|Protein name||Glycine amidinotransferase, mitochondrial||glycine amidinotransferasearginine-glycine amidinotransferasearginine-glycine transamidinaseglycine transamidinase|
|Synonyms||EC 18.104.22.168L-arginine:glycine amidinotransferaseTransamidinaseAT|
|MAP00220||Urea cycle and metabolism of amino groups|
|MAP00260||Glycine, serine and threonine metabolism|
|MAP00330||Arginine and proline metabolism|
|Activity||L-arginine + glycine = L-ornithine + guanidinoacetate.|
|Subunit||Homodimer. There is an equilibrium between the monomeric and dimeric forms, shifted towards the side of the monomer.|
|Subcellular location||Mitochondrion inner membrane, Peripheral membrane protein, Intermembrane side (Potential). Cytoplasm. Note=The mitochondrial form is found in the intermembrane space probably attached to the outer side of the inner membrane.|
|Type||amino acids,amine group,imine group,lipid||amino acids||amino acids,amine group,lipid||amino acids,amine group,imine group|
|References for Catalytic Mechanism|
|References||Sections||No. of steps in catalysis|
|Comments||ACTIVE SITE CYS-407|
|Authors||Humm A, Fritsche E, Mann K, Gohl M, Huber R|
|Title||Recombinant expression and isolation of human L-arginine:glycine amidinotransferase and identification of its active-site cysteine residue.|
|Comments||X-RAY CRYSTALLOGRAPHY (1.9 ANGSTROMS) OF 64-423|
|Authors||Humm A, Fritsche E, Steinbacher S, Huber R|
|Title||Crystal structure and mechanism of human L-arginine:glycine amidinotransferase: a mitochondrial enzyme involved in creatine biosynthesis.|
|Authors||Humm A, Fritsche E, Steinbacher S|
|Title||Structure and reaction mechanism of L-arginine:glycine amidinotransferase.|
|Journal||Eur J Biochem|
|Authors||Fritsche E, Humm A, Huber R|
|Title||Substrate binding and catalysis by L-arginine:glycine amidinotransferase--a mutagenesis and crystallographic study.|
|Authors||Fritsche E, Bergner A, Humm A, Piepersberg W, Huber R|
|Title||Crystal structure of L-arginine:inosamine-phosphate amidinotransferase StrB1 from Streptomyces griseus: an enzyme involved in streptomycin biosynthesis.|
|Comments||X-RAY CRYSTALLOGRAPHY (2.5 ANGSTROMS) OF 64-423|
|Journal||J Biol Chem|
|Authors||Fritsche E, Humm A, Huber R|
|Title||The ligand-induced structural changes of human L-Arginine:Glycine amidinotransferase. A mutational and crystallographic study.|
|The early papers,  & , proposed a catalytic mechanism as follows:|
The thiol group of Cys407, which plays a triple role as nucleophile, acid and base, makes a nucleophilic attack on the positively charged carbon atom of amidino group, and simultaneously protonates the leaving group, the epsilon-imino group of the arginine substrate.
A tetrahedral intermediate is formed, which can be stabilized by the interaction between the guanidino nitrogen atoms and acidic residues, Asp170 and Asp305, and between the epsilon-imino nitrogen and His303. The bond between the epsilon-imino group and the amidino-carbon atom is broken.
After the product of ornithine leaves the active site, another substrate, glycin can enter the site and bind to the amidino-carbon atom of the amidino-cysteine. The second half reaction may start with abstarction of proton from the positively charged glycine substrate by His303. The lone electron pair of the glycine nitorogen atom may make a nucleophilic attack at the amidino-carbon atom. In the final steps, formation of a tetrahedral intermediate and its breakdown by cleavage of the carbon-sulfur bond may occur. However, this second half reaction seemed to be speculative, according to the paper .
On the other hand, a more recent paper  revised this proposed mechanism. In the newly proposed mechanism, the reaction proceeds as follows:
(1) Asp305 was suggested to act as a catalytic acid-base, by abstracting a thiol proton from the nucleophile, Cys407.
(2) Cys407 makes a nucleophilic attack on the positively charged amidino carbon atom, leading to the formation of tetrahedral intermediate.
(3) The tetrahedral intermediate can be stabilized by the interaction with Asp170 and Asp305.
(4) In contrast, the sidechain of His303 seems to be protonated, activating the leaving group, the epsilon-imino group of arginine, as a general acid.
(5) The breakdown of the tetrahedral intermediate generates the product, ornithine, and the amidino-Cys407.
(6) A second substrate, glycine, binds to the active site as an acceptor group.
(7) Probably, His303 acts as a general base, to activate the acceptor amine group of glycine.
(8) The activated amine group makes a nucleophilic attack on the carbon atom of the planar amidino-Cys407.
(9) Probably, Asp305 acts as a general acid to protonate the leaving Cys407, resulting the formation of the product, guanidino-acetic acid.
(#) During the catalysis, Asp254 may be involved in the stablization and orientation of His303 towards the substrates (see ).