T. Simonson et al., A Poisson-Boltzmann study of charge insertion in an enzyme active site: The effect of dielectric relaxation, J PHYS CH B, 103(29), 1999, pp. 6142-6156
Continuum solvent models are playing an increasing role in the study of aqu
eous solutions, particularly those involving protein solutes. To estimate t
he magnitude of dielectric relaxation and clarify the microscopic meaning o
f the protein dielectric constant, charge insertion in the active site of t
he enzyme aspartyl-tRNA synthetase (AspRS) is analyzed using finite-differe
nce Poisson-Boltzmann calculations. The insertion process is a simplified m
odel that mimics qualitatively the mutation of substrate Asp into Asn, stud
ied earlier by free; energy simulations. A two-step insertion path gives th
e relaxation and nonrelaxation ("static") free energy components separately
. The assumption of linear response leads to a linear relation between the
two components, connecting the explicit structural differences between reac
tant and product structures with the relaxation free energy calculated from
either structure. This relation is verified here only if protein dielectri
c constants of 1 and 4-8 are used for the static and relaxation free energi
es, respectively. These are also the only conditions that give reasonable a
greement with the Asp --> Asn free energy simulated earlier and with a mole
cular dynamics/linear response estimate of the present charging free energy
. The use of two protein dielectric constants represents a significant depa
rture from standard continuum models. The values obtained are physically re
asonable: a protein dielectric of 4-8 for relaxation indicates that the act
ive site of AspRS, though highly polar, is only moderately polarizable. A d
ielectric of one for the static term indicates that the charge set, optimiz
ed for explicit solvent simulations, reproduces the equilibrium potential w
ithout the need for additional, implicit protein polarization. In contrast
to simple charge insertion, the binding free energies of Asp and Asn to Asp
RS are best calculated with a more standard protocol that uses a single pro
tein dielectric of 4, accurately reproducing free energy simulation results
. For this and other binding processes, additional fi ee energy components
are involved, related to desolvation of the binding site; the optimal diele
ctric constant represents an empirical compromise among these. A multistep
component analysis could also be used to analyze the role of relaxation in
these more complex processes. It is suggested that the use of more than one
dielectric constant in continuum models will lead to a more consistent and
robust description of dielectric phenomena in solution.