Glutamine synthetase (6.3.1.2) (GS) [1] plays an essential role in the metabolism of nitrogen by catalyzing the condensation of glutamate and ammonia to form glutamine.

There seem to be three different classes of GS [2, 3, 4]:

• Class I enzymes (GSI) are specific to prokaryotes, and are oligomers of 12 identical subunits. The activity of GSI-type enzyme is controlled by the adenylation of a tyrosine residue. The adenylated enzyme is inactive (see IPR001637).
• Class II enzymes (GSII) are found in eukaryotes and in bacteria belonging to the Rhizobiaceae, Frankiaceae, and Streptomycetaceae families (these bacteria have also a class-I GS). GSII are octamer of identical subunits. Plants have two or more isozymes of GSII, one of the isozymes is translocated into the chloroplast.
• Class III enzymes (GSIII) have been found in Bacteroides fragilis. in Butyrivibrio fibrisolvens. It is a hexamer of identical chains and in some protozoa. It is much larger (about 700 amino acids) than the GSI (450 to 470 amino acids) or GSII (350 to 420 amino acids) enzymes.

1. Some evolutionary relationships of the primary biological catalysts glutamine synthetase and RuBisCO. Cold Spring Harb. Symp. Quant. Biol. 52, 483-90
2. Evolution of the glutamine synthetase gene, one of the oldest existing and functioning genes. Proc. Natl. Acad. Sci. U.S.A. 90, 3009-13
3. Glutamine synthetase II in Rhizobium: reexamination of the proposed horizontal transfer of DNA from eukaryotes to prokaryotes. J. Mol. Evol. 29, 422-8
4. Evolutionary relationships of bacterial and archaeal glutamine synthetase genes. J. Mol. Evol. 38, 566-76

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