Fatty acyl carnitine, transport into

Fatty acyl carnitine is transported via a translocase that transports acylcarnitine into and carnitine out of the mitochondrion (Chapter 7). 

FIGURE17-6 Fatty acid entry into mitochondria via the acyl-carnitine/ carnitine transporter. After fatty acyl-carnitine is formed at the outer membrane or in the intermembrane space, it moves into the matrix by facilitated diffusion through the transporter in the inner membrane. In the matrix, the acyl group istransferred to mitochondrial coenzyme... 

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. 

Fig. 23.5. Transport of long-chain fatty acids into mitochondria. The fatty acyl CoA crosses the outer mitochondrial membrane. Carnitine pahnitoyl transferase I in the outer mitochondrial membrane transfers the fatty acyl group to carnitine and releases Co ASH. The fatty acyl carnitine is translocated into the mitochondrial matrix as carnitine moves out. Carnitine palmitoyl transferase II on the inner mitochondrial membrane transfers the fatty acyl group back to CoASH, to form fatty acyl CoA in the matrix.

All of the other enzymes of the /3-oxidation pathway are located in the mitochondrial matrix. Short-chain fatty acids, as already mentioned, are transported into the matrix as free acids and form the acyl-CoA derivatives there. However, long-chain fatty acyl-CoA derivatives cannot be transported into the matrix directly. These long-chain derivatives must first be converted to acylearnitine derivatives, as shown in Figure 24.9. Carnitine acyltransferase I, located on the outer side of the inner mitochondrial membrane, catalyzes the formation of... 

At this point, the acyl-CoA is still in the cytosol of the muscle cell. Entry of the acyl-CoA into the mitochondrial matrix requires two translocase enzymes, carnitine acyl transferase I and carnitine acyl transferase II (CAT I and CAT II), and a carrier molecule called carnitine the carnitine shuttles between the two membranes. The process of transporting fatty acyl-CoA into mitochondria is shown in Figure 7.15. 

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 ... 

After uptake by the cell, fatty acids are activated by conversion into their CoA derivatives—acyl CoA is formed. This uses up two energy-rich anhydride bonds of ATP per fatty acid (see p. 162). For channeling into the mitochondria, the acyl residues are first transferred to carnitine and then transported across the inner membrane as acyl carnitine (see B). 

The inner mitochondrial membrane has a group-specific transport system for fatty acids. In the cytoplasm, the acyl groups of activated fatty acids are transferred to carnitine by carnitine acyltransferase [1 ]. They are then channeled into the matrix by an acylcar-nitine/carnitine antiport as acyl carnitine, in exchange for free carnitine. In the matrix, the mitochondrial enzyme carnitine acyltransferase catalyzes the return transfer of the acyl residue to CoA. 

Fatty acids with an odd number of C atoms are treated in the same way as normal fatty acids—i. e., they are taken up by the cell with ATP-dependent activation to acyl CoA and are transported into the mitochondria with the help of the carnitine shuttle and broken down there by p-oxidation (see p. 164). In the last step, propionyl CoA arises instead of acetyl CoA. This is first carboxylated by propionyl CoA carboxylase into fSj-methylmalonyl CoA [3], which—after isomerization into the (i ) enantiomer (not shown see p. 411)—is isomerized into succinyl CoA [4]. 

In the third and final step of the carnitine shuttle, the fatty acyl group is enzymatically transferred from carnitine to intramitochondrial coenzyme A by carnitine acyltransferase II. This isozyme, located on the inner face of the inner mitochondrial membrane, regenerates fatty acyl-CoA and releases it, along with free carnitine, into the matrix (Fig. 17-6). Carnitine reenters the intermembrane space via the acyl-camitine/car-nitine transporter. 

This three-step process for transferring fatty acids into the mitochondrion—esterification to CoA, transesterification to carnitine followed by transport, and transesterification back to CoA—links two separate pools of coenzyme A and of fatty acyl-CoA, one in the cytosol, the other in mitochondria These pools have different functions. Coenzyme A in the mitochondrial matrix is largely used in oxidative degradation of pyruvate, fatty acids, and some amino acids, whereas cytosolic coenzyme A is used in the biosynthesis of fatty acids (see Fig. 21-10). Fatty acyl-CoA in the cytosolic pool can be used for membrane lipid synthesis or can be moved into the mitochondrial matrix for oxidation and ATP production. Conversion to the carnitine ester commits the fatty acyl moiety to the oxidative fate. 

