Electrostatic Potential Energy within a Protein Monitored by Metal Charge-Dependent Hydrogen Exchange

¹ Union College
² Wadsworth Center - NYSDOH / University of Albany - SUNY

^* To whom correspondence should be addressed. E-mail: griselda{at}wadsworth.org.

Submitted on July 28, 2006
Revised on September 8, 2006
Accepted on 21 September 2006

	Abstract

Hydrogen exchange measurements on Zn(II)-, Ga(III)- and Ge(IV)-substituted Pyrococcus furiosus rubredoxin demonstrate that the log ratio of the base-catalyzed rate constants (log k_ex) varies inversely with the distance out to at least 12 Å from the metal. This pattern is consistent with the variation of the amide nitrogen pK values with the metal charge-dependent changes in the electrostatic potential. Fifteen monitored amides lie within this range, providing an opportunity to assess the strength of electrostatic interactions simultaneously at numerous positions within the structure. Poisson-Boltzmann calculations predict an optimal effective internal dielectric constant of 6. The largest deviations between the experimentally estimated and the predicted pK values appear to result from the conformationally mobile charged sidechains of Lys 7 and Glu 48 and from differential shielding of the peptide units arising from their orientation relative to the metal site.

Key Words: electrostatic potential, hydrogen exchange, pK, rubredoxin