Investigation of a side-chain-side-chain hydrogen bond by mutagenesis, thermodynamics, and NMR spectroscopy.
Anomalous NMR behavior of the hydroxyl proton resonance for Ser 31 has been reported for histidine-containing protein (HPr) from two microorganisms: Escherichia coli and Staphylococcus aureus. The unusual slow exchange and chemical shift exhibited by the resonance led to the proposal that the hydroxyl group is involved in a strong hydrogen bond. To test this hypothesis and to characterize the importance of such an interaction, a mutant in which Ser 31 is replaced by an alanine was generated in HPr from Escherichia coli. The activity, stability, and structure of the mutant HPr were assessed using a reconstituted assay system, analysis of solvent denaturation curves, and NMR, respectively. Substitution of Ser 31 yields a fully functional protein that is only slightly less stable (delta delta G(folding) = 0.46 +/- 0.15 kcal mol-1) than the wild type. The NMR results confirm the identity of the hydrogen bond acceptor as Asp 69 and reveal that it exists as the gauche- conformer in wild-type HPr in solution but exhibits conformational averaging in the mutant protein. The side chain of Asp 69 interacts with two main-chain amide proteins in addition to its interaction with the side chain of Ser 31 in the wild-type protein. These results indicate that removal of the serine has led to the loss of all three hydrogen bond interactions involving Asp 69, suggesting a cooperative network of interactions. A complete analysis of the thermodynamics was performed in which differences in side-chain hydrophobicity and conformational entropy between the two proteins are accounted for.(ABSTRACT TRUNCATED AT 250 WORDS)
Hammen
P K
PK
Department of Biochemistry, University of Washington, Seattle 98195, USA.
Scholtz
J M
JM
Anderson
J W
JW
Waygood
E B
EB
Klevit
R E
RE
D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001483:Base Sequence; D004795:Enzyme Stability; D004926:Escherichia coli; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016296:Mutagenesis; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation; D011489:Protein Denaturation; D017433:Protein Structure, Secondary; D012694:Serine / Q000235:genetics; D013211:Staphylococcus aureus; D013816:Thermodynamics; D014508:Urea
D001426:Bacterial Proteins; D012694:Serine; D014508:Urea; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System; C030182:phosphocarrier protein HPr