Carnitine-palmitoyltransferase I

CPT1 refers to integral membrane proteins that are bound to the outer mitochondrial membrane through transmembrane regions in the peptide chain. Both terminal N and C domains are located on the cytoplasmic side of the membrane ^[4] .

All three isoforms of the enzyme are found in mammalian tissues. The hepatic isoform (CPT1A or CPTI-L) is localized in the mitochondria of all body cells, with the exception of skeletal muscle cells and brown adipose tissue cells ^{[5] [6]} . Muscle isoform (CPT1B or CPTI-M) is a highly expressive protein that forms in the myocardium of the heart, in skeletal muscle cells (myocytes) and brown adipose tissue cells ^{[4] [5] [6]} . The third isoform - cerebral (CPT1C), was isolated in 2002 and is located mainly in the brain and testes . Little is known about this form ^{[7] [8]} .

The exact structure of all CPT1 isoforms has not yet been determined, although in silico models have been created on the basis of closely related CPT1 enzymes — acyl carnitine transferases , such as carnitine acetyltransferase (CRAT) ^[9] .

An important structural difference between CPT1 and CPT2, CRAT and carnitine octanoyltransferase (COT) is that CPT1 contains an additional N-terminal domain of approximately 160 amino acids . It was found that this additional N-terminal domain is important for the molecule of the key enzyme inhibitor malonyl-CoA ^[10] .

Two different binding sites existing in CPT1A and CPT1B have been proposed. Section A or the CoA region seems to bind both malonyl-CoA and palmitoyl-CoA, as well as other molecules containing coenzyme A. It is believed that the enzyme binds these molecules through interaction with a coenzyme fragment. It has been suggested that malonyl-CoA may behave as a competitive inhibitor of CPT1A in this region. In the second, site O , it is believed that malonyl-CoA binds much more tightly than in site A. In contrast to site A , site O binds to malonyl-CoA via the dicarbonyl group of malonate ^[11] . The binding of malonyl-CoA to the enzyme by sites A and O inhibits the action of CPT1A by eliminating the binding of carnitine to the enzyme.

Due to the fact that there is no data on the crystal structure of the enzyme, the exact mechanism of CPT1 catalysis is still unknown. A couple of different possible mechanisms of CPT1 have been postulated, both of which include a histidine residue - His473 , which serves as a key catalytic center. One such mechanism is based on the carnitine acetyltransferase model shown below, in which the His473 residue deprotonizes carnitine, while the adjacent serine residue stabilizes the tetrahedral oxyanion intermediate .

Another mechanism is based on the assumption that there is a so-called catalytic triad consisting of amino acid residues Cys-305 , His-473 , and Asp-454 , which carry out catalytic acyl transfer ^[12] . This catalytic mechanism involves the formation of a covalent thioacyl enzyme intermediate with Cys-305.

Biological Functions

The carnitine-palmitoyltransferase system is an important step in the beta oxidation of long chain fatty acids. Long chain fatty acids, such as palmitoyl-CoA, unlike short- and medium-chain fatty acids, cannot freely diffuse through the inner mitochondrial membrane (it is impermeable); for this transition, there is a carnitine shuttle transporting them to the matrix ^[13] .

Carnitine-palmitoyltransferase I is the first component of the system and limits the rate of the chemical reaction of the carnitine transport system , catalyzing the transfer of the acyl group from coenzyme A to carnitine with the formation of palmitoylcarnitine. Using translocase ( carnitine-acylcarnitine translocase , CACT), palmitoylcarnitine is transferred via facilitated diffusion ( antiport ) through the inner mitochondrial membrane to the matrix.

Acting as an acceptor of the acyl group, carnitine may also play a role in the regulation of the intracellular pool of coenzyme A: the ratio of acyl-CoA / CoA ^[14] .

Regulation

CPT1 is inhibited by malonyl-CoA, although the exact mechanism of inhibition remains unknown. The CPT1 isoform of skeletal muscle and myocardium (CPT1B) has been shown to be 30-100 times more sensitive to malonyl-CoA inhibition than the similar CPT1A isoform. This inhibition is a good target for future attempts to regulate CPT1 in the treatment of metabolic disorders ^[15] .

Acetyl CoA carboxylase (ACC), an enzyme that catalyzes the formation of malonyl CoA from acetyl CoA, plays an important role in the regulation of fatty acid metabolism. Scientists have demonstrated that ACC2 knockout mice lead to reduced fat and weight compared to wild-type mice. This is the result of decreased ACC activity, which cause a subsequent decrease in malonyl-CoA concentrations. Such reduced levels of malonyl CoA, in turn, prevent inhibition of CPT1, resulting in a marginal increase in fatty acid oxidation ^[16] . Since the heart and skeletal muscle cells have a low ability to synthesize fatty acids, ACC can act exclusively as a regulatory enzyme in these cells.

In humans, only a deficiency of “CPT1A” (carnitine-palmitoyltransferase I deficiency) has been described ^[17] . This is a rare disease characterized by high risks of developing hypoketonymic hypoglycemia, seizures, and hepatic encephalopathy; in newborns, sudden infant death ^[18] .

CPT1 is also associated with type 2 diabetes mellitus and insulin resistance .

New research presented at the 2015 congress of the European Committee for the Treatment and Research of Multiple Sclerosis (ECTRIMS) suggests that CPT1 may also play a role in the pathology of multiple sclerosis (MS) ^[19] . Researchers have found that mutations in the CPT1 gene appear to protect against multiple sclerosis in certain populations. This hypothesis is that an increase in lipid metabolism can lead to nimble cell demyelination (destruction of myelin ), thereby allowing an autoimmune attack on nerve cells. Ph.D. John Dirk Niland and colleagues at Aalborg University in Denmark provided data showing that in animal models with MS, CPT1 is blocked by a molecule called etomoxir, most likely to alleviate the symptoms of this disease ^[20] . Researchers are currently seeking support for a clinical trial in sick people.

It is known that CPT1 interacts with many proteins, including those from the NDUF , PKC1, and ENO1 families ^[21] .

Knocked out CPT1A using shRNA library screening inhibits HIV-1 replication in Jurkat T cell culture ^[22] .

• Beta oxidation
• Carnitine acetyltransferase
• Carnitine shuttle