Carnitine palmitoyltransferase I (CPT1) also known as carnitine acyltransferase I, CPTI, CAT1, CoA:carnitine acyl transferase (CCAT), or palmitoylCoA transferase I, is a mitochondrial

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

responsible for the formation of acyl carnitines by catalyzing the transfer of the acyl group of a long-chain fatty acyl-CoA from coenzyme A to

l-carnitine Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids into mitochondria to be oxidized for energy production, ...

. The product is often

Palmitoylcarnitine Palmitoylcarnitine is an ester derivative of carnitine involved in the metabolism of fatty acids. During the tricarboxylic acid cycle (TCA), fatty acids undergo a process known as β-oxidation to produce energy in the form of ATP. β-oxidation occ ...

(thus the name), but other fatty acids may also be substrates. It is part of a family of enzymes called carnitine acyltransferases. This "preparation" allows for subsequent movement of the acyl carnitine from the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

into the intermembrane space of mitochondria. Three

isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some iso ...

of CPT1 are currently known: CPT1A, CPT1B, and CPT1C. CPT1 is associated with the

outer mitochondrial membrane A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is use ...

. This enzyme can be inhibited by

malonyl CoA Malonyl-CoA is a coenzyme A derivative of malonic acid. Functions It plays a key role in chain elongation in fatty acid biosynthesis and polyketide biosynthesis. Fatty acid biosynthesis Malonyl-CoA provides 2-carbon units to fatty acids and commi ...

, the first committed intermediate produced during fatty acid synthesis. Its role in fatty acid metabolism makes CPT1 important in many metabolic disorders such as

diabetes Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ...

. Since its

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...

is not known, its exact mechanism of action remains to be determined.

Structure

Carnitine Acetyltransferase and Carnitine

CPT1 is an

integral membrane protein An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All ''transmembrane proteins'' are IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a sign ...

that exists in three isoforms in mammalian tissues: CPT1A, CPT1B and CPT1C. The first two are expressed on the outer mitochondrial membrane of most tissues, but their relative proportions varies between tissues. CPT1A predominates in lipogenic tissues like liver, whereas CPT1B predominates in tissues like heart and skeletal muscle that have a high fatty acid oxidative capacity. brown adipose cells. Both isoforms are integral proteins of the mitochondrial outer membrane through two transmembrane regions in the

peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...

chain. The membrane topology of CPT1A was described by It is polytopic, with both the N- and C-termini exposed on the cytosolic aspect of the OMM, with a short loop linking the two transmembrane domains protruding into the mitochondrial inter-membrane space. The third isoform (CPT1C), was identified in 2002 and is expressed in both mitochondria and the endoplasmic reticulum. It is normally expressed only in neurones (brain), although its expression is altered in certain cancer cell types. The exact structure of any of the CPT1 isoforms has not yet been determined, although a variety of '' in silico'' models for CPT1 have been created based on closely related carnitine acyltransferases, such as carnitine acetyltransferase (CRAT). An important structural difference between CPT1 and CPT2, CRAT and carnitine octanoyltransferase (COT) is that CPT1 contains an additional domain at its N-terminal consisting of about 160 amino acids. It has been determined that this additional N-terminal domain is important for the key inhibitory molecule of CPT1, malonyl-CoA, and acts like a switch that makes CPT1A more or less sensitive to malonyl-CoA inhibition Two distinct

binding site In biochemistry and molecular biology, a binding site is a region on a macromolecule such as a protein that binds to another molecule with specificity. The binding partner of the macromolecule is often referred to as a ligand. Ligands may includ ...

s have been proposed to exist in CPT1A and CPT1B. The "A site" or "CoA site" appears to bind both malonyl-CoA and palmitoyl-CoA, as well as other molecules containing coenzyme A, suggesting that the enzyme binds these molecules via interaction with the coenzyme A moiety. It has been suggested that malonyl-CoA may behave as a

competitive inhibitor Competitive inhibition is interruption of a chemical pathway owing to one chemical substance inhibiting the effect of another by competing with it for binding or bonding. Any metabolic or chemical messenger system can potentially be affected b ...

of CPT1A at this site. A second "O site" has been proposed to bind malonyl-CoA more tightly than the A site. Unlike the A site, the O site binds to malonyl-CoA via the dicarbonyl group of the

malonate The conjugate acids are in :Carboxylic acids. {{Commons category, Carboxylate ions, Carboxylate anions Carbon compounds Oxyanions ...

moiety of malonyl-CoA. The binding of malonyl-CoA to either the A and O sites inhibits the action of CPT1A by excluding the binding of carnitine to CPT1A. Since a crystal structure of CPT1A has yet to be isolated and imaged, its exact structure remains to be elucidated.

Function

Enzyme mechanism

Because crystal structure data is currently unavailable, the exact mechanism of CPT1 is not currently known. A couple different possible mechanisms for CPT1 have been postulated, both of which include the

histidine Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the d ...

residue Residue may refer to: Chemistry and biology * An amino acid, within a peptide chain * Crop residue, materials left after agricultural processes * Pesticide residue, refers to the pesticides that may remain on or in food after they are applied ...

473 as the key catalytic residue. One such mechanism based upon a carnitine acetyltransferase model is shown below in which the His 473 deprotonates carnitine while a nearby serine residue stabilizes the tetrahedral

oxyanion An oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determine ...

intermediate. A different mechanism has been proposed that suggests that a

catalytic triad A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, acylases, li ...

composed of residues Cys-305, His-473, and Asp-454 carries out the acyl-transferring step of

catalysis Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

. This catalytic mechanism involves the formation of a thioacyl-enzyme

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

intermediate with Cys-305.

