Glutamine Synthetase

Glutamine synthetase is an important enzyme both in synthesis of glutamine and in ammonia metabolism. It catalyzes the following reaction (see here also):

> Glutamate + NH 3 + ATP -> Glutamine + ADP + Pi

The E. coli glutamine synthetase is a dodecamer, with 12 identical subunits and the complex has a molecular weight of about 600,000 Daltons. The amide nitrogen of glutamate is used for the synthesis of several amino acids, purine and pyrimidine nucleotides, and amino sugars, so glutamine synthetase plays a central role in nitrogen metabolism. In animals, the enzyme is a key participant in detoxifying ammonia, particularly in the brain, and in ammonia excretion in the kidney. Accumulation of glutamate and glutamine depletes -ketoglutarate, which would interfere with the citric acid cycle. As a result, glutamine synthetase is tightly regulated. Mechanisms controlling the activity of glutamine synthetase include the following:

> Cumulative feedback Inhibition - Eight specific feedback inhibitors, which are either metabolic end products of glutamine (tryptophan, histidine, glucosamine-6-phosphate, carbamoyl phosphate, CTP, or AMP) or indicators of the general status of amino acid metabolism (alanine or glycine), can bind to any of the subunits of the enzyme and at least partially inhibit it. The more inhibitors that bind, the greater the inhibition.

> Covalent modification (adenylylation) - A specific tyrosine residue in glutamine synthetase can react with ATP to form a phosphate ester with AMP (see here). Adenylylation renders the catalytic site of the enzyme inactive. Adenylylation and deadenylylation involve a complex series of regulatory cascades. Figure 20.9 shows regulatory mechanisms of the E. coli enzyme. Both processes are catalyzed by the same enzyme-a complex of adenylyl transferase (AT) and a regulatory protein, P II. The form of P II determines whether the AT-P II complex catalyzes adenylylation or deadenylylation. If P II is uridylyated, the AT-P II complex catalyzes deadenylylation. Deuridylylation of P II causes the AT-P II complex to catalyze adenylylation. The enzyme uridylyl transferase catalyzes uridylylation of P II, whereas deuridylylation is catalyzed by a different enzyme. Uridylyl transferase is allosterically regulated, with ATP and -ketoglutarate activating it and glutamine inhibits it.

See also: Utilization of Ammonia, Transamination in Amino Acid Metabolism