Fluorine-19 nuclear magnetic resonance study of 5-fluorotryptophan-labeled histidine-binding protein J of Salmonella typhimurium.

Fluorine-19 nuclear magnetic resonance has been used to investigate the histidine-binding protein J from Salmonella typhimurium. The protein has been labeled with fluorine-19 by growing the bacterial cells of a tryptophan auxotroph in the presence of 5-fluorotryptophan. Incorporation of up to 70% was achieved. The binding of L-histidine to the 19F-labeled protein is not affected by the isotopic labeling. The protein contains one tryptophan residue, giving rise to a single 19F resonance. Upon binding L-histidine to 19F-labeled histidine-binding protein J, the observed 19F resonance is shifted downfield by about 0.6 parts per million, indicating a conformational change of the protein molecule and a more hydrophobic environment for the 19F nucleus. Additional fluorescence experiments confirm that the tryptophan residue is located inside the hydrophobic core of the protein. 19F spin-lattice relaxation times of the 19F-labeled protein as a function of temperature show no difference between the free protein and the protein-histidine complex. However, the linewidth for the free protein is much larger than that of the protein-substrate complex. This can be explained by slow fluctuations between different conformations of the free protein molecule having slightly different 19F chemical shifts. Both with and without the substrate, the tryptophan residue is immobile inside the protein molecule as shown by the total disappearance of the 19F signal upon broadband irradiation at the 1H frequency. Also, the 19F spin-lattice relaxation times indicate that the protein is a rather rigid structure, in which rapid motions of the tryptophan residue on the time scale of 10(-8) second are not prominent.