In this study, a GlcNAc-6-O-Sulfotransferase, NodST and its complexation with the substrate 3Ј-phosphoadenosine 5Ј-phosphosulfate (PAPS) and the inhibitor 3Ј-phosphoadenosine 5Ј-phosphate (PAP) were studied using Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. In addition, using isotopically labeled substrate, we have successfully confirmed a sulfated enzyme intermediate, which was predicted by the MS kinetic measurement. It is also shown that information regarding solution binding affinities can be obtained using electrospray ionization (ESI)-FTICR mass spectrometry. The relative binding constants, K d (PAPS)/ K d (PAP), derived from the solution and gas phase were very similar, which suggests that the binding domain of this particular enzyme system, given known structures of other sulfotransferases, may be preserved during the transmission of the complex from solution to the gas