Carnitine O-acetyltransferase
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Carnitine O-acetyltransferase also called carnitine acetyltransferase (CRAT, or CAT) () is an
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
that encoded by the CRAT gene that
catalyzes 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 recycl ...
the
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...
:acetyl-CoA + carnitine \rightleftharpoons CoA +
acetylcarnitine Acetyl-L-carnitine, ALCAR or ALC, is an acetylated form of L-carnitine. It is naturally produced by the human body, and it is available as a dietary supplement. Acetylcarnitine is broken down in the blood by Blood plasma, plasma esterases to carn ...
where the acetyl group displaces the hydrogen atom in the central hydroxyl group of carnitine. Thus, the two substrates of this enzyme are
acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized fo ...
and
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, ...
, whereas its two
products Product may refer to: Business * Product (business), an item that serves as a solution to a specific consumer problem. * Product (project management), a deliverable or set of deliverables that contribute to a business solution Mathematics * Produ ...
are
CoA Coa may refer to: Places * Coa, County Fermanagh, a rural community in County Fermanagh, Northern Ireland * Côa River, a tributary of the Douro, Portugal ** Battle of Coa, part of the Peninsular War period of the Napoleonic Wars ** Côa Vall ...
and O-
acetylcarnitine Acetyl-L-carnitine, ALCAR or ALC, is an acetylated form of L-carnitine. It is naturally produced by the human body, and it is available as a dietary supplement. Acetylcarnitine is broken down in the blood by Blood plasma, plasma esterases to carn ...
. The reaction is highly reversible and does not depend on the order in which substrates bind. Different subcellular localizations of the CRAT mRNAs are thought to result from alternative splicing of the CRAT gene suggested by the divergent sequences in the 5' region of peroxisomal and mitochondrial CRAT cDNAs and the location of an
intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of the cistron .e., gene ...
where the sequences diverge. The alternatively splicing of this gene results in three distinct isoforms, one of which contains an
N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...
mitochondrial transit peptide, and has been shown to be located in mitochondria.


Nomenclature

This enzyme belongs to the family of
transferase A transferase is any one of a class of enzymes that catalyse the transfer of specific functional groups (e.g. a methyl or glycosyl group) from one molecule (called the donor) to another (called the acceptor). They are involved in hundreds of ...
s, to be specific those
acyltransferase Acyltransferase is a type of transferase enzyme that acts upon acyl groups. Examples include: * Glyceronephosphate O-acyltransferase * Lecithin-cholesterol acyltransferase *Long-chain-alcohol O-fatty-acyltransferase In enzymology, a long-chain ...
s transferring groups other than aminoacyl groups. The
systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...
of this enzyme class is acetyl-CoA:carnitine O-acetyltransferase. Other names in common use include acetyl-CoA-carnitine O-acetyltransferase, acetylcarnitine transferase, carnitine acetyl coenzyme A transferase, carnitine acetylase, carnitine acetyltransferase, carnitine-acetyl-CoA transferase, and CATC. This enzyme participates in
alanine and aspartate metabolism Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group side ...
.


Structure

In general, carnitine acetyltransferases have molecular weights of about 70 kDa, and contain approximately 600 residues1. CRAT contains two domains, an N domain and a C domain, and is composed of 20 α helices and 16 β strands. The N domain consists of an eight-stranded β sheet flanked on both sides by eight α helices. A six-stranded mixed β sheet and eleven α helices comprise the enzyme’s C domain. When compared, the cores of the two domains reflect significantly similar peptide backbone folding. This occurs despite the fact that only 4% of the amino acids that comprise those peptide backbones corresponds to one another.


Active site

His343 is the catalytic residue in CRAT. It is located at the interface between the enzyme’s C and N domains towards the heart of CRAT. His343 is accessible via two 15-18 Å channels that approach the residue from opposite ends of the CRAT enzyme. These channels are utilized by the substrates of CRAT, one channel for carnitine, and one for CoA. The side chain of His343 is positioned irregularly, with the δ1 ring nitrogen hydrogen bonded to the carbonyl oxygen on the amino acid backbone.


