Substrate synergism and the steady-state kinetic reaction mechanism for EPSP synthase from Escherichia coli.

Previous studies of Escherichia coli 5-enolpyruvoylshikimate-3-phosphate synthase (EPSPS, EC 2.5.1.19) have suggested that the kinetic reaction mechanism for this enzyme in the forward direction is equilibrium ordered with shikimate 3-phosphate (S3P) binding first followed by phosphoenolpyruvate (PEP). Recent results from this laboratory, however, measuring direct binding of PEP and PEP analogues to free EPSPS suggest more random character to the enzyme. Steady-state kinetic and spectroscopic studies presented here indicate that E. coli EPSPS does indeed follow a random kinetic mechanism. Initial velocity studies with S3P and PEP show competitive substrate inhibition by PEP added to a normal intersecting pattern. Substrate inhibition is proposed to occur by competitive binding of PEP at the S3P site [Ki(PEP) = 6-8 mM]. To test for a productive EPSPS.PEP binary complex, the reaction order of EPSPS was evaluated with shikimic acid and PEP as substrates. The mechanism for this reaction is equilibrium ordered with PEP binding first giving a Kia value for PEP in agreement with the independently measured Kd of 0.39 mM (shikimate Km = 25 mM). Results from this study also show that the 3-phosphate moiety of S3P offers 8.7 kcal/mol in binding energy versus a hydroxyl in this position. Over 60% of this binding energy is expressed in binding of substrate to enzyme rather than toward increasing kcat. Glyphosate inhibition of shikimate turnover was poor with approximately 8 x 10(4) loss in binding capacity compared to the normal reaction, consistent with the independently measured Kd of 12 mM for the EPSPS.glyphosate binary complex. The EPSPS.glyphosate complex induces shikimate binding, however, by a factor of 7 greater than EPSPS.PEP. Carboxyallenyl phosphate and (Z)-3-fluoro-PEP were found to be strong inhibitors of the enzyme that have surprising affinity for the S3P binding domain in addition to the PEP site as measured both kinetically and by direct observation with 31P NMR. The collective data indicate that the true kinetic mechanism for EPSPS in the forward direction is random with synergistic binding occurring between substrates and inhibitors. The synergism explains how the mechanism can be random with S3P and PEP, but yet equilibrium ordered with PEP binding first for shikimate turnover. Synergism also accounts for how glyphosate can be a strong inhibitor of the normal reaction, but poor versus shikimate turnover.