Carnitine Palmitoyltransferase I Mechanism

Biological function

The carnitine palmitoyltransferase system is an essential step in the beta-oxidation of

long chain fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

s. This transfer system is necessary because, while fatty acids are activated (in the form of a

thioester In organic chemistry, thioesters are organosulfur compounds with the functional group . They are analogous to carboxylate esters () with the sulfur in the thioester playing the role of the linking oxygen in the carboxylate ester, as implied by t ...

linkage to coenzyme A) on the outer mitochondrial membrane, the activated fatty acids must be oxidized within the

mitochondrial matrix In the mitochondrion, the matrix is the space within the inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contains the mitochondrial DNA, ribo ...

. Long chain fatty acids such as palmitoyl-CoA, unlike short- and medium-chain fatty acids, cannot freely

diffuse Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

through the

mitochondrial inner membrane The inner mitochondrial membrane (IMM) is the mitochondrial membrane which separates the mitochondrial matrix from the intermembrane space. Structure The structure of the inner mitochondrial membrane is extensively folded and compartmentalized. ...

, and require a shuttle system to be transported to the mitochondrial matrix. Carnitine palmitoyltransferase I is the first component and

rate-limiting step In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS or RD-step or r/d step) or rate-limiting step. For a given reaction mechanism, the prediction of th ...

of the carnitine palmitoyltransferase system, catalyzing the transfer of the acyl group from coenzyme A to carnitine to form

palmitoylcarnitine Palmitoylcarnitine is an ester derivative of carnitine involved in the metabolism of fatty acids. During the tricarboxylic acid cycle (TCA), fatty acids undergo a process known as β-oxidation to produce energy in the form of ATP. β-oxidation occ ...

. A

translocase Translocase is a general term for a protein that assists in moving another molecule, usually across a cell membrane. These enzymes catalyze the movement of ions or molecules across membranes or their separation within membranes. The reaction is des ...

then shuttles the acyl carnitine across the inner mitochondrial membrane where it is converted back into palmitoyl-CoA. By acting as an acyl group acceptor, carnitine may also play the role of regulating the intracellular CoA:acyl-CoA ratio.

Regulation

CPT1 is inhibited by malonyl-CoA, although the exact mechanism of inhibition remains unknown. The CPT1 skeletal muscle and heart isoform, CPT1B, has been shown to be 30-100-fold more sensitive to malonyl-CoA inhibition than CPT1A. This inhibition is a good target for future attempts to regulate CPT1 for the treatment of metabolic disorders. Acetyl-CoA carboxylase (ACC), the enzyme that catalyzes the formation of malonyl-CoA from acetyl-CoA, is important in the regulation of fatty acid metabolism. Scientists have demonstrated that ACC2 knockout mice have reduced body fat and weight when compared to

wild type The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, "m ...

mice. This is a result of decreased activity of ACC which causes a subsequent decrease in malonyl-CoA concentrations. These decreased malonyl-CoA levels in turn prevent inhibition of CPT1, causing an ultimate increase in fatty acid oxidation. Since heart and skeletal muscle cells have a low capacity for fatty acid synthesis, ACC may act purely as a regulatory enzyme in these cells.

Clinical significance

The "CPT1A" form is associated with

carnitine palmitoyltransferase I deficiency Carnitine palmitoyltransferase I deficiency is a rare metabolic disorder that prevents the body from converting certain fats called long-chain fatty acids(LCFA) into energy, particularly during periods without food. It is caused by a mutation in CP ...

. This rare disorder confers risk for

hepatic encephalopathy Hepatic encephalopathy (HE) is an altered level of consciousness as a result of liver failure. Its onset may be gradual or sudden. Other symptoms may include movement problems, changes in mood, or changes in personality. In the advanced stage ...

, hypoketotic hypoglycemia, seizures, and sudden unexpected death in infancy. CPT1 is associated with

type 2 diabetes Type 2 diabetes, formerly known as adult-onset diabetes, is a form of diabetes mellitus that is characterized by high blood sugar, insulin resistance, and relative lack of insulin. Common symptoms include increased thirst, frequent urinatio ...

and insulin resistance. Such diseases, along with many other health problems, cause free fatty acid (FFA) levels in humans to become elevated, fat to accumulate in skeletal muscle, and decreases the ability of muscles to oxidize fatty acids. CPT1 has been implicated in contributing to these symptoms. The increased levels of malonyl-CoA caused by hyperglycemia and

hyperinsulinemia Hyperinsulinemia is a condition in which there are excess levels of insulin circulating in the blood relative to the level of glucose. While it is often mistaken for diabetes or hyperglycaemia, hyperinsulinemia can result from a variety of metabo ...

inhibit CPT1, which causes a subsequent decrease in the transport of long chain fatty acids into muscle and heart mitochondria, decreasing fatty acid oxidation in such cells. The shunting of LCFAs away from mitochondria leads to the observed increase in FFA levels and the accumulation of fat in skeletal muscle. Its importance in fatty acid metabolism makes CPT1 a potentially useful enzyme to focus on in the development of treatments of many other metabolic disorders as well.

Interactions

CPT1 is known to interact with many proteins, including ones from the NDUF family, PKC1, and ENO1. In HIV, Vpr enhances PPARbeta/delta-induced PDK4, carnitine palmitoyltransferase I (CPT1) mRNA expression in cells. Knockdown of CPT1A by shRNA library screening inhibits HIV-1 replication in cultured Jurkat T-cells.

References

External links

GeneReviews/NCBI/NIH/UW entry on Carnitine Palmitoyltransferase 1A Deficiency
{{Mitochondrial enzymes