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* Glutamine amidotransferases class-I active site *
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A large group of biosynthetic enzymes are able to  catalyze the removal of the
ammonia group from glutamine and then to transfer this group to a substrate to
form  a new   carbon-nitrogen  group.  This catalytic  activity  is  known  as
glutamine amidotransferase (GATase) (EC 2.4.2.-) [1]. The GATase domain exists
either as a separate  polypeptidic subunit or  as part of a larger polypeptide
fused in  different  ways  to a  synthase domain.  On  the  basis  of sequence
similarities two classes of GATase domains have been identified [2,3]: class-I
(also known as trpG-type) and  class-II  (also  known  as  purF-type). Class-I
GATase domains have been found in the following enzymes:

 - The second  component of anthranilate  synthase (AS) (EC 4.1.3.27) [4].  AS
   catalyzes the biosynthesis  of  anthranilate from chorismate and glutamine.
   AS is generally a  dimeric  enzyme:  the  first  component  can  synthesize
   anthranilate using ammonia  rather  than  glutamine,  whereas  component II
   provides the GATase  activity.   In some bacteria and in fungi  the  GATase
   component of AS is  part  of a  multifunctional protein that also catalyzes
   other steps of the biosynthesis of tryptophan.
 - The second component of 4-amino-4-deoxychorismate (ADC) synthase (EC 4.1.3.
   -), a dimeric  prokaryotic  enzyme  that   function  in  the  pathway  that
   catalyzes the biosynthesis of para-aminobenzoate (PABA) from chorismate and
   glutamine.  The second  component (gene pabA)  provides the GATase activity
   [4].
 - CTP synthase (EC 6.3.4.2). CTP synthase catalyzes the final reaction in the
   biosynthesis of pyrimidine, the ATP-dependent formation of CTP from UTP and
   glutamine. CTP synthase is a single chain enzyme that contains two distinct
   domains; the GATase domain is in the C-terminal section [2].
 - GMP synthase (glutamine-hydrolyzing) (EC 6.3.5.2).  GMP synthase  catalyzes
   the   ATP-dependent  formation  of  GMP  from  xanthosine 5'-phosphate  and
   glutamine. GMP synthase is a single chain enzyme that contains two distinct
   domains; the GATase domain is in the N-terminal section [5].
 - Glutamine-dependent carbamoyl-phosphate synthase  (EC 6.3.5.5) (GD-CPSase);
   an enzyme  involved in  both arginine and pyrimidine biosynthesis and which
   catalyzes the ATP-dependent formation of carbamoyl phosphate from glutamine
   and carbon dioxide.  In bacteria GD-CPSase is composed of two subunits: the
   large chain (gene carB) provides the CPSase activity, while the small chain
   (gene carA) provides the GATase activity.  In yeast  the enzyme involved in
   arginine biosynthesis is also composed of two subunits:  CPA1 (GATase), and
   CPA2 (CPSase).  In most eukaryotes,  the  first  three steps  of pyrimidine
   biosynthesis are  catalyzed by a large multifunctional enzyme  (called URA2
   in yeast, rudimentary in Drosophila, and CAD in mammals). The GATase domain
   is located at the N-terminal extremity of this polyprotein [6].
 - Phosphoribosylformylglycinamidine synthase II (EC 6.3.5.3),  an enzyme that
   catalyzes the  fourth  step in the biosynthesis of purines. In some species
   of bacteria, FGAM  synthase  II  is composed of two subunits: a small chain
   (gene purQ)  which  provides  the  GATase  activity and a large chain (gene
   purL) which provides the aminator activity.
 - The histidine amidotransferase hisH, an  enzyme  that  catalyzes  the fifth
   step in the biosynthesis of histidine in prokaryotes.

In the second component of AS  a  cysteine  has been shown [7] to be essential
for the amidotransferase activity.    The sequence around this residue is well
conserved in all  the  above  GATase  domains and  can  be used as a signature
pattern for class-I GATase.

-Consensus pattern: [PA]-[LIVMFYT]-[LIVMFY]-G-[LIVMFY]-C-[LIVMFYN]-G-x-[QE]-x-
                    [LIVMFA]
                    [C is the active site residue]
-Sequences known to belong to this class detected by the pattern: ALL.
-Other sequence(s) detected in SWISS-PROT: NONE.

-Note: in the  first position of  the pattern Pro is found in all cases except
 in the slime mold GD-CPSase where it is replaced by Ala.

-Last update: June 1994 / Pattern and text revised.

[ 1] Buchanan J.M.
     Adv. Enzymol. 39:91-183(1973).
[ 2] Weng M., Zalkin H.
     J. Bacteriol. 169:3023-3028(1987).
[ 3] Nyunoya H., Lusty C.J.
     J. Biol. Chem. 259:9790-9798(1984).
[ 4] Crawford I.P.
     Annu. Rev. Microbiol. 43:567-600(1989).
[ 5] Zalkin H., Argos P., Narayana S.V.L., Tiedeman A.A., Smith J.M.
     J. Biol. Chem. 260:3350-3354(1985).
[ 6] Davidson J.N., Chen K.C., Jamison R.S., Musmanno L.A., Kern C.B.
     BioEssays 15:157-164(1993).
[ 7] Tso J.Y., Hermodson M.A., Zalkin H.
     J. Biol. Chem. 255:1451-1457(1980).