Glutamine Synthetase - Regulation & Inhibition

Regulation & Inhibition

Reversible Covalent Modification. A tyrosine residue in each subunit in GS can be modified by adenylylation. Adenylyl transferase catalyzes the adenylylation and phosphorolysis reactions. Adenyl transferase activity is influenced by two regulatory proteins: P_A and P_D. P_A reduces GS activity by attaching an AMP unit to GS. Adenylyl transferase and P_D removes the AMP unit. PA and PD may be interconverted via uridylyl transferase. Adenylylated GS is less active than unadenylated GS. In the majority of gram-negative bacteria, GS can be modified by adenylylation (some cyanobacteria and green algae or exceptions).

Inhibition of GS has largely focused on amino site ligands. Other inhibitors are the result of glutamine metabolism: tryptophan, histidine, carbamoyl phosphate, glucosamine-6-phosphate, cytidine triphosphate (CTD), and adenosine monophosphate (AMP). Other inhibitors/regulators are glycine and alanine. Alanine, glycine, and serine bind to the glutamate substrate site. GDP, AMP, ADP bind to the ATP site. L-serine, L-alanine, and glycine bind to the site for L-glutamate in unadenylated GS. The four amino acids bind to the site by their common atoms, “the main chain” of amino acids. Glutamate is another product of glutamine metabolism; however, glutamate is a substrate for GS inhibiting it to act as a regulator to GS.2 Each inhibitor can reduce the activity of the enzyme; once all final glutamine metabolites are bound to GS, the activity of GS is almost completely inhibited. Many inhibitory input signals allows for fine tuning of GS by reflecting nitrogen levels in the organism.

Feedback regulation distinguishes the difference between two eukaryotic types of GS: brain and non-brain tissues. Non-brain GS responds to end-product feedback inhibition, while brain GS does not. High concentrations of glutamine-dependent metabolites should inhibit GS activity, while low concentrations should activate GS activity.

Inhibitors:

• Methionine Sulfoximine (MSO): MSO is an inhibitor that binds to the glutamate site. Bound to GS, MSO is phosphorylated by ATP that results in an irreversible, non-covalent inhibition of GS. The S-isomer configuration is more inhibitory. Glutamate entry is blocked into the active site by a stabilization of the flexible loop in the active site by MSO.

• Phosphinothricin(PPT, Glufosinate): Phosphinothricin is an inhibitor that binds to the glutamate site. Glufosinate is used as an herbicide. Glufosinate treated plants die due to a buildup of ammonia and a cessation of photosynthesis.

• Many synthetic inhibitors are available today.