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# and pyruvate:orthophosphate dikinases (EC 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.

This family belongs to the .



Araki N., Suzuki T., Miyauchi K., Kasai D., Masai E. and Fukuda M. (201). Identification and characterization of uptake systems for glucose and fructose in Rhodococcus jostii RHA1. J Mol Microbiol Biotechnol. 20(3):125-36.

Brackenbury, W.J. and L.L. Isom. (2011). Na Channel β Subunits: Overachievers of the Ion Channel Family. Front Pharmacol 2: 53.

Pickl A., Johnsen U. and Schonheit P. (2012). Fructose degradation in the haloarchaeon Haloferax volcanii involves a bacterial type phosphoenolpyruvate-dependent phosphotransferase system, fructose-1-phosphate kinase, and class II fructose-1,6-bisphosphate aldolase. J Bacteriol. 194(12):3088-97.

Postma, P.W., J.W. Lengeler and G.R. Jacobson (1993). Phosphoenolpyruvate:carbohydrate phosphotransferase systems of bacteria. Microbiol. Rev. 57: 543-594.

Reizer, J., C. Hoischen, A. Reizer, T.N. Pham and M.H. Saier, Jr. (1993). Sequence analyses and evolutionary relationships among the energy-coupling proteins Enzyme I and HPr of the bacterial phosphoenolpyruvate:sugar phosphotransferase system. Prot. Sci. 2: 506-521.


TC#NameOrganismal TypeExample
8.A.7.1.1Enzyme I of the PTS Bacteria Enzyme I of E. coli

Enzyme I of the PTS (Araki et al., 2011)


Enzyme I of Rhodococcus jostii (Q0S1M9)


Phosphoenolpyruvate: protein phosphotransferase, PTS Enzyme I (based on homology)


Enzyme I of Haloterrigena turkmenica (D2RXA5)


Enzyme I, PtsI (functions with 4.A.2.1.15; Pickl et al., 2012)


Enzyme I of Haloferax volcanii (D4GYE2)


Enzyme I of 591 aas.  Enzyme I transfers the phosphoryl group from phosphoenolpyruvate (PEP) to the phosphoryl carrier protein (HPr).  This protein is encoded in the same operon with an HPr (TC# 8.A.8.1.5) and the IIC and IID genes of a mannose-type PTS system (TC# 4.A.6.1.20).

Enzyme I of Caldithrix abyssi


Enzyme I of an archaeal mannose (Man)-type PTS; 566 aas.

Enzyme I of Thermofilum pendens