Dihydrolipoyl transacetylase
Encyclopedia
Dihydrolipoyl transacetylase (or dihydrolipoamide acetyltransferase) is an enzyme
component of the multienzyme pyruvate dehydrogenase complex. The pyruvate dehydrogenase complex is responsible for the pyruvate decarboxylation step that links glycolysis
to the citric acid cycle
. This involves the transformation of pyruvate from glycolysis into acetyl-CoA
which is then used in the citric acid cycle to carry out cellular respiration
.
There are three different enzyme components in the pyruvate dehydrogenase complex. Pyruvate dehydrogenase
(EC 1.2.4.1) is responsible for the oxidation of pyruvate, dihydrolipoyl transacetylase (this enzyme; EC 2.3.1.12) transfers the acetyl group to coenzyme A
(CoA), and dihydrolipoyl dehydrogenase (EC 1.8.1.4) regenerates the lipoamide. Because dihydrolipoyl transacetylase is the second of the three enzyme components participating in the reaction mechanism for conversion of pyruvate into acetyl CoA, it is sometimes referred to as E2.
In humans, dihydrolipoyl transacetylase enzymatic activity resides in the pyruvate dehydrogenase complex component E2 (PDCE2) that is encoded by the DLAT (dihydrolipoamide S-acetyltransferase) gene
.
Other names in common use include:
Depending on the species, multiple subunits of dihydrolipoyl transacetylase enzymes can arrange together into either a cubic or dodecahedral shape. These structure then form the catalytic core of the pyruvate dehydrogenase complex which not only catalyzes the reaction that transfers an acetyl group to CoA, but also performs a crucial structural role in creating the architecture of the overall complex.
, is made up of 24 subunits total. The catalytic domains are assembled into trimers with the active site located at the subunit interface. The topology of this trimer active site is identical to that of chloramphenicol acetyltransferase
. Eight of these trimers are then arranged into a hollow truncated cube. The two main substrates, CoA and the lipoamide (Lip(SH)2), are found at two opposite entrances of a 30 Å long channel which runs between the subunits and forms the catalytic center. CoA enters from the inside of the cube, and the lipoamide enters from the outside.
and Enterococcus faecalis
as well as mammals such as humans and cows, the dodecahedral core structure is made up of 60 subunits total. The subunits are arranged in sets of three, similar to the trimers in the cubic core shape, with each set making up one of the 20 dodecahedral vertices.
(TPP), which is used by pyruvate dehydrogenase to oxidize pyruvate and to form a hydroxyethyl-TPP intermediate. This intermediate is taken up by dihydrolipoyl transacetylase and reacted with a second lipoamide cofactor to generate an acetyl-dihydrolipoyl intermediate, releasing TPP in the process. This second intermediate can then be attacked by the nucleophilic sulfur attached to Coenzyme A, and the dihydrolipoamide is released. This results in the production of acetyl CoA, which is the end goal of pyruvate decarboxylation. The dihydrolipoamide is taken up by dihydrolipoyl dehydrogenase, and with the additional cofactors FAD and NAD+, regenerates the original lipoamide (with NADH as a useful side product).
(ATP) which can then be used to carry out numerous biological reactions within a cell. The various parts of cellular respiration take place in different parts of the cell. In eukaryotes, glycolysis occurs in the cytoplasm, pyruvate decarboxylation in the mitochondria, the citric acid cycle within the mitochondrial matrix, and oxidative phosphorylation
via the electron transport chain
on the mitochondrial cristae. Thus pyruvate dehydrogenase complexes (containing the dihydrolipoyl transacetylase enzymes) are found in the mitochondria of eukaryotes (and simply in the cytosol of prokaryotes).
