Glutamate dehydrogenase
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Glutamate dehydrogenase (GLDH, GDH) is an
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 ...
observed in both prokaryotes and eukaryotic
mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
. The aforementioned reaction also yields ammonia, which in eukaryotes is canonically processed as a substrate in the
urea cycle The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea (NH2)2CO from ammonia (NH3). Animals that use this cycle, mainly amphibians and mammals, are called ureotelic. The urea cycle converts highl ...
. Typically, the α-ketoglutarate to glutamate reaction does not occur in mammals, as glutamate dehydrogenase equilibrium favours the production of ammonia and α-ketoglutarate. Glutamate dehydrogenase also has a very low affinity for ammonia (high
Michaelis constant Michaelis or Michelis is a surname. Notable people and characters with the surname include: * Adolf Michaelis, German classical scholar * Anthony R. Michaelis, German science writer * Edward Michelis, German theologian * Georg Michaelis, German p ...
K_m of about 1 mM), and therefore toxic levels of ammonia would have to be present in the body for the reverse reaction to proceed (that is, α-ketoglutarate and ammonia to glutamate and NAD(P)+). However, in brain, the NAD+/NADH ratio in brain mitochondria encourages oxidative deamination (i.e. glutamate to α-ketoglutarate and ammonia). In bacteria, the ammonia is assimilated to amino acids via glutamate and aminotransferases. In plants, the enzyme can work in either direction depending on environment and stress. Transgenic plants expressing microbial GLDHs are improved in tolerance to herbicide, water deficit, and pathogen infections. They are more nutritionally valuable. The enzyme represents a key link between
catabolic Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipids, ...
and
anabolic Anabolism () is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy, known also as an endergonic process. Anabolism is the building-up aspect of metabolism, whereas catabolism is the breaking ...
pathways, and is, therefore, ubiquitous in eukaryotes. In humans the relevant genes are called
GLUD1 GLUD1 (glutamate dehydrogenase 1) is a mitochondrial matrix enzyme, one of the family of glutamate dehydrogenases that are ubiquitous in life, with a key role in nitrogen and glutamate (Glu) metabolism and energy homeostasis. This dehydrogenase is ...
(glutamate dehydrogenase 1) and GLUD2 (glutamate dehydrogenase 2), and there are also at least 8 GLDH
pseudogene Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Most arise as superfluous copies of functional genes, either directly by DNA duplication or indirectly by Reverse transcriptase, reverse transcription of an mRNA trans ...
s in the
human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the n ...
as well, probably reflecting microbial influences on eukaryote evolution. Image:Glutaminsäure - Glutamic acid.svg,
Glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...
Image:Alpha-ketoglutaric acid.png , α-Ketoglutarate


Clinical application

GLDH can be measured in a
medical laboratory A medical laboratory or clinical laboratory is a laboratory where tests are conducted out on clinical specimens to obtain information about the health of a patient to aid in diagnosis, treatment, and prevention of disease. Clinical Medical labor ...
to evaluate the liver function. Elevated
blood serum Serum () is the fluid and solute component of blood which does not play a role in clotting. It may be defined as blood plasma without the clotting factors, or as blood with all cells and clotting factors removed. Serum includes all proteins not u ...
GLDH levels indicate liver damage and GLDH plays an important role in the differential diagnosis of liver disease, especially in combination with aminotransferases. GLDH is localised in
mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
, therefore practically none is liberated in generalised inflammatory diseases of the liver such as viral hepatitides. Liver diseases in which necrosis of hepatocytes is the predominant event, such as toxic liver damage or hypoxic liver disease, are characterised by high serum GLDH levels. GLDH is important for distinguishing between acute viral hepatitis and acute toxic liver necrosis or acute hypoxic liver disease, particularly in the case of liver damage with very high aminotransferases. In
clinical trials Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, dietar ...
, GLDH can serve as a measurement for the safety of a drug.
Enzyme immunoassay An enzyme immunoassay is any of several immunoassay methods that use an enzyme bound to an antigen or antibody. These may include: * Enzyme-linked immunosorbent assay (ELISA) * Enzyme multiplied immunoassay technique (EMIT) * Fluorescent enzyme ...
(EIA) for glutamate dehydrogenase (GDH) can be used as screening tool for patients with ''
Clostridioides difficile ''Clostridioides difficile'' ( syn. ''Clostridium difficile'') is a bacterium that is well known for causing serious diarrheal infections, and may also cause colon cancer. Also known as ''C. difficile'', or ''C. diff'' (), is Gram-positive spe ...
'' infection. The enzyme is expressed constitutively by most strains of C.diff, and can thus be easily detected in stool. Diagnosis is generally confirmed with a follow up EIA for C. Diff toxins A and B.


Cofactors

NAD+ (or
NADP Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NAD ...
+) is a cofactor for the glutamate dehydrogenase reaction, producing α-ketoglutarate and
ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...
as a byproduct. Based on which cofactor is used, glutamate dehydrogenase enzymes are divided into the following three classes: * EC 1.4.1.2: L-glutamate + H2O + NAD+ \rightleftharpoons 2-oxoglutarate + NH3 + NADH + H+ * EC 1.4.1.3: L-glutamate + H2O + NAD(P)+ \rightleftharpoons 2-oxoglutarate + NH3 + NAD(P)H + H+ * EC 1.4.1.4: L-glutamate + H2O + NADP+ \rightleftharpoons 2-oxoglutarate + NH3 + NADPH + H+


Role in flow of nitrogen

Ammonia incorporation in animals and microbes occurs through the actions of glutamate dehydrogenase and
glutamine synthetase Glutamine synthetase (GS) () is an enzyme that plays an essential role in the metabolism of nitrogen by catalyzing the condensation of glutamate and ammonia to form glutamine: Glutamate + Adenosine triphosphate, ATP + NH3 → Glutamine + Ad ...
. Glutamate plays the central role in
mammalian Mammals () are a group of vertebrate animals constituting the class (biology), class Mammalia (), characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk for feeding (nursing) their young, a ...
and microbe nitrogen flow, serving as both a nitrogen donor and a nitrogen acceptor.


