Pyruvate dehydrogenase kinase (also pyruvate dehydrogenase complex kinase, PDC kinase, or PDK; ) is a

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

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. A ...

which acts to inactivate the enzyme

pyruvate dehydrogenase Pyruvate dehydrogenase is an enzyme that catalyzes the reaction of pyruvate and a lipoamide to give the acetylated dihydrolipoamide and carbon dioxide. The conversion requires the coenzyme thiamine pyrophosphate. Pyruvate dehydrogenase is us ...

by phosphorylating it using ATP. PDK thus participates in the regulation of the

pyruvate dehydrogenase complex Pyruvate dehydrogenase complex (PDC) is a complex of three enzymes that converts 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 t ...

of which pyruvate dehydrogenase is the first component. Both PDK and the pyruvate dehydrogenase complex are located in 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 ...

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

s. The complex acts to convert

pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic aci ...

(a product of

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH ...

in 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 ...

) to

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 for ...

, which is then oxidized in the mitochondria to produce energy, in the

citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins ...

. By downregulating the activity of this complex, PDK will decrease the oxidation of pyruvate in mitochondria and increase the conversion of pyruvate to lactate in the cytosol. The opposite action of PDK, namely the dephosphorylation and activation of pyruvate dehydrogenase, is catalyzed by a phosphoprotein phosphatase called

pyruvate dehydrogenase phosphatase Pyruvate dehydrogenase phosphatase catalytic subunit 1 (PDPC 1), also known as protein phosphatase 2C, is an enzyme that in humans is encoded by the ''PDP1'' gene. PDPC 1 is an enzyme which serves to reverse the effects of pyruvate dehydrogenase k ...

. (Pyruvate dehydrogenase kinase should not be confused with

Phosphoinositide-dependent kinase-1 In the field of biochemistry, PDPK1 refers to the protein 3-phosphoinositide-dependent protein kinase-1, an enzyme which is encoded by the ''PDPK1'' gene in humans. It is implicated in the development and progression of melanomas. Function P ...

, which is also sometimes known as "PDK1".)

Phosphorylation sites

PDK can phosphorylate a

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...

residue on

at three possible sites. Some evidence has shown that phosphorylation at site 1 will nearly completely deactivate the enzyme while phosphorylation at sites 2 and 3 had only a small contribution to complex inactivation. Therefore, it is phosphorylation at site 1 that is responsible for pyruvate dehydrogenase deactivation.

Isozymes

There are four known

isozymes In biochemistry, isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters (e.g. dif ...

of PDK in humans: *

PDK1 Pyruvate dehydrogenase lipoamide kinase isozyme 1, mitochondrial is an enzyme that in humans is encoded by the ''PDK1'' gene. It codes for an isozyme of pyruvate dehydrogenase kinase (PDK). Pyruvate dehydrogenase (PDH) is a part of a mitochondri ...

PDK2 Pyruvate dehydrogenase kinase isoform 2 (PDK2) also known as pyruvate dehydrogenase lipoamide kinase isozyme 2, mitochondrial is an enzyme that in humans is encoded by the ''PDK2'' gene. PDK2 is an isozyme of pyruvate dehydrogenase kinase. Struct ...

PDK3 Pyruvate dehydrogenase lipoamide kinase isozyme 3, mitochondrial is an enzyme that in humans is encoded by the ''PDK3'' gene. It codes for an isozyme of pyruvate dehydrogenase kinase.The pyruvate dehydrogenase (PDH) complex is a nuclear-encoded m ...

PDK4 Pyruvate dehydrogenase lipoamide kinase isozyme 4, mitochondrial is an enzyme that in humans is encoded by the ''PDK4'' gene. It codes for an isozyme of pyruvate dehydrogenase kinase. This gene is a member of the PDK/BCKDK protein kinase family a ...

The primary sequencing between the four isozymes are conserved with 70% identity. The greatest differences occur near the

N-terminus 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 ami ...

. PDK1 is the largest of the four with 436 residues while PDK2, PDK3 and PDK4 have 407, 406, and 411 residues respectively. The isozymes have different activity and phosphorylation rates at each site. At site 1 in order from fastest to slowest, PDK2 > PDK4 ≈ PDK1 > PDK3. For site 2, PDK3 > PDK4 > PDK2 > PDK1. Only PDK1 can phosphorylate site 3. However, it has been shown that these activities are sensitive to slight changes in pH so the microenvironment of the PDK isozymes may change the reaction rates. Isozyme abundance has also been shown to be tissue specific. PDK1 is ample in

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...

cells. PDK3 is most abundant in

testis A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...

