General (non sugar-specific) component of the phosphoenolpyruvate-dependent sugar phosphotransferase system (sugar PTS). This major carbohydrate active-transport system catalyzes the phosphorylation of incoming sugar substrates concomitantly with their translocation across the cell membrane. Enzyme I transfers the phosphoryl group from phosphoenolpyruvate (PEP) to the phosphoryl carrier protein (HPr). L-histidyl-[protein] + phosphoenolpyruvate = N(pros)-phospho-L-histidyl-[protein] + pyruvate Homodimer. Transcription of ptsI appears to be regulated in response to sugars supplied, i.e. is four-fold lower when cells were grown on lactose than when grown on glucose. The N-terminal domain contains the HPr binding site, the central domain the pyrophosphate/phosphate carrier histidine, and the C-terminal domain the pyruvate binding site. The reaction takes place in three steps, mediated by a phosphocarrier histidine residue located on the surface of the central domain. The two first partial reactions are catalyzed at an active site located on the N-terminal domain, and the third partial reaction is catalyzed at an active site located on the C-terminal domain. For catalytic turnover, the central domain swivels from the concave surface of the N-terminal domain to that of the C-terminal domain. Belongs to the PEP-utilizing enzyme family.