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Biophys J, September 2002, p. 1341-1347, Vol. 83, No. 3
Department of Physics, Drexel University, Philadelphia, Pennsylvania 19104 USA
We carried our Poisson-Boltzmann (PB) calculations for
the effects of charge reversal at five exposed sites (K16E, R119E, K135E, K147E, and R154E) and charge neutralization and proton titration
of the H31-D70 semi-buried salt bridge on the stability of T4 lysozyme.
Instead of the widely used solvent-exclusion (SE) surface, we used the
van der Waals (vdW) surface as the boundary between the protein and
solvent dielectrics (a protocol established in our earlier study on
charge mutations in barnase). By including residual charge-charge
interactions in the unfolded state, the five charge reversal mutations
were found to have 
Gunfold from
1.6
to 1.3 kcal/mol. This indicates that the variable effects of charge
reversal observed by Matthews and co-workers are not unexpected.
The H31N, D70N, and H31N/D70N mutations were found to destabilize the
protein by 2.9, 1.3, and 1.6 kcal/mol, and the pKa values
of H31 and D70 were shifted to 9.4 and 0.6, respectively. These results
are in good accord with experimental data of Dahlquist and co-workers.
In contrast, if the SE surface were used, the H31N/D70N mutant would be
more stable than the wild-type protein by 1.3 kcal/mol. From these and
additional results for 27 charge mutations on five other proteins, we
conclude that 1) the popular view that electrostatic interactions are
generally destabilizing may have been based on overestimated
desolvation cost as a result of using the SE surface as the dielectric
boundary; and 2) while solvent-exposed charges may not reliably
contribute to protein stability, semi-buried salt bridges can provide
significant stabilization.
Biophys J, September 2002, p. 1341-1347, Vol. 83, No. 3
© 2002 by the Biophysical Society 0006-3495/02/09/1341/07 $2.00
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