CoA binding site

Due to the fact that CRAT binds CoA, rather than acetyl-CoA, it appears that CRAT possesses the ability to hydrolyze acetyl-CoA, before interacting with the lone CoA fragment at the binding site. CoA is bound in a linear conformation with its pantothenic arm binding at the active site. Here, the pantothenic arm’s terminal thiol group and the ε2 nitrogen on the catalytic His343 side chain form a hydrogen bond. The 3’-phosphate on CoA forms interactions with residues Lys419 and Lys423. Also at the binding site, the residues Asp430 and Glu453 form a direct hydrogen bond to one another. If either residue exhibits a mutation, can result in a decrease in CRAT activity.


Carnitine binding site

Carnitine binds to CRAT in a partially folded state, with its hydroxyl group and carboxyl group facing opposite directions. The site itself is composed of the C domain β sheet and particular residues from the N domain. Upon binding, a face of carnitine is left exposed to the space outside the enzyme. Like CoA, carnitine forms a hydrogen bond with the ε2 nitrogen on His343. In the case of carnitine, the bond is formed with its 3-hydroxyl group. This CRAT catalysis is stereospecific for carnitine, as the stereoisomer of the 3-hydroxyl group cannot sufficiently interact with the CRAT carnitine binding site. CRAT undergoes minor conformational changes upon binding with carnitine.


Function


Enzyme mechanism

The His343 residue at the active site of CRAT acts as a base that is able to deprotonate the CoA thiol group or the Carnitine 3’-hydroxyl group depending on the direction of the reaction. The structure of CRAT optimizes this reaction by causing direct hydrogen bonding between the His343 and both substrates. The deprotonated group is now free to attack the acetyl group of acetyl-CoA or acetylcarnitine at its carbonyl site. The reaction proceeds directly, without the formation of a His343-acetyl intermediate.


Hydrolysis

It is possible for catalysis to occur with only one of the two substrates. If either acetyl-CoA or acetylcarnitine binds to CRAT, a water molecule may fill the other binding site and act as an acetyl group acceptor.


Substrate-assisted catalysis

The literature suggests that the trimethylammonium group on carnitine may be a crucial factor in CRAT catalysis. This group exhibits a positive charge that stabilizes the oxyanion in the reaction’s intermediate. This idea is supported by the fact the positive charge of carnitine is unnecessary for active site binding, but vital for the catalysis to proceed. This has been proven to be the case through the synthesis of a carnitine analog lacking its trimethylammonium group. This compound was able to compete with carnitine in binding to CRAT, but was unable to induce a reaction. The emergence of subtrate-assisted catalysis has opened up new strategies for increasing synthetic substrate specificity.


Biological function

There is evidence that suggests that CRAT activity is necessary for the cell cycle to proceed from the G1 phase to the S phase.


Clinical significance

Those with an inherited deficiency in CRAT activity are at risk for developing severe heart and neurological problems. Reduced CRAT activity can be found in individuals suffering from Alzheimer’s disease. CRAT and its family of enzymes have great potential as targets for developing therapeutic treatments for Type 2 diabetes and other diseases.


Interactions

CRAT is known to interact with
NEDD8 NEDD8 is a protein that in humans is encoded by the ''NEDD8'' gene. (in ''saccharomyces cerevisiae'' this protein is known as Rub1) This ubiquitin-like (UBL) protein becomes covalently conjugated to a limited number of cellular proteins, in a proc ...
,
PEX5 Peroxisomal targeting signal 1 receptor (PTS1R) is a protein that in humans is encoded by the ''PEX5'' gene. PTS1R is a peroxisomal targeting sequence involved in the specific transport of molecules for oxidation inside the peroxisome. SKL bind ...
,
SUMO1 Small ubiquitin-related modifier 1 is a protein that in humans is encoded by the ''SUMO1'' gene. Function This gene encodes a protein that is a member of the SUMO (small ubiquitin-like modifier) protein family. It is a ubiquitin-like protein a ...
.


References


Further reading

* * *
AgRP Neurons Require Carnitine Acetyltransferase to Regulate Metabolic Flexibility and Peripheral Nutrient Partitioning
] {{Portal bar, Biology, border=no EC 2.3.1 Enzymes of known structure