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Localized thermodynamic coupling between hydrogen bonding and microenvironment polarity substantially stabilizes proteins.

ABSTRACT: The energetic contributions of hydrogen bonding to protein folding are still unclear, despite more than 70 years of study. This is due partly to the difficulty of extracting thermodynamic information about specific interactions from protein mutagenesis data and partly to the context dependence of hydrogen bond strengths. Herein, we test the hypothesis that hydrogen bond strengths depend on the polarity of their microenvironment, with stronger hydrogen bonds forming in nonpolar surroundings. Double-mutant cycle analysis using a combination of amide-to-ester backbone mutagenesis and traditional side chain mutagenesis revealed that hydrogen bonds can be stronger by up to 1.2 kcal mol(-1) when they are sequestered in hydrophobic surroundings than when they are solvent exposed. Such large coupling energies between hydrogen bond strengths and local polarity suggest that the context dependence of hydrogen bond strengths must be accounted for in any comprehensive account of the forces responsible for protein folding.

SUBMITTER: Gao J

PROVIDER: S-EPMC2754385 | biostudies-literature | 2009 Jul

REPOSITORIES: biostudies-literature

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Localized thermodynamic coupling between hydrogen bonding and microenvironment polarity substantially stabilizes proteins.

Gao Jianmin J Bosco Daryl A DA Powers Evan T ET Kelly Jeffery W JW

Nature structural & molecular biology 20090614 7

The energetic contributions of hydrogen bonding to protein folding are still unclear, despite more than 70 years of study. This is due partly to the difficulty of extracting thermodynamic information about specific interactions from protein mutagenesis data and partly to the context dependence of hydrogen bond strengths. Herein, we test the hypothesis that hydrogen bond strengths depend on the polarity of their microenvironment, with stronger hydrogen bonds forming in nonpolar surroundings. Doub ...[more]

PMID: 19525973

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Project description:Ultraviolet resonance Raman (UVRR) spectra of tryptophan compounds in various solvents and a model peptide are presented and reveal systematic changes that reflect solvent polarity, hydrogen bond strength, and cation-pi interaction. The commonly utilized UVRR spectral marker for environment polarity that has been based on off-resonance Raman data, the tryptophan Fermi doublet ratio I1360/I1340, exhibits different values in on- and off-resonance Raman spectra as well as for different tryptophan derivatives. Specifically, the UVRR Fermi doublet ratio for indole ranges from 0.3 in polar solvents to 0.8 in nonpolar solvents, whereas the respective values reported here and previously for off-resonance Raman spectra are 0.5-1.3. UVRR Fermi doublet ratios for the more biologically relevant molecule, N-acetyl tryptophan ethyl ester (NATEE), are in a smaller range of 1.1 (polar solvent) to 1.7 (nonpolar solvent) and correlate to the solvent polarity/polarization parameters pi* and ETN. As has been reported previously, several UVRR modes are also sensitive to the hydrogen bond strength of the indole N-H moiety. Here, we report a new unambiguous marker for H-bonding: the ratio of the W10 (approximately 1237 cm-1) intensity to that of the W9 (approximately 1254 cm-1) mode (RW10). This ratio is 0.7 for NATEE in the absence of hydrogen bond acceptors and increases to 3.1 in the presence of strong hydrogen bond acceptors, with a value of 2.3 in water. The W8 and W17 modes shift more than +10 and approximately -5 cm-1 upon increase in hydrogen bond strength; this range for W17 is smaller than that reported previously and reflects a more realistic range for proteins and peptides in solution. Finally, our data provide evidence for change in the W18 and W16 relative intensity in the presence of cation-pi interactions. These UVRR markers are utilized to interpret spectra of model membrane-bound systems tryptophan octyl ester and the peptide toxin melittin. These spectra reveal the importance of intra- and intermolecular hydrogen bonding and cation-pi interactions that likely influence the partitioning of membrane-associated biomolecules to lipid bilayers or self-associated soluble oligomers. The UVRR analysis presented here modifies and augments prior reports and provides an unambiguous set of spectral makers that can be applied to elucidate the molecular microenvironment and structure of a wide range of complex systems, including anchoring tryptophan residues in membrane proteins and peptides.

Project description:Computational studies are performed to analyze the physical properties of hydrogen bonds donated by Tyr16 and Asp103 to a series of substituted phenolate inhibitors bound in the active site of ketosteroid isomerase (KSI). As the solution pK(a) of the phenolate increases, these hydrogen bond distances decrease, the associated nuclear magnetic resonance (NMR) chemical shifts increase, and the fraction of protonated inhibitor increases, in agreement with prior experiments. The quantum mechanical/molecular mechanical calculations provide insight into the electronic inductive effects along the hydrogen bonding network that includes Tyr16, Tyr57, and Tyr32, as well as insight into hydrogen bond coupling in the active site. The calculations predict that the most-downfield NMR chemical shift observed experimentally corresponds to the Tyr16-phenolate hydrogen bond and that Tyr16 is the proton donor when a bound naphtholate inhibitor is observed to be protonated in electronic absorption experiments. According to these calculations, the electronic inductive effects along the hydrogen bonding network of tyrosines cause the Tyr16 hydroxyl to be more acidic than the Asp103 carboxylic acid moiety, which is immersed in a relatively nonpolar environment. When one of the distal tyrosine residues in the network is mutated to phenylalanine, thereby diminishing this inductive effect, the Tyr16-phenolate hydrogen bond becomes longer and the Asp103-phenolate hydrogen bond shorter, as observed in NMR experiments. Furthermore, the calculations suggest that the differences in the experimental NMR data and electronic absorption spectra for pKSI and tKSI, two homologous bacterial forms of the enzyme, are due predominantly to the third tyrosine that is present in the hydrogen bonding network of pKSI but not tKSI. These studies also provide experimentally testable predictions about the impact of mutating the distal tyrosine residues in this hydrogen bonding network on the NMR chemical shifts and electronic absorption spectra.

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Localized thermodynamic coupling between hydrogen bonding and microenvironment polarity substantially stabilizes proteins.

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Localized thermodynamic coupling between hydrogen bonding and microenvironment polarity substantially stabilizes proteins.

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