Inhibition of plant glutamine synthetases by substituted phosphinothricins

Glutamine synthetase (GS) utilizes various substituted glutamic acids as substrates. We have used this information to design herbicidal alpha- and gamma-substituted analogs of phosphinothricin (l-2-amino-4-(hydroxymethylphosphinyl)butanoic acid, PPT), a naturally occurring GS inhibitor and a potent herbicide. The substituted phosphinothricins inhibit cytosolic sorghum GS(1) and chloroplastic GS(2) competitively versusl-glutamate, with K(i) values in the low micromolar range. At higher concentrations, these inhibitors inactivate glutamine synthetase, while dilution restores activity through enzyme-inhibitor dissociation. Herbicidal phosphinothricins exhibit low K(i) values and slow enzyme turnover, as described by reactivation characteristics. Both the GS(1) and GS(2) isoforms of plant glutamine synthetase are similarly inhibited by the phosphinothricins, consistent with the broad-spectrum herbicidal activity observed for PPT itself as well as other active compounds in this series.

Inhibition of Escherichia coli glutamine synthetase by alpha- and gamma-substituted phosphinothricins

We have investigated the inhibition of Escherichia coli glutamine synthetase (GS) with alpha- and gamma-substituted analogues of phosphinothricin [L-2-amino-4-(hydroxymethylphosphinyl)butanoic acid (PPT)], a naturally occurring inhibitor of GS. These compounds display inhibition of bacterial GS that is competitive vs L-glutamate, with Ki values in the low micromolar range. At concentrations greater than Ki the phosphinothricins caused time-dependent loss of enzyme activity, while dilution after enzyme inactivation resulted in recovery of enzyme activity. ATP was required for inactivation; the nonhydrolyzable ATP analogue AMP-PCP failed to support inhibition of GS by the phosphinothricins. The binding of these inhibitors to the enzyme was also characterized by measurement of changes in protein fluorescence, which provided similar inactivation rate constants k1 and k2 for the entire series of compounds. Rate constants koff for recovery were also determined by fluorescence measurement and were comparable for both PPT and the gamma-hydroxylated analogue GHPPT and significantly greater for the alpha- and gamma-alkyl-substituted compounds. Electron paramagnetic resonance spectra provided information on the interaction of the phosphinothricins with the manganese form of the enzyme in the absence of ATP, and significant binding was observed for PPT and GHPPT. 31P NMR experiments confirmed that enzyme inactivation is accompanied by hydrolysis of ATP, although phosphorylated phosphinothricins could not be detected in solution. The kinetic behavior of these compounds is consistent with a mechanism involving inhibitor phosphorylation, followed by release from the active site and simultaneous hydrolysis to form Pi and free inhibitor.

Substrate variability as a factor in enzyme inhibitor design: inhibition of ovine brain glutamine synthetase by alpha- and gamma-substituted phosphinothricins

Ovine brain glutamine synthetase (GS) utilizes various substituted glutamic acids as substrates. We have used this information to design alpha- and gamma-substituted analogues of phosphinothricin [L-2-amino-4-(hydroxymethylphosphinyl)butanoic acid], a naturally occurring inhibitor of GS. These compounds display competitive inhibition of GS, and a correlation between the inhibitor Ki values and the Km/Vmax values of the analogously substituted glutamates supports the hypothesis that the phosphinothricins participate in transition-state analogue inhibition of GS. At concentrations greater than Ki these inhibitors caused biphasic time-dependent loss of enzyme activity, with initial pseudo-first-order behavior; k'inact parameters were determined for several compounds and were similar to the 2.1 X 10(-2)s-1 value measured for PPT. Dilution after GS inactivation caused a non-first-order recovery of activity. Reactivation kinetics were insensitive to inhibitor and ADP concentrations over wide ranges, although very high postdilution concentrations of inhibitor suppressed reactivation. The burst activity level, beta, as well as the concentration of inhibitor required to suppress reactivation to this level, mu, expressed as a multiple of the Ki value, was characteristic for each compound in the phosphinothricin series. Increasing substitution of the phosphinothricin parent structure caused an increase in Ki values as well as in the inactivation/reactivation parameters. The kinetic behavior of these inhibitors is consistent with a mechanistic scheme involving initial phosphorylation and rapid partial inhibitor dissociation, followed by slow release of remaining bound inhibitor.