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Reorganization in apo‐ and holo‐β‐lactoglobulin upon protonation of Glu89: Molecular dynamics and pK a calculations
Author(s) -
Eberini Ivano,
Baptista António M.,
Gianazza Elisabetta,
Fraternali Franca,
Beringhelli Tiziana
Publication year - 2004
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/prot.10643
Subject(s) - protonation , molecular dynamics , chemistry , electrostatics , crystallography , proton , hydrogen bond , crystal structure , computational chemistry , chemical physics , physics , molecule , quantum mechanics , ion , organic chemistry
Molecular dynamics (MD) simulations starting from crystallographic data allowed us to directly account for the effects of the protonation state of Glu89 on the conformational stability of apo‐ and holo‐β‐lactoglobulin (BLG). In apo‐BLG simulations starting from the protonated crystal structure, we observe a long‐lived H‐bond interaction between the protonated Glu89 and Ser116. This interaction, sequestering the proton from the aqueous medium, explains a pK half value evaluated at pH 7.3 by continuum electrostatics/Monte Carlo computation on MD data, using linear response approximation. A very large root‐mean‐square deviation (RMSD; 5.11 Å) is observed for the EF loop between protonated and unprotonated apo‐BLG. This results from a quite different orientation of the EF loop that acts either as a closed or as an open lid above the protein calyx. Proton exchange by Glu89 in apo‐ but not in holo‐BLG is associated with a reorganization energy of 4.7 kcal/mol. A 3‐ns MD simulation starting from the crystal structure of protonated apo‐BLG, but considering the Glu89 as unprotonated, shows the progressive opening of the lid giving rise to the Tanford transition. In both holo‐BLG forms, the lid is most probably held in place by hydrophobic interactions of amino acid side‐chains of the EF loop with the palmitate hydrocarbon tail. Proteins 2004;00:000–000. © 2004 Wiley‐Liss, Inc.