Design, Synthesis, and Biochemical Evaluation of Phosphonate and Phosphonamidate Analogs of Glutathionylspermidine as Inhibitors of Glutathionylspermidine Synthetase/Amidase from Escherichia coli

Three phosphapeptides designed to mimic two distinct tetrahedral intermediates formed during either the synthesis or hydrolysis of glutathionylspermidine (Gsp) were synthesized and evaluated as inhibitors of the bifunctional enzyme Gsp synthetase/amidase. While the polyamine-containing phosphapeptides were determined to be potent and selective inhibitors, they selectively inhibit the synthetase activity over the amidase domain. A phosphonate-containing tetrahedral mimic is a reversible mixed-type inhibitor of Gsp synthetase with an inhibition constant of 6 μM for the inhibitor binding to the free enzyme (K i) and 14 μM for the inhibitor binding to the enzyme−substrate complex (K i‘). The corresponding phosphonamidate is a slow-binding inhibitor with a K i of 24 μM and a K i* (isomerization inhibition constant) of 0.88 μM. A non-polyamine-containing phosphonamidate exhibits no significant inhibition of the synthetase or amidase activity.