Quantifying Additive Interactions of the Osmolyte
Proline with Individual Functional Groups of Proteins: Comparisons
with Urea and Glycine Betaine, Interpretation of m‑Values
posted on 2013-09-03, 00:00authored byRoger
C. Diehl, Emily J. Guinn, Michael W. Capp, Oleg V. Tsodikov, M. Thomas Record
To quantify interactions of the osmolyte l-proline with
protein functional groups and predict their effects on protein processes,
we use vapor pressure osmometry to determine chemical potential derivatives
dμ2/dm3 = μ23, quantifying the preferential interactions of proline (component
3) with 21 solutes (component 2) selected to display different combinations
of aliphatic or aromatic C, amide, carboxylate, phosphate or hydroxyl
O, and amide or cationic N surface. Solubility data yield μ23 values for four less-soluble solutes. Values of μ23 are dissected using an ASA-based analysis to test the hypothesis
of additivity and obtain α-values (proline interaction potentials)
for these eight surface types and three inorganic ions. Values of
μ23 predicted from these α-values agree with
the experiment, demonstrating additivity. Molecular interpretation
of α-values using the solute partitioning model yields partition
coefficients (Kp) quantifying the local
accumulation or exclusion of proline in the hydration water of each
functional group. Interactions of proline with native protein surfaces
and effects of proline on protein unfolding are predicted from α-values
and ASA information and compared with experimental data, with results
for glycine betaine and urea, and with predictions from transfer free
energy analysis. We conclude that proline stabilizes proteins because
of its unfavorable interactions with (exclusion from) amide oxygens
and aliphatic hydrocarbon surfaces exposed in unfolding and that proline
is an effective in vivo osmolyte because of the osmolality increase
resulting from its unfavorable interactions with anionic (carboxylate
and phosphate) and amide oxygens and aliphatic hydrocarbon groups
on the surface of cytoplasmic proteins and nucleic acids.