Lysozymes are part of the defense system against bacteria and have also been implied in the midgut digestion of bacteria in organisms like ruminants and insects. Digestive lysozymes from insects are "c" type, so they share structure and mechanistic characteristics with hen egg white lysozyme (HEWL). On the other hand, using Micrococcus lysodeikticus as substrate, digestive lysozymes present a pH optimum more acidic than defense lysozymes.
In order to elucidate the molecular basis of this difference, a digestive lysozyme from Musca domestica midgut (Lys2 - AF344589) was expressed in Pichia pastoris and purified. The pH effect on the lytic activity of Lys2 in two different ionic strengths (m = 0.25 and 0.071) was determined indicating that increment in m results in decrement of the catalytic activity (4.7 fold) and pH optimum (4.5 to 3.9) of Lys2.
Differently, the characterization of Lys2 using methylumbelliferyl N-acetyl-chitotriose (MUQ3) revealed that in both ionic strengths (0.25 and 0.071) Lys2 catalytic activity and pH optimum (4.9) are the same, indicating that these effects result from a direct influence of the ionic strength on the electrostatic potential and effective pH in the microenvironment of the negatively charged M. lysodeikticus cell wall.
The pH effect on V_max/K_m of the hydrolysis of MUQ3 showed that catalytic activity of Lys2 depends on two ionizable groups presenting pK_E´s of 3.6 and 6.4. These values are just 0.2 units lower than those determined for HEWL (pH optimum 5.1).
Lys2 was crystallized and X-ray diffraction data collected at resolution of 2.7Ǻ. Lys2 spatial structure may provide data about the differential interaction of this enzyme and polyeletrolytic or non-charged substrates.

Supported by Fapesp, CNPq and LNLS.