Role of the triad N46, S106 and T107 and the surface charges in the determination of the acidic pH optimum of digestive lysozymes from Musca domestica

Structures of digestive lysozymes 1 and 2 from housefly (MdL1 and MdL2) show that S106-T107 delimit a polar pocket around E32 (catalytic acid/base) and N46 contributes to the positioning of 050 (catalytic nucleophile), whereas those residues are replaced by V109-A110 and D48 in the non-digestive lysozyme from hen egg-white (HEWL). Further analyses revealed that MdL1 and MdL2 surfaces are less positively charged than HEWL surface. To verify the relevance of these differences to the acidic pH optimum of digestive lysozymes it was determined that pKas of the catalytic residues of the triple mutant MdL2 (N46D-S106V-T107A) are similar to HEWL pKas and higher than those for MdL2. In agreement, triple mutant MdL2 and HEWL exhibits the same pH optimum upon methylumbelliferylchitotrioside. In addition to that, the introduction of six basic residues on MdL1 surface increased by 1 unit the pH optimum for the activity upon bacterial walls. Thus, the acidic pH optimum for MdL2 and MdL1 activities upon methylumbelliferylchitotrioside is determined by the presence of N46, S106 and T107 in the environment of their catalytic residues, which favors pKas reduction. Conversely, acidic pH optimum upon bacterial walls is determined by a low concentration of positive charges on the MdL2 and MdL1 surfaces. (C) 2010 Elsevier Inc. All rights reserved.