ACTIVE-CENTER TORSION-ANGLE STRAIN REVEALED IN 1.6 ANGSTROM-RESOLUTION STRUCTURE OF HISTIDINE-CONTAINING PHOSPHOCARRIER PROTEIN

THE histidine-containing phosphocarrier protein (HPr) is a central component of the phosphoenolpyruvate: sugar phosphotransferase system that transports carbohydrates across the cell membrane of bacteria1. A typical phosphotransfer sequence is phosphoenolpyruvate --> enzyme I --> HPr --> enzyme II/III(sugar) -->. This is thermodynamically favourable owing to the participation of the high-energy phosphoenolpyruvate. We report here the structure of HPr from Streptococcus faecalis determined at 1.6 angstrom resolution. Remarkable disallowed Ramachandran torsion angles at the active centre, revealed by the X-ray structure, demonstrate a unique example of torsion-angle strain that is probably directly involved in protein function. During phosphorylation, the active-centre torsion-angle strain should facilitate the phosphotransfer reaction by lowering the activation-energy barrier. A recently reported Bacillus subtilis HPr structure2, which represents the phosphorylated state of HPr with no torsion-angle strain, provides direct evidence supporting our hypothesis that torsion-angle strain plays a direct part in the function of HPr. An HPr phosphotransfer cycling mechanism is proposed, based primarily on the structures of HPr and other phosphotransferase system proteins.