L-Methionine-S-sulfoximine phosphate

Studies on the interaction of the enzyme with many substrates and non-substrates showed that L-glutamate attaches to the enzyme in an extended conformatigg. which the a-hydrogen atom is directed away from the active site—-—. Computer analysis of the active site permitted the selection of.a phosphorylation site and of an ammonia binding site on the enzyme—. The position of the latter site was derived from the calculated position of the nitrogen atom of the tetrahedral intermediate (or transition state) presumably formed in the reaction of ammonia with enzyme-bound Y gl tamyl phosphate. Comparison of the structure of the tetrahedral intermediate with that of L-methionine-S-sulfoximine phosphate showed that these structures are very similar. L-Methionine-S-sulfoximine phosphate was found to fit as closely to the calculated enzyme sites as does the tetrahedral intermediate. These studies support the view that L-methionine-S-sulf oximine serves as an inhibitory analog of the enzyme-bound intermediate or transition state formed in the normal catalytic reaction. That methionine sulfoximine is phosphorylated also provides additional evidence for the formation of an acyl phosphate intermediate in the normal catalytic reaction. 

Figure 2. Comparison of the structure of the tetrahedral intermediates formed in the reactions catalyzed by glutamine and Y glutamylcysteine synthetase and of L-methionine-S-sulfoximine phosphate. (From 34).

Other possible analogs, which will only be mentioned here, are L-methionine-(S)-sulfoximine phosphate and L-benzylsuccinate these compoimds bind tightly to glutamine ssmthetase and carbo peptidase, respectively, and can resemble to some extent the tetrahedral-like transition states. 

Our studies of the substrates [glutamate (I) and ATP] and of substrate analogs [AMP-P-(CH2)-P and methionine sulfoximine] reveal interactions between both substrate sites and both metal ion sites. Previously mentioned studies by Meister s group showed that the irreversible inhibition of glutamine synthetase in the presence of L-methionine (S)-sulfoximine and ATP was due to formation of the sulfoximine phosphate (IV). The tetrahedral geometry at the sulfur atom of the sulfoximine was suggested to be a mimic of the active structure of the adduct of y-glutamyl phosphate and ammonia (III). Data in our laboratory provide spectroscopic evidence that methionine... 

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