Mammalian glutamate dehydrogenase (GDH) is a mitochondrial enzyme that catalyzes the reversible oxidative deamination of L-glutamate to 2-oxoglutarate using NAD(P)+ as coenzyme.  The enzyme is a homohexamer that is tightly regulated by a large number of positive and negative allosteric effectors as well as by cooperative interactions between subunits.
Structure:
The Figure to the right is a ribbon diagram of the entire boGDH hexamer.  In this diagram, each of the identical subunits are represented by different colors.  The N-terminal GLU binding domain forms all of the interactions across the non-crystallographic 2-fold axes.  The NAD+ binding domain rests on top of the GLU binding domain giving the subunit a ‘clam-like’ appearance with the active site lying in the cleft between the two domains.  The NAD+ binding domain makes very little contact with the other subunits in the hexamer.  In contrast, the antenna region, extending from the top of the NAD+ binding domain, forms extensive interactions with other subunits in the trimers.  The long helix is quite amphipathic with a large number of leucines and valines contacting the other helices that form this right-handed helical bundle.  This large, 48-residue antenna region is not found in non-mammalian GDH and may be intimately involved in allosteric regulation.
Regulation:
We have determined the structures of several structures of both bovine and human GDH in the presence of a number of ligands.  Shown on the right is one of the 6 subunits colored from red to blue as the chain extends from the N to the C terminus.  The yellow, grey, red, and green sphere models are glutamate, NAD, GTP, and ADP, respectively.  During each catalytic turnover, the entire NAD binding domain moves up and down by ~20° as the enzyme clamps down on the substrate and then opens it to release the produces.  We propose that GTP acts as an inhibitor by binding to the side of the mouth after it closes down on substrates and prevents the enzyme from opening its mouth to release product.  Conversely, we argue that ADP binds to either the open or closed forms of the enzyme and facilitates the motion during catalysis.  During this cycle, the antennae undergo a great deal of conformational change - which we argue is necessary for regulation.
Evolution:
It was long thought that only GDH from the animal kingdom was regulated by a number of allosteric ligands and that the unusual antenna region was also only found in animal GDH.  Animal GDH is inhibited by GTP, ATP, and palmatoyl CoA and activated by leucine and ADP.  However, recent sequencing of tetrahymena and paramecium GDH clearly demonstrated that the antenna evolved first in these protists.  Upon analysis of the enzymes it was found that tetrahymena GDH was regulated by palmatoyl CoA and ADP but not GTP, ATP, or leucine.  When the antenna was genetically removed, the enzyme lost much of this regulation.  As noted below, GTP and leucine regulate GDH that in turn affects insulin secretion.  Therefore, we suggested that animal GDH regulation is a result of evolutionary exaptation.  The protists evolved the antenna for regulation as fatty acid oxidation was moving from the peroxisomes to the mitochondria.  The regulation afforded by the antenna allowed them to link fatty acid and amino acid oxidation.  Animals then used this feature and further evolved it to also control insulin homeostasis.
Insulin:
Although the allosteric regulation of GDH has been intensively studied, its role in human physiology has remained unclear until recent demonstration of the involvement of GDH in a newly discovered form of congenital hypoglycemia; hyperinsulinism and hyperammonemia (HI / HA) syndrome.  In some children, however, hyperinsulinism is also associated with hyperammonemia.  Patients were found to be heterozygous for mutant forms of GDH that were unresponsive to the inhibitor, GTP.  In the beta cells of the pancreas, the increased glutamate to a-ketoglutarate conversion will increase respiration rate via the Krebs cycle, increase the ATP/ADP ratio, leading to inhibition of K+ channel activity, and result in excessive release of insulin.  This may also explain why these patients are particularly sensitive to protein-induced hypoglycemia.  Leucine is an activator of GDH and therefore ingestion of protein could exacerbate the already hyperactive state of GDH. Shown here is the location of the HI/HA mutants with respect to the bound GTP inhibitor.  Most mutations directly interact with GTP but some mutations are on the portion of the antenna that moves as the mouth opens and closes.
A cup of Tea:
Our goal was to find a non-toxic pharmacological agent that could be used to test these models and access whether GDH might be an appropriate target in the treatment of insulin disorders.  To that end, naturally occurring compounds from green tea were examined.  According to legend, green tea was discovered by the Chinese Emperor Shen-Nung in 2737 B.C. and for centuries has been used as a folk remedy to treat a number of ailments including diabetes mellitus.  Green tea is a significant source of a type of flavonoids called catechins; including epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC).  One 200 ml cup of green tea supplies 140, 65, 28, and 17 mg of these polyphenols, respectively.      We found that EGCG specifically and allosterically inhibits GDH and, in turn, affects insulin secretion by pancreatic ß-cells.  Kinetic analysis demonstrates that EGCG and ECG, but not EGC and EC, inhibit purified GDH with nanomolar ED50’s.  This inhibition is dependent upon the antenna-like protrusion on the enzyme but is unlikely to bind to the GTP inhibitory site since EGCG inhibits forms of GDH with non-functional GTP sites.  In-situ studies with pancreatic ß-cells demonstrated that EGCG specifically affects insulin secretion under conditions where GDH is known important for insulin homeostasis.  These results demonstrate that EGCG can be used a pharmacological tool to examine the complex regulation of insulin secretion by specifically blocking GDH activity.

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