Glutamine synthetase (EC 22.214.171.124) (GS)  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.
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) has, currently, only been found in Bacteroides
fragilis and in butyrivibrio fibrisolvens. It is a hexamer of identical
chains. It is much larger (about 700 amino acids) than the GSI (450 to 470
amino acids) or GSII (350 to 420 amino acids) enzymes.
While the three classes of GS's are clearly structurally related, the sequence
similarities are not so extensive. As signature patterns we selected three
conserved regions. The first pattern is based on a conserved tetrapeptide in
the N-terminal section of the enzyme, the second one is based on a glycine-rich region which is thought to be involved in ATP-binding. The third pattern
is specific to class I glutamine synthetases and includes the tyrosine residue
which is reversibly adenylated.
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