Glutamate dehydrogenase

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glutamate dehydrogenase (GLDH)
Identifiers
EC number1.4.1.2
CAS number9001-46-1
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
glutamate dehydrogenase [NAD(P)+]
Identifiers
EC number1.4.1.3
CAS number9029-12-3
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
glutamate dehydrogenase (NADP+)
Identifiers
EC number1.4.1.4
CAS number9029-11-2
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO

Glutamate dehydrogenase (GLDH, GDH) is an enzyme, present in most microbes and the mitochondria of eukaryotes, as are some of the other enzymes required for urea synthesis, that converts glutamate to α-ketoglutarate, and vice versa. In animals, the produced ammonia is usually used as a substrate in the urea cycle. 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 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)[1]. In bacteria, the ammonia is assimilated to amino acids via glutamate and aminotransferases.[2] In plants, the enzyme can work in either direction depending on environment and stress.[3][4] Transgenic plants expressing microbial GLDHs are improved in tolerance to herbicide, water deficit, and pathogen infections.[5] They are more nutritionally valuable.[6]

The enzyme represents a key link between catabolic and anabolic pathways, and is, therefore, ubiquitous in eukaryotes. In humans the relevant genes are called GLUD1 (glutamate dehydrogenase 1) and GLUD2 (glutamate dehydrogenase 2), and there are also at least 8 GLDH pseudogenes in the human genome as well, probably reflecting microbial influences on eukaryote evolution.

Clinical application[edit]

GLDH can be measured in a medical laboratory to evaluate the liver function. Elevated blood serum 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, 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, GLDH can serve as a measurement for the safety of a drug.

Cofactors[edit]

NAD+(or NADP+) is a cofactor for the glutamate dehydrogenase reaction, producing α-ketoglutarate and ammonium as a byproduct.[4][7]

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+ 2-oxoglutarate + NH3 + NADH + H+
  • EC 1.4.1.3: L-glutamate + H2O + NAD(P)+ 2-oxoglutarate + NH3 + NAD(P)H + H+
  • EC 1.4.1.4: L-glutamate + H2O + NADP+ 2-oxoglutarate + NH3 + NADPH + H+

Role in flow of nitrogen[edit]

Ammonia incorporation in animals and microbes occurs through the actions of glutamate dehydrogenase and glutamine synthetase. Glutamate plays the central role in mammalian and microbe nitrogen flow, serving as both a nitrogen donor and a nitrogen acceptor.

Regulation of glutamate dehydrogenase[edit]

In humans, the activity of glutamate dehydrogenase is controlled through ADP-ribosylation, a covalent modification carried out by the gene sirt4. This regulation is relaxed in response to caloric restriction and low blood glucose. 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 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) of the enzyme.[8]

The control of GLDH through ADP-ribosylation is particularly important in insulin-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 levels.

Bovine liver glutamate dehydrogenase was found to be regulated by nucleotides in the late 1950s and early 1960s by Carl Frieden.[9] [10] [11] [12] 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. [13]

Mutations alter the allosteric binding site of GTP cause permanent activation of glutamate dehydrogenase lead to disorder known as hyperinsulinism-hyperammonemia.

Regulation[edit]

Allosteric regulation:

This protein may use the morpheein model of allosteric regulation.[7][14]

Allosteric inhibitors:

Activators:

Guanosine diphosphate

Additionally, Mice GLDH shows substrate inhibition by which GLDH activity decreases at high glutamate concentrations.[7]

Isozymes[edit]

Humans express the following glutamate dehydrogenase isozymes:

glutamate dehydrogenase 1
Identifiers
SymbolGLUD1
Alt. symbolsGLUD
Entrez2746
HUGO4335
OMIM138130
RefSeqNM_005271
UniProtP00367
Other data
EC number1.4.1.3
LocusChr. 10 q21.1-24.3
glutamate dehydrogenase 2
Identifiers
SymbolGLUD2
Alt. symbolsGLUDP1
Entrez2747
HUGO4336
OMIM300144
RefSeqNM_012084
UniProtP49448
Other data
EC number1.4.1.3
LocusChr. X q25

See also[edit]

References[edit]

