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'''Glutathione synthetase''' (GSS) is an homo-dimeric, ATP-depending ligase responsible for the condensation of γ-Glutamylcysteine and glycine to form Glutathione (GSH) during the second step of the glutathione biosynthesis pathway <ref>PMID:19672693</ref>. '''Glutathione''' is considered to be one of the most abundant and important antioxidants present across many bacterial (cyano- and proteobacteria), and all plant & mammalian cells <ref>http://www.ncbi.nlm.nih.gov/protein/NP_000169.1</ref>. In addition to protecting cells from the oxidative damage caused by free radicals, it is believed to be involved in the detoxification of xenobiotics, toxins in the blood, and even amino acid transport <ref>PMID:21683691</ref>.

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==Substrate and ATP Binding Residues==

===Aspartate 458 <ref>PMID:21771585</ref>===

'''Val44''' and '''Val45''' are two other residues which have been theorized to be important to the catalytic function of GSS due to their location on the dimerization site of the homogenous subunits. Early computer studies suggested that mutation to Val45 would have a larger detrimental effect than a mutation to Val44, and these predictions have since been verified by experimental studies. Differential scanning calorimetry has demonstrated that mutations to either of these two valines results in a loss of structural stability, with Val45 mutants being less stable than the Val44 mutants. Kinetic experiments suggest little effect on the affinity of GSS for γ-Glutamylcysteine by mutating one of these two residues, therefore it is assumed that the dimerization site is a part of the allosteric pathway rather than involved in the active site of the enzyme. It can be said with confidence, however, that they are integral to the stability of the biologically active protein.  

===Glycine Triad <ref>PMID:20800579</ref>===

As stated previously, the catalytic active site of GSS is composed of the G-loop, S-loop, and A-loop. The G-loop has been termed the “<scene name='56/564047/Glycine_triad/1'>Glycine Triad</scene>” due to the contribution of three glycine residues in this loop to the enzymatic activity of GSS – '''Gly369''', '''Gly370''', and '''Gly371'''. While all three residues are essential to the activity of the enzyme, kinetic experiments have shown Gly369 and Gly370 to have much more critical roles than Gly371. G369V and G370V variants were found to contain a mere 0.7% and 0.3% of the activity of the wild type GSS enzyme, respectively. G371V mutants still contained approximately 13% of the wild type activity, indicating a level of importance similar to the Asp458 residue of the A-loop. These experimental results suggest that the mechanism of activity interference lies in a decreased ligand binding and failure to close the active site once the ligand has bound.  

==Alternative Splicing Variants <ref>PMID:19672693</ref>==

Though reduced levels of GSH have been observed in patients with '''Alzheimers''' and '''Parkinsons''', inborn errors in the endogenous GSS enzyme resulting in significantly low levels of GSH is believed to be the cause of a very rare metabolic deficiency in which there is a large build up of 5-oxoproline in the urine - termed '''5-oxoprolinuria'''. It is so rare that, as of 2006, it had been diagnosed in less than 100 people worldwide.

Studies suggest that the rarity of this disorder can be attributed to the fact it is autosomal recessive and thus both copies of the cell's chromosome must contain the genetic coding for the disorder. Each of the parents must carry a single copy of the mutated <i>gss</i> gene, thus displaying no physical symptoms, and both must pass their mutated copy on to the child.  

==References==