarnitine splitting... 

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. 

Fatty acids are utilized as fuels by most tissues, although the brain, red and white blood cells, the retina, and adrenal medulla are important exceptions. Catabolism of fatty acids requires extramitochondrial activation, transport into mitochondria, and then oxidation via the /3-oxidative pathway. The initial step is catalyzed by fatty acyl-CoA synthetase (also called thiokinase and fatty acyl-CoA ligase), as shown in Equation (19.5). The product, fatty acyl-CoA, then exchanges the CoA for carnitine, as shown in Equation (19.6) ... 

Mold, P. A., Oscal, L. B. and Holloszy, J, 0. (1971) Adaptation of muscle to exercise. Increase in levels of palmltyl CoA synthetase, carnitine palmltyl-transferase and palmltyl CoA dehydrogenase and In the capacity to oxidize fatty acids. J. Clin. Invest. 50 2323-30. 

In normal myocardium, the first step in fatty acid utilization is thioesterification catalyzed by acylCoA synthetase. There are three potential metabolic fates of the synthesized acylCoA including 1) transport into the mitochondrial matrix for subsequent P-oxidation, 2) utilization as an intermediate in polar and nonpolar lipid synthesis, and 3) hydrolysis by acylCoA hydrolase (i.e., a net futile cycle). In normal myocardium, the major fraction of synthesized acylCoA is transported into the mitochondrial matrix by sequential transesterification reactions catalyzed by carnitine acyltransferase. AcylCoA in the mitochondrial matrix space is sequentially oxidized in two-carbon units to produce acetylCoA, which is accompanied by the production of the reducing equivalents, NADH and FADH2. 

Fig. 1 The pathway for L-carnitine production in Agrobacterium/Rhizob um HK4. (1) 4-butyrobetainyl-CoA synthetase (2) 4-butyrobetainyl-CoA dehydrogenase (3) crotonobetainyl-CoA hydrolase (4) thioesterase (5) carnitine dehydrogenase.

Fatty acids are activated to form acyl-CoA by acyl-CoA synthetase, an enzyme in the outer mitochrondrial membrane. Acyl-CoA then reacts with carnitine to form an acylcamitine derivative. Carnitine acyltrans-ferase I catalyzes this reaction. After acylcamitine is transported across the inner membrane by a carrier protein, it is subsequently reconverted to carnitine and acyl-CoA by carnitine acyltransferase II. 

Fig. 1. Camitine-dependent transfer of acyl groups across the inner mitochondrial membrane. Abbreviations ACS, acyl-CoA synthetase CPT I and CPT II, carnitine palmitoyltransferase I and II, respectively T, carnitine acylcamitine translocase.

Short-chain and medium-chain fatty acids with less than 10 carbon atoms can enter mitochondria as free acids independent of carnitine. They are activated by short-chain and medium-chain acyl-CoA synthetases that are present in the mitochondrial matrix. 

Fig. 23.1. Overview of mitochondrial long-chain fatty acid metabolism. (1) Fatty acid binding proteins (FaBP) transport fatty acids across the plasma membrane and bind them in the cytosol. (2) Fatty acyl CoA synthetase activates fatty acids to fatly acyl CoAs. (3) Carnitine transports the activated fatty acyl group into mitochondria. (4) p-oxidation generates NADH, FAD(2H), and acetyl CoA (5) In the liver, acetyl CoA is converted to ketone bodies...

Within the liver, they bind to fatty acid-binding proteins and are then activated on the outer mitochondrial membrane, the peroxisomal membrane, and the smooth endoplasmic reticulum by fatty acyl CoA synthetases. The fatty acyl group is transferred from CoA to carnitine for transport through the inner mitochondrial membrane, where it is reconverted back into fatty acyl CoA and oxidized to acetyl CoA in the (3-oxidation spiral (see Chapter 23). 

The enzymes in the pathways of fatty acid activation and p-oxidation (the synthetases, the carnitine acyltransferases, and the dehydrogenases of p-oxidation) are somewhat specific for the length of the fatty acid carbon chain. The chain length specificity is divided into enzymes for long-chain fatty acids (C20 to approximately C12), medium-chain (approximately C12 to C4), and short-chain (C4-C2). The major lipids oxidized in the liver as fuels are the long-chain fatty acids (palmitic, stearic, and oleic acids), because these are the lipids that are synthesized in the liver, are the major lipids ingested from meat or dairy sources, and are the major form of fatty acids present in adipose tissue triacylglycerols. The liver, as well as many other tissues, uses fatty acids as fuels when the concentration of the fatty acid-albumin complex is increased in the blood. 

Other genes involved in mitochondrial lipid metabolism, such as acyl-CoA synthetase" or medium-chain acyl-CoA dehydrogenase (MCAD), are also targets of PPAR, indicating that different steps of fatty acid metabolism, like activation, oxidation, and utilization of fatty acids are regulated by the levels of the substrate, the fatty acids. Therefore, we speculated that the main control step in fatty acid P-oxidation, the outer membrane component of carnitine palmitoyltransferase enzyme system, CPT 1, could also be a PPAR target. 

After transport across the plasma membrane, FAs must be esterified to coenzyme A, on the outer mitochodrial membrane by long chain acyl-CoA synthetase activity (ACSL C12 to C20) before they can undergo oxidative degradation. This reaction is coupled with two ATP hydrolysis to AMP and 2Pi. The mitochondrial membrane is not permeable to long chain acyl-CoA (i.e., C16-C18), therefore requires the initial conversion of acyl-CoA to an ester acylcamitine, followed by transport of the acylcamitine across the inner mitochondrial membrane into the mitochondrial matrix and subsequent delivery of acyl-CoA [126], This process is referred to as carnitine shuttle and requires the concerted action of 3 proteins 6 ... 

Figure 3. Model showing possible fates for lipid and carbohydrate carbon during nitrogen assimilation in P. tricomutum. Ruxes shown do not represent actual stoichiometries. (1) glutamine synthetase (2) nitrite reductase (3) nitrate reductase (4) malate glycolysis (5) cytosolic malate dehydrogenase (6) anaplerotic carbon flux (7) gluconeogenesis (8) glycolysis (9) carnitine acyltransferase (10) isocitrate lyase.

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