Regulation of glutamate dehydrogenase

In humans, the activity of glutamate dehydrogenase is controlled through ADP-ribosylation, a covalent modification carried out by the gene
sirt4 Sirtuins are a family of signaling proteins involved in metabolic regulation. They are ancient in animal evolution and appear to possess a highly conserved structure throughout all kingdoms of life. Chemically, sirtuins are a class of proteins t ...
. This regulation is relaxed in response to
caloric restriction Calorie restriction (caloric restriction or energy restriction) is a dietary regimen that reduces intake of energy from caloric foods & beverages without incurring malnutrition. "Reduce" can be defined relative to the subject's previous intake be ...
and low
blood glucose Glycaemia, also known as blood sugar level, blood sugar concentration, or blood glucose level is the measure of glucose concentrated in the blood of humans or other animals. Approximately 4 grams of glucose, a simple sugar, is present in the blo ...
. Under these circumstances, glutamate dehydrogenase activity is raised in order to increase the amount of α-ketoglutarate produced, which can be used to provide energy by being used 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 ...
to ultimately produce ATP. In microbes, the activity is controlled by the concentration of ammonium and or the like-sized rubidium ion, which binds to an allosteric site on GLDH and changes the Km (
Michaelis constant Michaelis or Michelis is a surname. Notable people and characters with the surname include: * Adolf Michaelis, German classical scholar * Anthony R. Michaelis, German science writer * Edward Michelis, German theologian * Georg Michaelis, German p ...
) of the enzyme. The control of GLDH through ADP-ribosylation is particularly important in
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 ...
-producing β cells. Beta cells secrete insulin in response to an increase in the ATP: ADP ratio, and, as amino acids are broken down by GLDH into α-ketoglutarate, this ratio rises and more insulin is secreted. SIRT4 is necessary to regulate the metabolism of amino acids as a method of controlling insulin secretion and regulating blood
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
levels. Bovine liver glutamate dehydrogenase was found to be regulated by nucleotides in the late 1950s and early 1960s by Carl Frieden. In addition to describing the effects of nucleotides like ADP, ATP and GTP he described in detail the different kinetic behavior of NADH and NADPH. As such it was one of the earliest enzymes to show what was later described as allosteric behavior. Mutations which alter the allosteric binding site of GTP cause permanent activation of glutamate dehydrogenase, and lead to
hyperinsulinism-hyperammonemia syndrome Hyperinsulinism-hyperammonemia syndrome (HI/HA) is an autosomal dominant disorder that results in the excess production of insulin and ammonia in mammals. HI/HA is caused by increased Glutamate dehydrogenase 1 (GDH) activity due to the presence ...
.


Regulation

Allosteric regulation In biochemistry, allosteric regulation (or allosteric control) is the regulation of an enzyme by binding an effector molecule at a site other than the enzyme's active site. The site to which the effector binds is termed the ''allosteric site ...
: This protein may use the morpheein model of
allosteric regulation In biochemistry, allosteric regulation (or allosteric control) is the regulation of an enzyme by binding an effector molecule at a site other than the enzyme's active site. The site to which the effector binds is termed the ''allosteric site ...
. Allosteric inhibitors: *
Guanosine triphosphate Guanosine-5'-triphosphate (GTP) is a purine nucleoside triphosphate. It is one of the building blocks needed for the synthesis of RNA during the transcription process. Its structure is similar to that of the guanosine nucleoside, the only diffe ...
(GTP) *
Adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
(ATP) * Palmitoyl-CoA * Zn2+ Activators: *
Adenosine diphosphate Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbon ...
(ADP) *
Leucine Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α- ca ...
*
l-isoleucine Isoleucine (symbol Ile or I) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...
* l-valine *
Guanosine diphosphate Guanosine diphosphate, abbreviated GDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside guanosine. GDP consists of a pyrophosphate group, a pentose sugar ribose, and the nucleobase guanine. GDP is the product ...
Other Inhibitors: *
EGCG Epigallocatechin gallate (EGCG), also known as epigallocatechin-3-gallate, is the ester of epigallocatechin and gallic acid, and is a type of catechin. EGCG – the most abundant catechin in tea – is a polyphenol under basic research for its ...
Additionally, Mice GLDH shows substrate inhibition by which GLDH activity decreases at high glutamate concentrations.


Isozymes

Humans express the following glutamate dehydrogenase
isozyme 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 ...
s:


See also

*
Anaplerotic reactions Anaplerotic reactions, a term coined by Hans Kornberg and originating from the Greeἀνά 'up' anπληρόω 'to fill', are chemical reactions that form intermediates of a metabolic pathway. Examples of such are found in the citric acid cycle (TC ...


References


External links

* {{Portal bar, Biology, border=no EC 1.4.1 Glutamate (neurotransmitter)