(PBC) is an autoimmune disease
characterized by autoantibodies against mitochondrial and nuclear antigens. These are called anti-mitochondrial antibodies
(AMA) and anti-nuclear antibodies (ANA), respectively. These antibodies are detectable in the sera of PBC patients and vary greatly with regards to epitope
specificity from patient to patient. Of the mitochondrial antigens that can generate autoantibodiy reactivity in PBC patients, the E2 subunit of the pyruvate dehydrogenase complex, dihydrolipoyl transacetylase, is the most common epitope (other antigens include enzymes of the 2-oxoacid dehydrogenase complexes as well as the other enzymes of the pyruvate dehydrogenase complexes). Recent evidence has suggested that peptides within the catalytic site may present the immunodominant epitopes recognized by the anti-PDC-E2 antibodies in PBC patients. There is also evidence of anti-PDC-E2 antibodies in autoimmune hepatitis
(AIH) patients.
(PDH) is a genetic disease resulting in lactic acidosis
as well as neurological dysfunction in infancy and early childhood. Typically PDH is the result of a mutation in the X-linked gene for the E1 subunit of the pyruvate dehydrogenase complex. However, there have been a few rare cases in which a patient with PDH actually has a mutation in the autosomal gene for the E2 subunit instead. These patients have been reported to have much less severe symptoms, with the most prominent disease manifestation being episodic dystonia, though both hypotonia
and ataxia
were also present.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
component of the multienzyme pyruvate dehydrogenase complex. The pyruvate dehydrogenase complex is responsible for the pyruvate decarboxylation step that links glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
to the citric acid cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
. This involves the transformation of pyruvate from glycolysis into acetyl-CoA
Acetyl-CoA
Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
which is then used in the citric acid cycle to carry out cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
.
There are three different enzyme components in the pyruvate dehydrogenase complex. Pyruvate dehydrogenase
Pyruvate dehydrogenase
Pyruvate dehydrogenase complex is a complex of three enzymes that transform pyruvate into acetyl-CoA by a process called pyruvate decarboxylation. Acetyl-CoA may then be used in the citric acid cycle to carry out cellular respiration, and this complex links the glycolysis metabolic pathway to the...
(EC 1.2.4.1) is responsible for the oxidation of pyruvate, dihydrolipoyl transacetylase (this enzyme; EC 2.3.1.12) transfers the acetyl group to coenzyme A
Coenzyme A
Coenzyme A is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All sequenced genomes encode enzymes that use coenzyme A as a substrate, and around 4% of cellular enzymes use it as a substrate...
(CoA), and dihydrolipoyl dehydrogenase (EC 1.8.1.4) regenerates the lipoamide. Because dihydrolipoyl transacetylase is the second of the three enzyme components participating in the reaction mechanism for conversion of pyruvate into acetyl CoA, it is sometimes referred to as E2.
In humans, dihydrolipoyl transacetylase enzymatic activity resides in the pyruvate dehydrogenase complex component E2 (PDCE2) that is encoded by the DLAT (dihydrolipoamide S-acetyltransferase) gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
.
Nomenclature
The systematic name of this enzyme class is acetyl-CoA:enzyme N6-(dihydrolipoyl)lysine S-acetyltransferase.Other names in common use include:
- acetyl-CoA:dihydrolipoamide S-acetyltransferase,
- acetyl-CoA:enzyme 6-N-(dihydrolipoyl)lysine S-acetyltransferase.
- dihydrolipoamide S-acetyltransferase,
- dihydrolipoate acetyltransferase,
- dihydrolipoic transacetylase,
- dihydrolipoyl acetyltransferase,
- enzyme-dihydrolipoyllysine:acetyl-CoA S-acetyltransferase,
- lipoate acetyltransferase,
- lipoate transacetylase,
- lipoic acetyltransferase,
- lipoic acid acetyltransferase,
- lipoic transacetylase,
- lipoylacetyltransferase,
- thioltransacetylase A, and
- transacetylase X.
