Lysozyme, active site catalytic mechanism

Weaver et al. (1985) noted some similarities in the active site of the three lysozymes, but with the following striking difference. Residue 73 (Glu) in goose corresponds with residue 35 (Glu) in chick and with residue 11 (Glu) in bacteriophage T4. On the other hand, there are two Asp residues at positions 86 and 97 in the goose active site, neither of which corresponds exactly with Asp-52 of chick nor Asp-20 of T4. The implications for potential differences in the mechanism of catalytic action by the three lysozymes were discussed by Johnson et al. (1988) and by et al. (1985). The latter authors discussed the unresolved question as to whether the c-type lysozyme exons correspond to distinct structural and/or functional entities that are conserved during evolution of the three types of lysozyme considered. 

Genetic methods provide a powerful approach to the biosynthetic introduction of redox groups [including cysteine residues as well as unnatural redox active amino acids (79)] into proteins. As an example, a disulfide bridge inserted across the active site of T4 lysozyme has been used to create a redox mechanism for regulating enzyme activity (SO). Oxidation of the cysteines to form the disulfide closes the active site region, whereas reduction exposes the active site and restores catalytic activity. 

Lysozyme has two catalytic groups at the active site Glu 35 and Asp 52 (Figure 24.9). Once it was determined that the enzyme-catalyzed reaction takes place with retention of configuration at the anomeric carbon, it could be concluded that it cannot be a one-step Sn2 reaction the reaction must involve at least two steps and, therefore, must form an intermediate. Although lysozyme was the first enzyme to have its mechanism studied— and the mechanism has been studied extensively for almost 40 years—only recently have data been obtained that support the mechanism shown in Figrue 24.9 ... 

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