. PDK2 is present in most tissues but low in

spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .

and

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

cells. PDK4 is predominantly found in

skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

and

tissues.

Mechanism

Pyruvate dehydrogenase is deactivated when phosphorylated by PDK. Normally, the active site of pyruvate dehydrogenase is in a stabilized and ordered conformation supported by a network of

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

s. However, phosphorylation by PDK at site 1 causes steric clashes with another nearby serine residue due to both the increased size and negative charges associated with the phosphorylated residue. This disrupts the hydrogen bond network and disorders the conformation of two phosphorylation loops. These loops prevent the reductive

acetylation : In organic chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed ''acetate esters'' or simply '' acetates''. Deacetylation is the oppo ...

step, thus halting overall activity of the enzyme. The conformational changes and mechanism of deactivation for phosphorylation at sites 2 and 3 are not known at this time.

Regulation

Pyruvate dehydrogenase kinase isozyme 4 with bound ADP

Pyruvate dehydrogenase kinase is activated by ATP,

NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...

and

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 for ...

. It is inhibited by ADP, NAD+, CoA-SH and

. Each isozyme responds to each of these factors slightly differently. NADH stimulates PDK1 activity by 20% and PDK2 activity by 30%. NADH with acetyl-CoA increases activity in these enzymes by 200% and 300% respectively. In similar conditions, PDK3 is unresponsive to NADH and inhibited by NADH with acetyl-CoA. PDK4 has a 200% activity increase with NADH, but adding acetyl-CoA does not increase activity further.

Disease relevance

PDK isoforms are elevated in obesity, diabetes, heart failure, and cancer. Some studies have shown that cells that lack

insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the ''INS'' gene. It is considered to be the main anabolic hormone of the body. It regulates the metabolism o ...

(or are insensitive to insulin) overexpress PDK4. As a result, the pyruvate formed from glycolysis cannot be oxidized which leads to

hyperglycaemia Hyperglycemia is a condition in which an excessive amount of glucose circulates in the blood plasma. This is generally a blood sugar level higher than 11.1 mmol/L (200 mg/dL), but symptoms may not start to become noticeable until even ...

due to the fact that glucose in the blood cannot be used efficiently. Therefore, several drugs target PDK4 hoping to treat

type II 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 ...

. PDK1 has shown to have increased activity in hypoxic cancer cells due to the presence of

HIF-1 Hypoxia-inducible factors (HIFs) are transcription factors that respond to decreases in available oxygen in the cellular environment, or hypoxia. They are only present in parahoxozoan animals. Discovery The HIF transcriptional complex w ...

. PDK1 shunts pyruvate away from the citric acid cycle and keeps the hypoxic cell alive. Therefore, PDK1 inhibition has been suggested as an antitumor therapy since PDK1 prevents

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...

in these cancerous cells. Similarly, PDK3 has been shown to be overexpressed in colon cancer cell lines. Three proposed inhibitors are AZD7545 and

dichloroacetate Dichloroacetic acid (DCA), sometimes called bichloroacetic acid (BCA), is the chemical compound with formula . It is an acid, an analogue of acetic acid, in which 2 of the 3 hydrogen atoms of the methyl group have been replaced by chlorine at ...

which both bind to PDK1, and

Radicicol Radicicol, also known as monorden, is a natural product that binds to Hsp90 (Heat Shock Protein 90) and alters its function. HSP90 client proteins play important roles in the regulation of the cell cycle, cell growth, cell survival, apoptosis, ang ...

which binds to PDK3. Mutations in the PDK3 gene are a rare cause of X-linked Charcot-Marie-Tooth disease (CMTX6). In dogs, specifically

Doberman Pinschers The Dobermann (; ), or Doberman Pinscher in the United States and Canada, is a medium-large dog breed, breed of domestic dog that was originally developed around 1890 by Karl Friedrich Louis Dobermann, Louis Dobermann, a tax collector fro ...

, a mutation in the PDK4 gene is associated with

dilated cardiomyopathy Dilated cardiomyopathy (DCM) is a condition in which the heart becomes enlarged and cannot pump blood effectively. Symptoms vary from none to feeling tired, leg swelling, and shortness of breath. It may also result in chest pain or fainting. Co ...

(DCM).

References

External links

* * {{Portal bar, Biology, border=no EC 2.7.11 Citric acid cycle Glycolysis