Carnitine shuttle

Long-chain fatty acids can slowly cross the mitochondrial membrane by themselves, but this is too slow to keep up with their metabolism. The carnitine shuttle provides a transport mechanism and allows control of (3 oxidation. Malonyl-CoA, a precursor for fatty acid synthesis, inhibits the carnitine shuttle and slows down (3 oxidation (Fig. 13-5). 

The CARNITINE SHUTTLE is used to transport fatty acids into the mitochondria. 

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. 

The most important process in the degradation of fatty acids is p-oxidation—a metabolic pathway in the mitochondrial matrix (see p. 164). initially, the fatty acids in the cytoplasm are activated by binding to coenzyme A into acyl CoA [3]. Then, with the help of a transport system (the carnitine shuttle [4] see p. 164), the activated fatty acids enter the mitochondrial matrix, where they are broken down into acetyl CoA. The resulting acetyl residues can be oxidized to CO2 in the tricarboxylic acid cycle, producing reduced... 

The carnitine shuttle is the rate-determining step in mitochondrial fatty acid degradation. Malonyl CoA, a precursor of fatty acid biosynthesis, inhibits carnitine acyltransferase (see p. 162), and therefore also inhibits uptake of fatty acids into the mitochondrial matrix. 

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]. 

The carnitine shuttle for transporting acyl residues into the mitochondrial matrix is discussed on p. 164. 

Carnitine deficiency leads to impaired carnitine shuttle activity the resulting decreased LCFA metabolism and accumulation of LCFAs In tissues and wasting of acyl-carnitine in urine can produce cardiomyopathy, skeletal muscle myopathy, encephalopathy, and impaired liver function. 

Figure 8-3. The carnitine shuttle. A long-chain fatty acyl CoA (LCFA CoA) can diffuse across the outer mitochondrial membrane but must be carried across the inner membrane as acyl-carnitine. The active sites of CPT-I and CPT-II are oriented toward the interiors of their respective membranes. CPT, carnitine palmitoyltransferase.

Both these disorders are treated by avoidance of fasting, dietary restriction of LCFAs, and carnitine supplementation the objective is to stimulate whatever carnitine shuttle activity is present. 

The enzymes of fatty acid oxidation in animal cells are located in the mitochondrial matrix, as demonstrated in 1948 by Eugene P. Kennedy and Albert Lehninger. The fatty acids with chain lengths of 12 or fewer carbons enter mitochondria without the help of membrane transporters. Those with 14 or more carbons, which constitute the majority of the FFA obtained in the diet or released from adipose tissue, cannot pass directly through the mitochondrial membranes—they must first undergo the three enzymatic reactions of the carnitine shuttle. The first reaction is catalyzed by a family of isozymes (different isozymes specific for fatty acids having short, intermediate, or long carbon chains) present... 

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. 

Once inside cells, fatty acids are activated at the outer mitochondrial membrane by conversion to fatty acyl-CoA thioesters. Fatty acyl-CoA to be oxidized enters mitochondria in three steps, via the carnitine shuttle. 

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 acid degradation ((3-oxidation) occurs in mitochondria. The carnitine shuttle is... 

Acylcamitine then moves across the mitochondrial membrane via an antiport, which also transports carnitine in the opposite direction. In the mitochondria, carnitine is once more exchanged with CoA, which is a reversal of Equation (19.6), yielding acyl-CoA. Free carnitine is then returned to the extramitochondrial space by the antiport. The carnitine shuttle is shown in Figure 19.5. Carnitine is synthesized in the organism from lysine. The symptoms of carnitine deficiency are muscle weakness, cardiac myopathy, and hypertriglyceridemia. These are observed in certain genetic disorders, alcoholism, hemo-... 

Figure 36-4. The activation of fatty acids and transport into the mitochondrion via the carnitine shuttle. (Reproduced, with permission, from D.B. Marks, et al, Basic Medical Biochemistry A Clinical Approach, Philadelphia Lippincott...

Oleate is an abundant 18-carbon monounsaturated fatty acid with a cis double bond between C-9 and C-10 (denoted A ). In the first step of oxidation, oleate is converted to oleoyl-CoA and, like the saturated fatty acids, enters the mitochondrial matrix via the carnitine shuttle (Fig. 17-6). Oleoyl-CoA then undergoes three passes through the fatty acid oxidation cycle to yield three molecules of acetyl-CoA and the coenzyme A ester of a A, 12-carbon unsaturated fatty acid, cis-A -dodecenoyl-CoA (Fig. 17-9). This product cannot serve as a substrate for enoyl-CoA hydratase, which acts only on trans double bonds. The auxiliary enzyme A, A -enoyl-CoA isomerase isomerizes the ci5-A -enoyl-CoA to the fra/J5-A -enoyl-CoA, which is converted by enoyl-CoA hydratase into the corresponding L-/3-hydroxyacyl-CoA (fra/75-A -dodecenoyl-CoA). This intermediate is now acted upon by the remaining enzymes of /3 oxidation to yield acetyl-CoA and the coenzyme A ester of a 10-carbon saturated fatty acid, decanoyl-CoA. The latter undergoes four more passes through the pathway to yield five more molecules of acetyl-CoA. Altogether, nine acetyl-CoAs are produced from one molecule of the 18-carbon oleate. 

Figure 20.1 The carnitine shuttle. This brings fatty acyl chains into the mitochondrial matrix and because it exchanges acyl carnitine going in for carnitine going out, the carnitine supply doesn t run out in either cellular compartment.

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