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). 

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) ... 

The mitochondrial inner membrane is impermeable to fatty acyl-CoA. To be transported into mitochondria, the acyl moiety must first be esterified to L-carnitine (hereafter referred to simply as carnitine) to form the corresponding fatty acylcarnitine. This reaction is catalyzed by carnitine palmitoyltransferase I (CPTI) localized... 

A specific transport protein, the carnitine-acylcarnitine translocase, moves the fatty acylcarnitine into the mitochondrial matrix while returning carnitine from the matrix to the cytoplasm. Once inside the mitochondria, another enzyme, carnitine palmitoyltransferase II (CPT II), located on the matrix side of the mitochondrial inner membrane, catalyzes the reconversion of fatty acylcarnitine to fatty acyl-CoA. Intramitochondrial fatty acyl-CoA then undergoes (3-oxidation to generate acetyl-CoA.Acetyl-CoA can enter the Kreb s cycle for complete oxidation or, in the liver, be used for the synthesis of acetoacetate and P-hydroxybutyrate (ketone bodies). 

Transport of the fatty acyl-CoA into the mitochondrion is accomplished via an acyl-carnitine intermediate inside the mitochondrion the fatty acyl-CoA molecule is regenerated. 

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. 

Carititine (3-hydroxy,4-AT-trimethylaminobutyric acid) has a central role in the transport of fatty acids across the mitochondrial membrane for p -oxidation. At the outer face of the outer mitochondrial membrane, carnitine acyltransferase I catalyzes the reaction shown in Figure 14.1, the transfer of fatty acids from coenzyme A (CoA) to form acyl carnitine esters that cross into the mitochondrial matrix. At the timer face of the inner mitochondrial membrane, carnitine acyltransferase II catalyzes the reverse reaction. 

Triacylglycerols can be mobilized by the hydrolytic action of lipases that are under hormonal control. Fatty acids are activated to acyl CoAs, transported across the inner mitochondrial membrane by carnitine, and degraded in the mitochondrial matrix by a recurring sequence of four reactions oxidation by FAD, hydration, oxidation by NAD+, and thiolysis by CoA. The FADH2 and NADH formed in the oxidation steps transfer their electrons to O2 by means of the respiratory chain, whereas the acetyl CoA formed in the thiolysis step normally enters the citric acid cycle by condensing with oxaloacetate. Mammals are unable to convert fatty acids into glucose, because they lack a pathway for the net production of oxaloacetate, pyruvate, or other gluconeogenic intermediates from acetyl CoA. 

A third classification includes those transporters which catalyze transport of metabolites which are neutral at physiological pH. These include the transporters for glutamine [31], and for the other neutral amino acids [32]. It also includes the one required for the oxidation of fatty acids which catalyze the exchange of carnitine for acyl carnitine [33]. This carrier also catalyzes net uptake of carnitine or acyl carnitines into the mitochondrial membrane, but at a rate only 1-2% of the exchange rate [34-36]. 

Carnitine is used mainly for facilitating the transport of long-chain fatty adds into the mitochondria. As shown in Figure4.53, this transport system requires the participation of two different carnitine acyl transferases. One is located on the outside of the mitochondrial membrane, the other on the inner side. Once fatty acyl-camitine is inside the organelle, its carnitine is released. A separate transport system is used to transport this carnitine from the interior of the mitochondrion back to the cytoplasm for reuse. 

Carnitine is required for transport of longoxidative metabolism as well as in the formation of ketone bcidies, The concentration of free carnitine in muscle is about 4,0 mmol/kg. The concentration of carnitine bound to long-chain fatty adds (fatty acyl-camitine) is lower, about 0,2 mmol/kg. Short-chain fatty adds, including acetic, are also esterified to carnitine, but the functions of these complexes are not clear. There is some indication that keto forms of BCAAs (BCKAs) can also be esterified to carnitine. These complexes can then be transported into the mitochondria for complete oxidation of the BCKAs, The importance of this mode of BCKA transport is not dear (Takakura et ai., 1997). 

Prior to oxidation, long-chain fatty acids are activated, forming fatty acyl CoA, which is transported into mitochondria by a carnitine carrier system. 

