8.A.7 The Phosphotransferase System Enzyme I (EI) Family
Enzymes I (EC# 2.A.7.9.3) of the phosphotransferase system (PTS) serve as energy coupling proteins for PTS porters. Phosphoenolpyruvate (PEP) is the energy source and phosphoryl donor. PEP phosphorylates a histidyl residue in EI; this is then passed to a histidyl residue in a heat stable phopho-carrier protein called HPr, and the phosphoryl group is then transferred to a histidyl residue in the IIA domain/protein of a PTS porter (TC #4.A.1-4.A.7) (Brackenbury and Isom, 2011).
Enzymes I are homologous to PEP synthases (EC# 220.127.116.11) and pyruvate:orthophosphate dikinases (EC 18.104.22.168). Five Enzyme I paralogues are encoded within the genome of E. coli. The functions of several of these proteins are not known, but they probably function in PTS-related transport, phosophorylation or regulatory capacities.