1 research outputs found
Charges in Hydrophobic Environments: A Strategy for Identifying Alternative States in Proteins
In
the V23E variant of staphylococcal nuclease, Glu-23 has a p<i>K</i><sub>a</sub> of 7.5. At low pH, Glu-23 is neutral and buried
in the hydrophobic interior of the protein. Crystal structures and
NMR spectroscopy experiments show that when Glu-23 becomes charged,
the protein switches into an open state in which strands β1
and β2 separate from the β-barrel; the remaining structure
is unaffected. In the open state the hydrophobic interior of the protein
is exposed to bulk water, allowing Glu-23 to become hydrated. This
illustrates several key aspects of protein electrostatics: (1) The
apparent p<i>K</i><sub>a</sub> of an internal ionizable
group can reflect the average of the very different p<i>K</i><sub>a</sub> values (open ≈4.5, closed ≫7.5) sampled
in the different conformational states. (2) The high apparent dielectric
constant reported by the p<i>K</i><sub>a</sub> value of
internal ionizable group reflects conformational reorganization. (3)
The apparent p<i>K</i><sub>a</sub> of internal groups can
be governed by large conformational changes. (4) A single charge buried
in the hydrophobic interior of a protein is sufficient to convert
what might have been a transient, partially unfolded state into the
dominant state in solution. This suggests a general strategy for examining
inaccessible regions of the folding landscape and for engineering
conformational switches driven by small changes in pH. These data
also constitute a benchmark for stringent testing of the ability of
computational algorithms to predict p<i>K</i><sub>a</sub> values of internal residues and to reproduce pH-driven conformational
transitions of proteins