Carnitine acyltransferase I, which catalyzes the transfer of acyl groups from coenzyme A to carnitine, is inhibited by malonyl CoA, an intermediate in fatty acid synthesis. Therefore, when fatty acids are being synthesized in the cytosol, malonyl CoA inhibits their transport into mitochondria and, thus, prevents a futile cycle—synthesis followed by immediate degradation. 

The answer is c. (Murray, pp 123-148. Scriver, pp 2367-2424. Sack, pp 159-175. Wilson, pp 287-317.) The most likely cause of the symptoms observed is carnitine deficiency. Under normal circumstances, long-chain fatty acids coming into muscle cells are activated as acyl coenzyme A and transported as acyl carnitine across the inner mitochondrial membrane into the matrix. A deficiency in carnitine, which is normally synthesized in the liver, can be genetic but it is also observed in preterm babies with liver problems and dialysis patients. Blockage of the transport of long-chain fatty acids into mitochondria not only deprives the patient of energy production, but also disrupts the structure of the muscle cell with the accumulation of lipid droplets. Oral dietary supplementation usually can effect a cure. Deficiencies in the carnitine acyltransferase enzymes I and II can cause similar symptoms. 

The answer is a. (Murray, pp 258-297. Scriver, pp 2705-2716. Sack, pp 121—138. Wilson, pp 362—367.) Under conditions of active synthesis of fatty acids in the cytosol of hepatocytes, levels of malonyl CoA are high. Malonyl CoA is the activated source of two carbon units for fatty acid synthesis. Malonyl CoA inhibits carnitine acyltransferase 1, which is located on the cytosolic face of the inner mitochondrial membrane. Thus, long-chain fatty acyl CoA units cannot be transported into mitochondria where p oxidation occurs, and translocation from cytosol to mitochondrial matrix is prevented. In this situation compartmentalization of membranes as well as inhibition of enzymes comes into play. 

Thioesterification takes place within the cytoplasm but the rest of the reactions which constitute /5-oxidation occur within the mitochondria. However, fatty acyl CoA cannot penetrate the mitochondrial membrane, so for the transport to occur, the CoA group is substituted by a compound called carnitine, which carries the fatty acyl group into the mitochondria. Once inside it reverts to acyl CoA. 

In muscle, most of the fatty acids undergoing beta oxidation are completely oxidized to C02 and water. In liver, however, there is another major fate for fatty acids this is the formation of ketone bodies, namely acetoacetate and b-hydroxybutyrate. The fatty acids must be transported into the mitochondrion for normal beta oxidation. This may be a limiting factor for beta oxidation in many tissues and ketone-body formation in the liver. The extramitochondrial fatty-acyl portion of fatty-acyl CoA can be transferred across the outer mitochondrial membrane to carnitine by carnitine palmitoyltransferase I (CPTI). This enzyme is located on the inner side of the outer mitochondrial membrane. The acylcarnitine is now located in mitochondrial intermembrane space. The fatty-acid portion of acylcarnitine is then transported across the inner mitochondrial membrane to coenzyme A to form fatty-acyl CoA in the mitochondrial matrix. This translocation is catalyzed by carnitine palmitoyltransferase II (CPTII Fig. 14.1), located on the inner side of the inner membrane. This later translocation is also facilitated by camitine-acylcamitine translocase, located in the inner mitochondrial membrane. The CPTI is inhibited by malonyl CoA, an intermediate of fatty-acid synthesis (see Chapter 15). This inhibition occurs in all tissues that oxidize fatty acids. The level of malonyl CoA varies among tissues and with various nutritional and hormonal conditions. The sensitivity of CPTI to malonyl CoA also varies among tissues and with nutritional and hormonal conditions, even within a given tissue. Thus, fatty-acid oxidation may be controlled by the activity and relative inhibition of CPTI. 

Carnitine tablets are sold in health food stores. It is claimed that carnitine will enhance the breakdown of body fat. Carnitine is a tertiary amine found in mitochondria, cell organelles in which food molecules are completely oxidized and ATP is produced. Carnitine is involved in transporting the acyl groups of fatty acids from the cytoplasm into the mitochondria. The fatty acyl group is transferred from a fatty acyl CoA molecule and esterified to carnitine. Inside the mitochondria the reachon is reversed and the fatty acid is completely oxidized. The structure of carnitine is shown here ... 

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