Lysozyme active site structure

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. 

LD model, see Langevin dipoles model (LD) Linear free-energy relationships, see Free energy relationships, linear Linear response approximation, 92,215 London, see Heitler-London model Lysine, structure of, 110 Lysozyme, (hen egg white), 153-169,154. See also Oligosaccharide hydrolysis active site of, 157-159, 167-169, 181 calibration of EVB surfaces, 162,162-166, 166... 

From crystal-structure analysis of hen-egg lysozyme and of its complex with the competitive inhibitor tri-Af-acetylchitotriose, the following conclusions were drawn the active site consists of a cleft containing six sub-sites, A to F, of which each could accommodate a) -( 1 — 4)-linked A-acetylglucosa-... 

Following myoglobin and lysozyme, bovine carboxypeptidase A was the third protein to have its 3-D structure solved at high resolution. The active site zinc is bound to His69, Glu72 and Hisl96 (Figure 12.4), and to a water molecule, which is displaced when a... 

Fig. 1. Representation of the active site region of hen lysozyme, from the X-ray structure (adapted from Ref. 31). The lanthanide cations bind in the region between asp 52 and glu 35.

Lysozyme is a natural antibacterial agent found in tears and egg whites. The hen egg white lysozyme (Mr 14,296) is a monomer with 129 amino acid residues. This was the first enzyme to have its three-dimensional structure determined, by David Phillips and colleagues in 1965. The structure revealed four stabilizing disulfide bonds and a cleft containing the active site (Fig. 6-24a see also Fig. 4-18). More than five decades of lysozyme investigations have provided a detailed picture of the structure and activity of the enzyme, and an interesting story of how biochemical science progresses. 

With the active site residues identified and a detailed structure of the enzyme available, the path to understanding the reaction mechanism seemed open in the 1960s. However, definitive evidence for a particular mechanism eluded investigators for nearly four decades. There are two chemically reasonable mechanisms that could generate the observed product of lysozyme-mediated cleavage of the glycosidic bond. Phillips and... 

The active site is in a cleft between a large domain with a nonpolar core and a smaller (3-sheet domain that contains many hydrogen-bonded polar side chains (Figs. 12-3,12-4). Human lysozyme has a similar structure and properties.57-59 The T4 lysozyme has an additional C-terminal domain whose function may be to bind the crosslinking peptide of the E. coli peptidoglycan. Goose lysozyme is similar in part to both hen lysozyme and T4 lysozyme. All three enzymes, as well as that of our own tears, may have evolved from a common ancestral protein.60 On the other hand, Streptomyces erythaeus has developed its own lysozyme with a completely different structure.61 An extensive series of T4 lysozyme mutants have been studied in efforts to understand protein folding and stability.61-63... 

The structure of the complex formed between the enzyme lysozyme and its substrate. The crevice that forms the site for substrate binding (the active site) runs horizontally across the enzyme molecule. The individual hexose sugars of the hexasaccharide substrate are shown in a darker color and labeled A-F. (Coordinates courtesy of D. C. Philips, Oxford, England.) (Illustration copyright by Irving Geis. Reprinted by permission.)... 

Figure 7.8. The substrate interaction at the active site of an enzyme. The interaction of tetra-A/,A/,A/,A/-acetylchitotetraose (NAG4) with amino acid residues at the active site of lysozyme (ILZC.pdb) can be viewed/saved at PDBsum server (Enyme Structure Database->PDBsum->LIGPLOT of interactions under Ligand) linked to the Enzyme Structure Database.

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