Glutamate Dehydrogenase

Glutamate dehydrogenases are enzymes that are responsible for the catalysis of an important step between carbohydrate and amino acid metabolism, the oxidative deamination of L-glutamate to alpha-ketoglutarate and ammonia. The reaction is driven by the consumption of ammonia in the urea cycle, so along with aminotransferases, glutamate dehydrogenases “serve as clearing house[s] for alpha-amino groups” (5.) To that extent this enzyme is located in the mitochondria, where the toxic free ammonia by-product can be isolated until converted. The enzyme is also readily affected by energy levels in the cell as it is allostericlly regulated by ADP and GTP (8.) When energy levels are high in the cell, the levels of GTP are high. The greater concentration correlates with an increase in binding to glutamate dehydrogenase, resulting in an induced inhibition. ADP has the opposite effect when bound in times of low energy.

“Glutamate dehydrogenases have been isolated and sequenced from a number of sources and fall into two oligomeric classes. The bacterial and fungal NADP⁺-linked and vertebrate dual-specificity [proteins] have six identical subunits” whereas “the NAD⁺-linked enzymes have either four identical subunits” or six identical subunits (5.) In each subunit two domains parted by a profound cleft are visibly distinguishable. Each domain is comprised of a central beta-sheet that is surrounded by alpha-helices. The cleft is the site of cofactor and substrate binding and is lined with amino acids involved in determing substrate specifity. Certain residues involved include Lys89, Ser380, Val377, Ala163 that all interact with glutamte through its carbonyl to hold it into the pocket, as well as "five glycine residues that shape the active site (Gly122 and Gly123), orient functional groups (Gly90 and Gly91), and lie close to the binding site for the nicotinamide ring (Gly376)" (5.)

Jmol image 1BGV showing domain and secondary structure when complexed with glutamate.

The mechanism of the reaction catalyzed by glutamate dehydrogenase in Clostridium symbiosum is discussed in the paper Stillman, et al. The process begins with the removal of a proton from the alpha-amino group of the glutamate by Asp165, resulting in the subsequent hydride transfer to the NAD⁺ enabled by the formation of an iminoglutarate intermediate that brings the cofactor and substrate close enough for interaction. With the aid of a basic Lys125, a water molecule then attacks the iminogluterate intermediate generating a carbinolamine intermediate. Asp165 is crucial for proton transfer to and from the substrate during this and the consequent collapse to 2-oxoacid. The final step involves the loss of a proton from both of the engaged residues to the water molecule.

This Jmol image (1V9L) shows the binding of the cofactor NAD⁺ to the enzyme.

[Home] [Introduction to Oxidoreductases] [Glutamate Dehydrogenase] [Aldehyde Reductase]

[Chloroperoxidase] [Summary] [References]