Solution NMR structures of productive and non-productive complexes between the A and B domains of the cytoplasmic subunit of the mannose transporter of the Escherichia coli phosphotransferase system

Solution structures of complexes between the isolated A (IIA(Man)) and B (IIB(Man)) domains of the cytoplasmic component of the mannose transporter of Escherichia coli have been solved by NMR. The complex of wild-type IIA(Man) and IIB(Man) is a mixture of two species comprising a productive, phosphoryl transfer competent complex and a non-productive complex with the two active site histidines, His-10 of IIA(Man) and His-175 of IIB(Man), separated by approximately 25A. Mutation of the active site histidine, His-10, of IIA(Man) to a glutamate, to mimic phosphorylation, results in the formation of a single productive complex. The apparent equilibrium dissociation constants for the binding of both wild-type and H10E IIA(Man) to IIB(Man) are approximately the same (K(D) approximately 0.5 mM). The productive complex can readily accommodate a transition state involving a pentacoordinate phosphoryl group with trigonal bipyramidal geometry bonded to the Nepsilon2 atom of His-10 of IIA(Man) and the Ndelta1 atom of His-175 of IIB(Man) with negligible (<0.2A) local backbone conformational changes in the immediate vicinity of the active site. The non-productive complex is related to the productive one by a approximately 90 degrees rotation and approximately 37A translation of IIB(Man) relative to IIA(Man), leaving the active site His-175 of IIB(Man) fully exposed to solvent in the non-productive complex. The interaction surface on IIA(Man) for the non-productive complex comprises a subset of residues used in the productive complex and in both cases involves both subunits of IIA(Man). The selection of the productive complex by IIA(Man)(H10E) can be attributed to neutralization of the positively charged Arg-172 of IIB(Man) at the center of the interface. The non-productive IIA(Man)-IIB(Man) complex may possibly be relevant to subsequent phosphoryl transfer from His-175 of IIB(Man) to the incoming sugar located on the transmembrane IIC(Man)-IID(Man) complex.