Studies on the Reaction Mechanism of Adenosine Triphosphate : Glutamine Synthetase Adenylyl- transferase from Escherichia coli B

Glutamine establishes feedback control of glutamine synthetase (EC 6.3.1.2) in Escherichia coli over a novel loop, viz. by its effect on an enzymatic adenylylation of glutamine synthetase, whose activity is thus modulated. Positive effecters of the adenylyl transfer reaction, magnesium ion and glutamine, increase the number of -SH groups in adenylyltransferase (EC 2.7.7. -) titratable by 5,5’-dithiobis(2-nitrobenzoic acid); inhibitors of the transfer reaction, or-ketoglutarate and 3-phosphoglycerate, decrease the number of titratable -SH groups. It is concluded that effector control of the adenylylation reaction is exerted at the level of adenylyltransferase. The substrates of the adenylyltransfer reaction, MgATP and nonadenylylated glutamine synthetase also expose additional -SH groups of adenylyltransferase to titration. A substrate of the reverse reaction (pyrophosphorolysis of AMP-glutamine synthetase) MgPPi’buries -SH groups. The relationship between the number of -SH groups modified and residual enzyme activity is established.