Biochemistry of the Reaction:

As far as is known, AGAT needs three particular amino acid residues in order to carry out the proper reaction.  These three form a disjoint triad, and are His 303, Cys 407, and Asp 254.  Other amino acid residues in the active site are fairly important in forming a narrow channel to the centrally located active site, but mutations in these typically result in malformation or closure of the active site rather than a loss of reactivity.

You can see just how narrow this channel is by this spacefilling model (ornithine is green and is in the active site):

The disjoint triad and ornithine can be seen here:

The active site mutants studied so far include the following (wild type first, then mutant): Cys407:Ser ; Asp254:Asn ; His303:Val ; Asp305:Ala; Arg322:Glu; Ser355:Ala; Asp170:Asn; and Cys410:Ala.  The first three mutations are of key residues in the reaction.  It has been shown that what the enzyme needs is the disjoint triad, with a cysteine on one side and a paired histidine and aspartic acid residue on the other side.  The other mutants seem to only affect the reaction by the conformational changes of the protein mentioned above.

It has been shown that the transamidination reaction follows a ping-pong mechanism and induces large conformational changes in the native protein.  Also, the enzyme can react with canavanine, a chemical that differs from arginine only by an ether oxygen between the delta carbon and the epsilon nitrogen.   Beta-mercaptoethanol has also been incorporated into the active site, but it will not be transamidinated as glycine would be. 

Refresh the enzyme by pushing this:

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