Solution structure of the 40,000 Mr phosphoryl transfer complex between the N-terminal domain of enzyme I and HPr

The solution structure of the first protein–protein complex of the bacterial phosphoenolpyruvate: sugar phosphotransferase system between the N–terminal domain of enzyme I (EIN) and the histidine–containing phosphocarrier protein HPr has been determined by NMR spectroscopy, including the use of residual dipolar couplings that provide long–range structural information. The complex between EIN and HPr is a classical example of surface complementarity, involving an essentially all helical interface, comprising helices 2, 2', 3 and 4 of the α–subdomain of EIN and helices 1 and 2 of HPr, that requires virtually no changes in conformation of the components relative to that in their respective free states. The specificity of the complex is dependent on the correct placement of both van der Waals and electrostatic contacts. The transition state can be formed with minimal changes in overall conformation, and is stabilized in favor of phosphorylated HPr, thereby accounting for the directionality of phosphoryl transfer.