Phosphocarrier protein
Histidine-containing phosphocarrier protein is a small cytoplasmic protein that is a component of the phosphoenolpyruvate-dependent sugar phosphotransferase system.
The phosphoenolpyruvate-dependent sugar phosphotransferase system is a major carbohydrate transport system in bacteria. The PTS catalyses the phosphorylation of sugar substrates during their translocation across the cell membrane. The mechanism involves the transfer of a phosphoryl group from phosphoenolpyruvate via enzyme I to enzyme II of the PTS system, which in turn transfers it to a phosphocarrier protein. In some bacteria HPr is a domain in a larger protein that includes an EIII domain and in some cases also an EI domain.
There is a conserved histidine in the N-terminus of HPr, which serves as an acceptor for the phosphoryl group of EI. In the central part of HPr there is a conserved serine which, in most Gram-positive bacteria and certain Gram-negative bacteria, is phosphorylated by an ATP-dependent protein kinase, a process which probably plays a regulatory role in sugar transport.