Structure
All dihydrolipoyl transacetylases have a unique multidomain structure consisting of (from N to C): 3 lipoyl domains, an interaction domain, and the catalytic domain (see the domain architecture at Pfam). Interestingly all the domains are connected by disordered, low complexity linker regions.Depending on the species, multiple subunits of dihydrolipoyl transacetylase enzymes can arrange together into either a cubic or dodecahedral shape. These structure then form the catalytic core of the pyruvate dehydrogenase complex which not only catalyzes the reaction that transfers an acetyl group to CoA, but also performs a crucial structural role in creating the architecture of the overall complex.
Cube
The cubic core structure, found in species such as Azotobacter vinelandiiAzotobacter vinelandii
Azotobacter vinelandii is diazotroph that can fix nitrogen while grown aerobically. It is a genetically tractable system that is used to study nitrogen fixation...
, is made up of 24 subunits total. The catalytic domains are assembled into trimers with the active site located at the subunit interface. The topology of this trimer active site is identical to that of chloramphenicol acetyltransferase
Chloramphenicol acetyltransferase
Chloramphenicol acetyltransferase is a bacterial enzyme that detoxifies the antibiotic chloramphenicol and is responsible for chloramphenicol resistance in bacteria. This enzyme covalently attaches an acetyl group from acetyl-CoA to chloramphenicol, which prevents chloramphenicol from binding to...
. Eight of these trimers are then arranged into a hollow truncated cube. The two main substrates, CoA and the lipoamide (Lip(SH)2), are found at two opposite entrances of a 30 Å long channel which runs between the subunits and forms the catalytic center. CoA enters from the inside of the cube, and the lipoamide enters from the outside.
Dodecahedron
In many species, including bacteria such as Geobacillus stearothermophilusBacillus stearothermophilus
Bacillus stearothermophilus is a rod-shaped, Gram-positive bacterium and a member of the division Firmicutes. The bacteria is a thermophile and is widely distributed in soil, hot springs, ocean sediment, and is a cause of spoilage in food products. It will grow within a temperature range of 30-75...
and Enterococcus faecalis
Enterococcus faecalis
Enterococcus faecalis – formerly classified as part of the Group D Streptococcus system – is a Gram-positive, commensal bacterium inhabiting the gastrointestinal tracts of humans and other mammals. It is among the main constituents of some probiotic food supplements. Like other species in the genus...
as well as mammals such as humans and cows, the dodecahedral core structure is made up of 60 subunits total. The subunits are arranged in sets of three, similar to the trimers in the cubic core shape, with each set making up one of the 20 dodecahedral vertices.
Mechanism
Pyruvate decarboxylation requires a few cofactors in addition to the enzymes that make up the complex. The first is thiamine pyrophosphateThiamine pyrophosphate
Thiamine pyrophosphate , or thiamine diphosphate , is a thiamine derivative which is produced by the enzyme thiamine pyrophosphatase. Thiamine pyrophosphate is a coenzyme that is present in all living systems, in which it catalyzes several biochemical reactions...
(TPP), which is used by pyruvate dehydrogenase to oxidize pyruvate and to form a hydroxyethyl-TPP intermediate. This intermediate is taken up by dihydrolipoyl transacetylase and reacted with a second lipoamide cofactor to generate an acetyl-dihydrolipoyl intermediate, releasing TPP in the process. This second intermediate can then be attacked by the nucleophilic sulfur attached to Coenzyme A, and the dihydrolipoamide is released. This results in the production of acetyl CoA, which is the end goal of pyruvate decarboxylation. The dihydrolipoamide is taken up by dihydrolipoyl dehydrogenase, and with the additional cofactors FAD and NAD+, regenerates the original lipoamide (with NADH as a useful side product).
Biological function and location
As mentioned above, dihydrolipoyl transacetylase participates in the pyruvate decarboxylation reaction that links glycolysis to the citric acid cycle. These metabolic processes are important for cellular respiration—the conversion of biochemical energy from nutrients into adenosine triphosphateAdenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
(ATP) which can then be used to carry out numerous biological reactions within a cell. The various parts of cellular respiration take place in different parts of the cell. In eukaryotes, glycolysis occurs in the cytoplasm, pyruvate decarboxylation in the mitochondria, the citric acid cycle within the mitochondrial matrix, and oxidative phosphorylation
Oxidative phosphorylation
Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...
via the electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
on the mitochondrial cristae. Thus pyruvate dehydrogenase complexes (containing the dihydrolipoyl transacetylase enzymes) are found in the mitochondria of eukaryotes (and simply in the cytosol of prokaryotes).
