Glutamate dehydrogenase mechanism

A three-substrate, three-product enzyme-catalyzed reaction scheme in which the three substrates (A, B, and C) and three products (P, Q, and R) can bind to and be released in any order. A number of enzymes have been reported to have this mechanism for example, adenylosuccinate synthetase , glutamate dehydrogenase, glutamine synthetase , formyltetrahydrofolate synthetase, and tubulin tyrosine ligase . See Multisubstrate Mechanisms... 

P. C. Engel, and D. W. Rice, Insights into the mechanism of domain closure and substrate sof glutamate dehydrogenase from Clostridium symbosium, J. Mol. Biol. 1999, 285, 875-885. 

The major enzyme involved in the formation of ammonia in the liver, brain, muscle, and kidney is glutamate dehydrogenase, which catalyzes the reaction in which ammonia is condensed with 2-oxoglutarate to form glutamate (Sec. 15.1). Small amounts of ammonia are produced from important amine metabolites such as epinephrine, norepinephrine, and histamine via amine oxidase reactions. It is also produced in the degradation of purines and pyrimidines (Sec. 15.6) and in the small intestine from the hydrolysis of glutamine. The concentration of ammonia is regulated within narrow limits the upper limit of normal in the blood in humans is 70/tmol L-1. It is toxic to most cells at quite low concentrations hence there are specific chemical mechanisms for its removal. The reasons for ammonia toxicity are still not understood. The activity of the urea cycle in the liver maintains the concentration of ammonia in peripheral blood at 20/ molL. ... 

Rife JE, Qeland WW. Kinetic mechanism of glutamate dehydrogenase. Biochemistry 1980 19 2321-2328. 

Behavioral disorders such as anorexia, sleep disturbances, and pain insensitivity associated with hyperammonemia have been attributed to increased tryptophan transport across the blood-brain barrier and the accumulation of its metabolites. Two of the tryptophan-derived metabolites are serotonin and quinolinic acid (discussed later). The latter is an excitotoxin at the N-methyl-D-aspartate (NMDA) glutamate receptors. Thus, the mechanism of the ammonium-induced neurological abnormalities is multifactorial. Normally only small amounts of NH3 (i.e., NH4 ) are present in plasma, since NH3 is rapidly removed by reactions in tissues of glutamate dehydrogenase, glutamine synthase, and urea formation. 

For glyceraldehyde-3-phosphate dehydrogenase, glutamate dehydrogenase, and the NADP-linked oxidative decarboxylases, which have three substrates in one direction, initial rate measurements with a fixed concentration of any one of the three substrates also conform to Eq. (1). This again rules out any form of enzyme-substitution mechanism in which free product is formed before all the substrates have combined with the enzyme and indicates the involvement of a quaternary enzyme complex. The appropriate generalized form of Eq. (1) is... 

It is noteworthy that glutamate synthase occurs in a complex with glutamate dehydrogenase in E. coli (4 ). The two activities can be copurified 30-fold. An important mechanism of regulation may be provided by association of the two enzymes in vivo. 

Only one single concentration term present, and another Michaelis constant equal to zero (B, C, A bAC terms missing). This is an ordered mechanism with addition of A in steady state and B in rapid equilibrium, and examples include glutamate dehydrogenase with a-ketovalerate or a-ketobutyrate as substrates (14). 

The intracellular levels of ammonium ions, in S. noursei, are also influenced by the action of alanine dehydrogenase [102]. Ammonium liberated by alanine oxidation is available for the biosynthesis of glutamine via glutamine synthetase. 0-aminobenzoic acid represses both enzymes while stimulating formation of glutamate dehydrogenase and the antibiotic. Cephalosporin biosynthesis appears to be regulated by the same mechanism [103]. 

These data supported the hypothesis that acetaminophen-induced liver toxicity is mediated by covalent binding to critical proteins. In an attempt to further understand the mechanism of hepatotoxicity of acetaminophen, specific proteins to which acetaminophen was covalently bound were isolated and sequenced by our laboratory and by Cohen s laboratory (Cohen et al. 1997). The proteins that were identified by this approach were glutamine synthase, glutamate dehydrogenase,... 

---