Allosteric regulation of monocyclic interconvertible enzyme cascade systems: Use of Escherichia coli glutamine synthetase as an experimental model

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The interconversion of Escherichia coli glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] between its adenylylated and unadenylylated forms has been used to verify the prediction derived from a theoretical analysis of the steady-state functions of a model for a monocyclic interconvertible enzyme cascade system [Stadtman, E. R. & Chock, P. B. (1977) Proc. Natl. Acad. Sci. USA 74, 2761-2770]. Because glutamine and α-ketoglutarate are multifunctional effectors and because three active enzyme complexes are involved in both adenylylation and deadenylylation of glutamine synthetase, at least 28 constants are required to describe the glutamine synthetase monocyclic cascade. Of these, 22 constants were determined experimentally and 6 were estimated via computer curve fitting. Despite the complexity, when both adenylylation and deadenylylation reactions are functioning, the number of adenylyl groups bound per mole of enzyme, n̄, assumes a steady-state level as is predicted by the model. This n̄ value is determined by the mole fraction of PIIA—given by ([PIIA]/([PIIA] + [PIID])—and the ratio of glutamine to α-ketoglutarate (PIID and PIID are the unmodified and the uridylylated forms of the PII regulatory protein). In the presence of 0.5 mM glutamine and 2 mM α-ketoglutarate, the value of n̄ increases as a nearly hyperbolic function in response to increasing mole fractions of PIIA. When the constant level of α-ketoglutarate is gradually increased to 40 μM, the hyperbolic function converts slowly to a parabolic function. When the PIIA mole fraction was maintained at 0.6 and α-ketoglutarate levels were varied from 1 mM to 4 μM, an 800-fold increase in signal amplification was observed with respect to glutamine activation. In addition, because glutamine activates the adenylylation and inhibits the deadenylylation reaction, a sensitivity index of 2.1 (corresponding to a Hill number of 1.5) was obtained for the variation of n̄ values in response to increasing glutamine concentration.

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