Hexa- , lysozyme hydrolysis

Hen egg-white lysozyme catalyzes the hydrolysis of various oligosaccharides, especially those of bacterial cell walls. The elucidation of the X-ray structure of this enzyme by David Phillips and co-workers (Ref. 1) provided the first glimpse of the structure of an enzyme-active site. The determination of the structure of this enzyme with trisaccharide competitive inhibitors and biochemical studies led to a detailed model for lysozyme and its hexa N-acetyl glucoseamine (hexa-NAG) substrate (Fig. 6.1). These studies identified the C-O bond between the D and E residues of the substrate as the bond which is being specifically cleaved by the enzyme and located the residues Glu 37 and Asp 52 as the major catalytic residues. The initial structural studies led to various proposals of how catalysis might take place. Here we consider these proposals and show how to examine their validity by computer modeling approaches. 

The shape of the difference spectra attributed to the lysozyme—hexa-NAG complex at low temperature (-20°C) is pH dependent, and should depend on the ionization of chromophores near the binding site. The maximum amplitude of these spectra corresponds to the known pH of the maximum activity of lysozyme in the hydrolysis of hexa-NAG. 

When tetra-NAG is treated with lysozyme, some hexa-NAG and di-NAG are formed. This reaction, transglycosylation, was first proposed by Maksimov et al. (1965) to account for hydrolysis of tri- and tetrasac-charides, during which an insoluble chitinlike carbohydrate was formed. Thus, short-chain oligosaccharides are cleaved by lysozyme, with the products transferred to longer-chain oligosaccharides, which are subsequently broken down. Much work has focused on this phenomenon. 

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