Lysozymes glycosylated

Note should be made of the fact that all of the mechamsilis for lysozyme in which a carbonium ion intermediate is formed are depicted as proceeding through a cyclic glycosyl oxo-carbonium ion. In the acid-catalysed hydrolysis of glucosides the carbonium ion formed is probably cyclic rather thEin an open-chain ion (Capon, 1969), even though the cyclic ion would necessarily have to adopt a... 

It is reasonable that a cyclic glycosyl carbonium ion would be formed in lysozyme-catalysed reactions, but this expectation must be viewed in the light of the postulated straining of the substrate during... 

If aspartic acid-52 acts as a nucleophile in lysozyme reactions a glycosyl enzyme intermediate will be formed [60]. There is no evidence, kinetic or otherwise, for substituted enzyme intermediates, but rapid breakdown might preclude attainment of detectable concentrations. Formation of a substituted enzyme could explain the observed retention of configuration at the anomeric carbon in transglycosidation reactions, provided backside attack in a subsequent reaction is chemically reasonable. It has therefore been important to attempt to understand the chemistry of acylal hydrolysis so as to assess the properties that would be expected of an acylal intermediate in reactions catalysed by the enzyme. 

General acid catalysis by glutamic acid-35 represents at present the mechanism for lysozyme best able to explain the kinetic and structural data. For it to occur, however, distortion of the hexose ring in subsite D to a half-chair must take place so that relief of strain in the transition state will make bond breaking sufficiently easy. A question that must be answered for this picture to be tenable is whether relief of strain can so greatly facilitate bond breaking when the carbonium ion is a glycosyl ion [see also the discussion in Fife (1972) and Atkinson and Bruice (1974)]. 

The chymotiypsin reaction is one example of acyl group transfer (see Fig. 6-21). Glycosyl group transfers involve nucleophilic substitution at C-l of a sugar ring, which is the central atom of an acetal. In principle, the substitution could proceed by an SnI or Sn2 path, as described for the enzyme lysozyme (see Fig. 6-25). 

Hen egg white lysozyme is a small protein of Mr 14 500 and 129 amino acid residues. This enzyme was introduced in Chapter 1, where it was pointed out that examination of the crystal structure of the enzyme stimulated most of the solution studies. Hen egg white lysozyme has the distinction of being the first enzyme to have had its structure solved by x-ray crystallography.207 It is an atypical member of the hexosaminidase class of glycosyl transfer enzymes. It catalyzes the hydrolysis of substrates with retention of stereochemistry. T4 lysozyme was for many years thought to have the same fold and mechanism of lysozyme, despite there being no sequence homology. But it has now been found that the T4 enzyme has inversion of configuration and so operates by a different mechanism.208,209 A mechanism proposed for the enzymatic reaction was based on the structure of the... 

Figure 7. Molecular ion envelope of glycosylated lysozyme Amadori products (top) and cesium iodide reference mass cluster ions (bottom). The centroided masses of the tiro labeled peaks are 16,265 and 16,427 corresponding to lysozyme Amadori adducts containing 12 (G 12) and 13 (G=13) glucose residues respectively.

Elofsson M, Roy S, Walse B, Kihlberg J, Solid-phase synthesis and conformational studies of glycosylated derivatives of helper-T-cell immunogenic peptides from hen-egg lysozyme., Carbohydr. Res., 17, 246 89-103, 1993. 

Schematic drawing of monosaccharide binding sites in various glycosyl hydrolases, (a) Subsites of an endo-hydrolase, e. g., hen egg white lysozyme, (b) subsites of a disaccharide hydrolase, e. g., ((x-glucosidase, (c) subsites of a monosaccharide-releasing exo-hydrolase, e. g., glucoamylase, (d) subsites of a disaccharide-releasing exe-hydrolase, e.g., / -amylase (after [18])...

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