Carnitine

Carnitine (β-hydroxy-γ-N-trimethylaminobutyric acid, 3-hydroxy-4-N,N,N-trimethylaminobutyrate) is a quaternary ammonium compound involved in metabolism in most mammals, plants and some bacteria. Carnitine may exist in two isomers, labeled D-carnitine and L-carnitine, as they are optically active. At room temperature, pure carnitine is a white powder, and a water-soluble zwitterion with low toxicity. Carnitine only exists in animals as the L-enantiomer, and D-carnitine is toxic because it inhibits the activity of L-carnitine. Carnitine, derived from an amino acid, is found in nearly all organisms and animal tissue. Carnitine is the generic expression for a number of compounds that include L-carnitine, acetyl-L-carnitine, and propionyl-L-carnitine. It is most accumulated in cardiac and skeletal muscles as it accounts for 0.1% of its dry matter. It was first derived from meat extracts in 1905, therefore the name carnitine is derived from Latin 'carnus' or flesh. The body synthesizes enough carnitine from lysine side chains to keep up with the needs of energy production in the body as carnitine acts as a transporter of long-chain fatty acids into the mitochondria to be oxidized and produce energy. Some individuals with genetic or medical disorders (like preterm infants) cannot make enough, so this makes carnitine a conditionally essential nutrient for them. Carnitine (β-hydroxy-γ-N-trimethylaminobutyric acid, 3-hydroxy-4-N,N,N-trimethylaminobutyrate) is a quaternary ammonium compound involved in metabolism in most mammals, plants and some bacteria. Carnitine may exist in two isomers, labeled D-carnitine and L-carnitine, as they are optically active. At room temperature, pure carnitine is a white powder, and a water-soluble zwitterion with low toxicity. Carnitine only exists in animals as the L-enantiomer, and D-carnitine is toxic because it inhibits the activity of L-carnitine. Carnitine, derived from an amino acid, is found in nearly all organisms and animal tissue. Carnitine is the generic expression for a number of compounds that include L-carnitine, acetyl-L-carnitine, and propionyl-L-carnitine. It is most accumulated in cardiac and skeletal muscles as it accounts for 0.1% of its dry matter. It was first derived from meat extracts in 1905, therefore the name carnitine is derived from Latin 'carnus' or flesh. The body synthesizes enough carnitine from lysine side chains to keep up with the needs of energy production in the body as carnitine acts as a transporter of long-chain fatty acids into the mitochondria to be oxidized and produce energy. Some individuals with genetic or medical disorders (like preterm infants) cannot make enough, so this makes carnitine a conditionally essential nutrient for them. Many eukaryotes have the ability to synthesize carnitine, including humans. Humans synthesize carnitine from the substrate TML (6-N-trimethyllysine), which is in turn derived from the methylation of the amino acid lysine. TML is then hydroxylated into hydroxytrimethyllysine (HTML) by trimethyllysine dioxygenase, requiring the presence of ascorbic acid and iron. HTML is then cleaved by HTML aldolase (a pyridoxal phosphate requiring enzyme), yielding 4-trimethylaminobutyraldehyde (TMABA) and glycine. TMABA is then dehydrogenated into gamma-butyrobetaine in an NAD+-dependent reaction, catalyzed by TMABA dehydrogenase. Gamma-butyrobetaine is then hydroxylated by gamma butyrobetaine hydroxylase (a zinc binding enzyme) into L-carnitine, requiring iron in the form of Fe2+. Carnitine is involved in transporting fatty acids across the mitochondrial membrane, by forming a long chain acetylcarnitine ester and being transported by carnitine palmitoyltransferase I and carnitine palmitoyltransferase II. Carnitine also plays a role in stabilizing Acetyl-CoA and coenzyme A levels through the ability to receive or give an acetyl group. Rebouche and Engel had investigated the tissue distribution of carnitine-biosynthetic enzymes in humans. They found TMLD to be active in the liver, heart, muscle, brain and highest in kidney. HTMLA activity is found primarily in the liver. The rate of TMABA oxidation is greatest in the liver, with considerable activity also found in the kidney, however is low in brain, heart and muscle. These results indicate that all the investigated tissues have the ability to convert TML into butyrobetaine through containing the required enzymes for it but not all of them can convert butyrobetaine into carnitine, only the kidney, liver and brain are capable of that. The free-floating fatty acids, released from adipose tissues to the blood, bind to carrier protein molecule known as serum albumin that carry the fatty acids to the cytoplasm of target cells such as the heart, skeletal muscle, and other tissue cells, where they are used for fuel. But before the target cells can use the fatty acids for ATP production and β oxidation, the fatty acids with chain lengths of 14 or more carbons must be activated and subsequently transported into mitochondrial matrix of the cells in three enzymatic reactions of the carnitine shuttle. The first reaction of the carnitine shuttle is a two-step process catalyzed by a family of isozymes of acyl-CoA synthetase that are found in the outer mitochondrial membrane, where they promote the activation of fatty acids by forming a thioester bond between the fatty acid carboxyl group and the thiol group of coenzyme A to yield a fatty acyl–CoA. In the first step of the reaction, acyl-CoA synthetase catalyzes the transfer of adenosine monophosphate group (AMP) from an ATP molecule onto the fatty acid generating a fatty acyl–adenylate intermediate and a pyrophosphate group (PPi). The pyrophosphate, formed from the hydrolysis of the two high-energy bonds in ATP, is immediately hydrolyzed to two molecule of Pi by inorganic pyro phosphatase. This reaction is highly exergonic which drives the activation reaction forward and makes it more favorable. In the second step, the thiol group of a cytosolic coenzyme A attacks the acyl-adenylate, displacing AMP to form thioester fatty acyl-CoA. In the second reaction, the activated fatty acids that are intended for mitochondrial oxidation are transported into the matrix by a carrier protein, but first the acyl-CoA must be transiently attached to the hydroxyl group of carnitine to form fatty acyl–carnitine. This transesterification is catalyzed by an enzyme found in the outer membrane of the mitochondria known as carnitine acyltransferase 1 (also called carnitine palmitoyltransferase 1, CPT1). The fatty acyl–carnitine ester formed then diffuses across the intermembrane space of the mitochondria and enters the matrix by passive transport through the acyl-carnitine/carnitine cotransporter that is found in inner mitochondrial membrane. This cotransporter return one molecule of carnitine from the matrix to the intermembrane space as one molecule of fatty acyl– carnitine moves into the matrix.