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Glutamate Dehydrogenase

General InformationGeneral Information

Glutamate Dehydrogenase (GluDH) is a member of the superfamily of amino acid dehydrogenase and functions in the cell to dehydrate α-ketoglutarate to the amino acid glutamate and also to perform the reverse reaction. GluDH feeds α-ketoglutarate into the tricarboxylic acid cycle (TCA) and the amine product is thought to be utilized by other biosynthetic pathways.. Likely due to its prominent position on the threshold between catalytic and biosynthetic pathways, GluDH is ubiquitously expressed in both complex and simple organisms.

• 
  Glutamic Acid
• α-Ketoglutarate
  α-Ketoglutarate

Reductive amination of α-ketoglutarate (α-KG) is the process by which the ketone is converted to an amine via an imine intermediate. The reverse reaction, oxidative deamination, is the conversion of the amine functional group to a ketone.

Glutamate dehydrogenase shares sequence homology and structural homology to the superfamily of amino acid dehydrogenases, which supports the idea that this superfamily formed by divergent evolution. Because of the homology among all proteins in this superfamily, many dehydrogenases can work on multiple substrates. Nonetheless, GluDH appears to be very specific towards its substrates. NAD(P)H are cofactors for the reaction and serve to reduce α-KG/ oxidize Glu when they have been oxidized. Though procaryotic GluDH has not been found to be allosterically inhibited, Mammalian GluDH has been found to accomodate allosteric inhibition from GTP and ATP.

ProkaryoteProkaryote

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General StructureGeneral Structure

Prokaryotic glutamate dehydrogenase (GDH) does not have any common quaternary structure among crystallized structures (1EUZ is a hexamer, 1HRD a trimer); however, every prokaryotic structure so far elucidated shows a common overall tertiary structure.

Each monomer (reguardless of quaternary structure) has two domains: a domain that is a variant of the Rossmann dinucleotide binding fold (), and a domain involved in oligomerization (when it occurs) and contains most of the substrate binding residues ().

SpecificitySpecificity

is made up of polar interactions from K89 and S380 and hydrophobic interactions from G90, V377 and A163. The last three residues that make this interaction are highly conserved among amino acid dehydrogenases. The polar residues make specific contacts with the glutamine substrate.

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Domain II is in Blue and Domain I is in Purple

Allosteric InteractionsAllosteric Interactions

m m m m

...more to come

EukaryoteEukaryote

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General StructureGeneral Structure

Unlike the prokaryotic GluDH, mammalian GluDH has been found to always hexamerize as a dimer of . Also unlike prokaryotic GluDH, the has 48 residue "antenna" that assists in the trimerization process. . These antennae appear to undergo conformational changes as the "mouth" of GluDH opens and closes (see morph below). Also unlike prokarytoic GluDH, Mammalian GluDH is allosterically controlled by GTP (-), ATP (-), GDP(+) and ADP(+). Buried proximally (from the center of the protein) to the antennae is the allosteric binding site.

Like the prokaryotic GluDH, the mammalian GluDH is composed of two domains... one that

SpecificitySpecificity

GluDH makes non-coavalent and specific contacts with its substrates, cofactors and allosteric inhibitors.

For example, glutamate and α-KG both bind via hydrogen bonding within the catalytic cleft between the two distinct clamping domains (see description above in prokaryotes).

Both make contact with K126, K90, S381, R211, and N349. However, α-KG binding is thought to be stabilized also by N374 (through a water molecule) and K114 and Glu by K114 in a different crystal structure.

NADH, as it binds within the to the catalytic cleft, makes specific hydrogen bonds with D168, S170, E275, S276, N349, A326 and S327 in all crystal structures. Q250 contacts both NADPH and NADH, but not NAD+. Instead Q330 makes a similar contact to NAD+ in that region. roughly illustrates the binding pocket in which the dinucleotide cofactor sits. Directly above the cofactor is evident the space where Glu would sit.

Allosteric InteractionsAllosteric Interactions

Mammalian GluDH is allosterically regulated by ATP and GTP, among other agents that are not likely used in a cell's natural process - like...

It is thought that GTP causes negative allosteric regulation of GluDH by increasing the enzyme's affinity for the product to the extent that the release of the product is the rate limiting step of the overall reaction.

When the enzyme is highly saturated, the enzyme has been found to form an "abortive complex" that is the cofactor and the reagent locked in a non-catalytic conformation. Upon the binding of a positive regulator, like GDP or ADP, the reaction is allowed to go to completion. ADP has also been shown to decrease the affinity of the enzyme to its products.

via hydrogen bonding. Most of the contacts are with the triphosphate moeity - the sidechains of H209, H450, Y262, R217, R265, R261 - however, the sidechains of K281 and E292 make specific contacts with the adenosine ring (to the carbonyl and the N1 imino, respectively) and S213 makes contact with the 2' hydroxyl on the sugar. The guanidinium of R261 is thought to stack against the purine ring.

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