@article { ISI:000295988100003,
	title = {Dissecting electrostatic interactions in Bacillus circulans xylanase through NMR-monitored pH titrations},
	journal = {JOURNAL OF BIOMOLECULAR NMR},
	volume = {51},
	number = {1-2},
	year = {2011},
	month = {SEP},
	pages = {5-19},
	abstract = {NMR-monitored pH titration curves of proteins provide a rich source of structural and electrostatic information. Although relatively straightforward to measure, interpreting pH-dependent chemical shift changes to obtain site-specific acid dissociation constants (pK(A) values) is challenging. In order to analyze the biphasic titrations exhibited by the side chain (13)C(gamma) nuclei of the nucleophilic Glu78 and general acid/base Glu172 in Bacillus circulans xylanase, we have revisited the formalism for the ionization equilibria of two coupled acidic residues. In general, fitting NMR-monitored pH titration curves for such a system will only yield the two macroscopic pK(A) values that reflect the combined effects of both deprotonation reactions. However, through the use of mutations complemented with ionic strength-dependent measurements, we are able to extract the four microscopic pK(Ai) values governing the branched acid/base equilibria of Glu78 and Glu172 in BcX. These data, confirmed through theoretical calculations, help explain the pH-dependent mechanism of this model GH11 xylanase by demonstrating that the kinetically determined pK(A) values and hence catalytic roles of these two residues result from their electrostatic coupling.},
	issn = {0925-2738},
	doi = {10.1007/s10858-011-9537-x},
	author = {McIntosh, Lawrence P. and Naito, Daigo and Baturin, Simon J. and Okon, Mark and Joshi, Manish D. and Nielsen, Jens E.}
}