  1. ^ McKenna & Ferreira (2016) Enzyme complexes important for the glutamate-glutamine cycle. The Glutamate/ GABA-Glutamine Cycle. Springer Int.
  2. ^ Lightfoot DA, Baron AJ, Wootton JC (May 1988). "Expression of the Escherichia coli glutamate dehydrogenase gene in the cyanobacterium Synechococcus PCC6301 causes ammonium tolerance". Plant Molecular Biology. 11 (3): 335–44. doi:10.1007/BF00027390. PMID 24272346.
  3. ^ Mungur R, Glass AD, Goodenow DB, Lightfoot DA (June 2005). "Metabolite fingerprinting in transgenic Nicotiana tabacum altered by the Escherichia coli glutamate dehydrogenase gene". Journal of Biomedicine & Biotechnology. 2005 (2): 198–214. doi:10.1155/JBB.2005.198. PMC 1184043. PMID 16046826.
  4. ^ a b Grabowska A, Nowicki M, Kwinta J (2011). "Glutamate dehydrogenase of the germinating triticale seeds: gene expression, activity distribution and kinetic characteristics". Acta Physiol. Plant. 33 (5): 1981–90. doi:10.1007/s11738-011-0801-1.
  5. ^ Lightfoot DA, Bernhardt K, Mungur R, Nolte S, Ameziane R, Colter A, Jones K, Iqbal MJ, Varsa E, Young B (2007). "Improved drought tolerance of transgenic Zea mays plants that express the glutamate dehydrogenase gene (gdhA) of E. coli". Euphytica. 156 (1–2): 103–116. doi:10.1007/s10681-007-9357-y.
  6. ^ Lightfoot DA (2009). "Genes for use in improving nitrogen use efficiency in crops". In Wood, Andrew; Matthew A. Jenks. Genes for Plant Abiotic Stress. Wiley-Blackwell. pp. 167–182. ISBN 978-0-8138-1502-2.
  7. ^ a b c Botman D, Tigchelaar W, Van Noorden CJ (November 2014). "Determination of glutamate dehydrogenase activity and its kinetics in mouse tissues using metabolic mapping (quantitative enzyme histochemistry)". The Journal of Histochemistry and Cytochemistry. 62 (11): 802–12. doi:10.1369/0022155414549071. PMC 4230541. PMID 25124006.
  8. ^ Wootton JC (February 1983). "Re-assessment of ammonium-ion affinities of NADP-specific glutamate dehydrogenases. Activation of the Neurospora crassa enzyme by ammonium and rubidium ions". The Biochemical Journal. 209 (2): 527–31. doi:10.1042/bj2090527. PMC 1154121. PMID 6221721.
  9. ^ Frieden C (April 1959). "Glutamic dehydrogenase. II. The effect of various nucleotides on the association-dissociation and kinetic properties". The Journal of Biological Chemistry. 234 (4): 815–20. PMID 13654269.
  10. ^ Frieden C (May 1962). "The unusual inhibition of glutamate dehydrogenase by guanosine di- and triphosphate". Biochimica et Biophysica Acta. 59 (2): 484–6. doi:10.1016/0006-3002(62)90204-4. PMID 13895207.
  11. ^ Frieden C (1963). L-Glutamate Dehydrogenase, in The Enzymes, Vol VII. Academic Press. pp. 3–24.
  12. ^ Frieden C (May 1965). "Glutamate Dehydrogenase. VI. Survey of Purine Nucleotide and Other Effects on the Enzyme from Various Sources". The Journal of Biological Chemistry. 240: 2028–35. PMID 14299621.
  13. ^ Monod J, Wyman J, Changeux JP (1965). "On the Nature of Allosteric Transitions: A Plausible Model". J Mol Biol. 12: 88–118. doi:10.1016/s0022-2836(65)80285-6. PMID 14343300.
  14. ^ Selwood T, Jaffe EK (March 2012). "Dynamic dissociating homo-oligomers and the control of protein function". Archives of Biochemistry and Biophysics. 519 (2): 131–43. doi:10.1016/j.abb.2011.11.020. PMC 3298769. PMID 22182754.

External links[edit]