Primary biliary cirrhosis
Primary biliary cirrhosisPrimary biliary cirrhosis
Primary biliary cirrhosis, often abbreviated PBC, is an autoimmune disease of the liver marked by the slow progressive destruction of the small bile ducts within the liver. When these ducts are damaged, bile builds up in the liver and over time damages the tissue. This can lead to scarring,...
(PBC) is an autoimmune disease
Autoimmune disease
Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body actually attacks its own cells. The immune system mistakes some part of the body as a pathogen and attacks it. This may be restricted to...
characterized by autoantibodies against mitochondrial and nuclear antigens. These are called anti-mitochondrial antibodies
Anti-mitochondrial antibodies
Anti-mitochondrial antibodies are autoantibodies, consisting of immunoglobulins formed against mitochondria, primarily mitochondria in cells of the liver...
(AMA) and anti-nuclear antibodies (ANA), respectively. These antibodies are detectable in the sera of PBC patients and vary greatly with regards to epitope
Epitope
An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that recognizes the epitope is called a paratope...
specificity from patient to patient. Of the mitochondrial antigens that can generate autoantibodiy reactivity in PBC patients, the E2 subunit of the pyruvate dehydrogenase complex, dihydrolipoyl transacetylase, is the most common epitope (other antigens include enzymes of the 2-oxoacid dehydrogenase complexes as well as the other enzymes of the pyruvate dehydrogenase complexes). Recent evidence has suggested that peptides within the catalytic site may present the immunodominant epitopes recognized by the anti-PDC-E2 antibodies in PBC patients. There is also evidence of anti-PDC-E2 antibodies in autoimmune hepatitis
Autoimmune hepatitis
Autoimmune Hepatitis is a disease of the liver that occurs when the body's immune system attacks cells of the liver. Anomalous presentation of human leukocyte antigen class II on the surface of hepatocytes, possibly due to genetic predisposition or acute liver infection, causes a cell-mediated...
(AIH) patients.
Pyruvate dehydrogenase deficiency
Pyruvate dehydrogenase deficiencyPyruvate dehydrogenase deficiency
Pyruvate Dehydrogenase Deficiency is a genetic disease that involves human metabolism.-Inheritance and pathophysiology:It most commonly follows an X-linked inheritance pattern, but is approximately equally prevalent in both males and females. This is because heterozygote females commonly manifest...
(PDH) is a genetic disease resulting in lactic acidosis
Lactic acidosis
Lactic acidosis is a physiological condition characterized by low pH in body tissues and blood accompanied by the buildup of lactate especially D-lactate, and is considered a distinct form of metabolic acidosis. The condition typically occurs when cells receive too little oxygen , for example...
as well as neurological dysfunction in infancy and early childhood. Typically PDH is the result of a mutation in the X-linked gene for the E1 subunit of the pyruvate dehydrogenase complex. However, there have been a few rare cases in which a patient with PDH actually has a mutation in the autosomal gene for the E2 subunit instead. These patients have been reported to have much less severe symptoms, with the most prominent disease manifestation being episodic dystonia, though both hypotonia
Hypotonia
Hypotonia is a state of low muscle tone , often involving reduced muscle strength. Hypotonia is not a specific medical disorder, but a potential manifestation of many different diseases and disorders that affect motor nerve control by the brain or muscle strength...
and ataxia
Ataxia
Ataxia is a neurological sign and symptom that consists of gross lack of coordination of muscle movements. Ataxia is a non-specific clinical manifestation implying dysfunction of the parts of the nervous system that coordinate movement, such as the cerebellum...
were also present.