Here we show that, as a consequence of binding the drug trifluoperazine, a major conformational movement occurs in Ca(2+)-calmodulin (CaM). The tertiary structure changes from an elongated dumb-bell, with exposed hydrophobic surfaces, to a compact globular form which can no longer interact with its target enzymes. It is likely that inactivation of Ca(2+)-CaM by trifluoperazine is due to this major tertiary-structural alteration in Ca(2+)-CaM, which is initiated and stabilized by drug binding. This conformational change is similar to that which occurs on the binding of Ca(2+)-CaM to target peptides. Two hydrophobic binding pockets, created by amino acid residues adjacent to Ca(2+)-coordinating residues, form the key recognition sites on Ca(2+)-CaM for both inhibitors and target enzymes.
The histidine-containing phosphocarrier protein (HPr) is a central component of the phosphoenolpyruvate: sugar phosphotransferase system that transports carbohydrates across the cell membrane of bacteria. A typical phosphotransfer sequence is phosphoenolpyruvate-->enzyme I-->HPr-->enzyme II/IIIsugar-->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 A 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 structure, 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.
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