Abstract
Abstract The regulatory enzyme ATP:glutamine synthetase adenylyltransferase (EC 2.7.7.-) catalyzes the transfer of AMP groups to glutamine synthetase (EC 6.3.1.2) from Escherichia coli, thereby diminishing the latter's ability to synthesize glutamine. The course of the transfer reaction has been investigated with particular reference to the possible existence of an AMP-adenylyltransferase intermediate. Direct attempts to isolate such a species were unsuccessful. Measurements of [32P]pyrophosphate-ATP exchange failed to support the existence of the reaction ATP + adenylyltransferase ⇌ AMP-adenylyltransferase + PPi A kinetic analysis showed initial velocities of the transfer conformed to the rate equation of an ordered mechanism. It was concluded therefore that the reaction occurs in a ternary complex of glutamine synthetase, MgATP, and adenylyltransferase, and that a stable AMP-adenylyltransferase does not exist. Values for the Michaelis constants with respect to glutamine synthetase and MgATP were found to be 1.1 and 3.9 mm, respectively. For purposes of correcting the kinetic data for MgATP bound to cosubstrate glutamine synthetase, the association constant between these two substances was measured.
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