Mechanism of inactivation of ornithine transcarbamoylase by N-delta-(N '-sulfodiaminophosphinyl)-L-ornithine, a true transition state analogue? Crystal structure and implications for catalytic mechanism

The crystal structure is reported at 1.8 Angstrom resolution of Escherichia coli ornithine transcarbamoylase in complex with the active derivative of phaseolotoxin from Pseudomonas syringae pv. phaseolicola, N-delta-(N'-sulfo diaminophosphinyl)-L-ornithine. Electron density reveals that the complex i s not a covalent adduct as previously thought. Kinetic data confirm that N- delta-(N'-sulfodiaminophosphinyl)-L-ornithine exhibits reversible inhibitio n with a half-life in the order of similar to 22 h and a dissociation const ant of K-D = 1.6 x 10(-12) M at 37 degrees C and pH 8.0. Observed hydrogen bonding about the chiral tetrahedral phosphorus of the inhibitor is consist ent only with the presence of the R enantiomer, A strong interaction is als o observed between Arg(57) N epsilon and the P-N-S bridging nitrogen indica ting that imino tautomers of N-delta-(N'-sulfodiaminophosphinyl)-L-ornithin e are present in the bound state. An imino tautomer of N-delta-(N'-sulfodia minophosphinyl)-L-ornithine is structurally analogous to the proposed react ion transition state. Hence, we propose that N-delta-(N'-sulfodiaminophosph inyl)-L-ornithine, with its three unique N-P bonds, represents a true trans ition state analogue for ornithine transcarbamoylases, consistent with the tight binding kinetics observed.