Action patterns of feedback modifiers on equilibrium exchanges and applications to glutamine synthetase (Escherichia coli W).

Theoretical and experimental results are presented to illustrate the applicability of equilibrium rate measurements by isotope exchange to the understanding of the effect of modifiers on enzymic catalysis. A table summarizes the expected theoretical patterns for a two-substrate, two-product system with random substrate addition and ternary complex interconversion. Such patterns are used as a first approximation model for interpreting rate measurements with Escherichia coli glutamine synthetase for the glutamate ti glutamine, ATP & Pi, NH3 $ glutamine, and Oglutamate F! Opi exchanges. These measurements with nine different modifiers showed strikingly different inhibitory patterns, including apparent complete block of chemical interconversion, and differential interference with various substrate-binding and release steps. A new type of modifier action by GDP and AMP is reported, in which the glutamate F! glutamine exchange is unimpaired but the ATP * Pi exchange is completely inhibited. This is interpreted as a cryptic catalysis in which the modifier prevents dissociation of the ATP, but still allows the rapid chemical interconversion of bound substrates and rapid exchange between the NH3glutamine and glutamate-glutamine pools, as well as 180 between glutamate and Pi.