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.



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