Methionine residues chemical modification

The sulfur atom of methionine residues may be modified by formation of sulfonium salts or by oxidation to sulfoxides or the sulfone. The cyanosulfonium salt is not particularly useful for chemical modification studies because of the tendency for cyclization and chain cleavage (129). This fact, of course, makes it very useful in sequence work. Normally, the methionine residues of RNase can only be modified after denaturation of the protein, i.e., in acid pH, urea, detergents, etc. On treatment with iodoacetate or hydrogen peroxide, derivatives with more than one sulfonium or sulfoxide group did not form active enzymes on removal of the denaturing agent (130) [see, however, Jori et al. (131)]. There was an indication of some active monosubstituted derivatives (130, 132). 

Cigarette smoke decreases the < i-PI activity in rat lung and produces a functional deficiency of protease inhibitors in the lower respiratory tract of humans (11,12). It is quite reasonable to assume that this is due to the oxidation of the essential methionine residue in i-PI. Thus, chemical modification of ax-PI by oxidation of a methionine residue to a methionine sulfoxide residue by some component of cigarette smoke or by myeloperoxidase which is released by cigarette smoke, results in inactivation of this essential protease inhibitor. The resulting imbalance of proteases and protease inhibitors in the lung then results in the development of emphysema. 

Selective chemical modification. The usefulness of this technique is also limited to those residues, such as lysine, methionine and tyrosine, for which modification reactions are known. In some cases assignment of spectral lines to specific residues by this technique has been possible, but more often the selectivity of the reaction in situ is inadequate to permit an unequivocal interpretation. 

It is worth noting that the radical damage to methionine-containing peptides and proteins consists of a desulfurization process, which leads to the replacement of a methionine residue with an a-aminobutyric acid in the sequence. This could be a posttranslational modification, which is linked to a postsynthetic modification of lipids by the above-reported tandem mechanism. A chemical biology approach can be proposed involving lipidomics and proteomics, in order to configure the metabolic changes related to a radical stress. 

Peptides that contain amino acid residues that are prone to chemical modification (Bronstrup, 2004), such as methionine, tryptophan, or cysteine, should not be considered. 

Despite this rather nebulous state of affairs, some changes can be made which affect the enzyme in a predictable fashion. The exchange of amino acids which are easily oxidized (e.g. methionine residues) can improve the stability of the enzyme, but usually at the expense of lower catalytic activity. The incorporation of additional disulphide bridges will also increase stability, but usually not beyond what is available from the native enzyme under ideal conditions the conditions have to be rigged to show the improvement. Changes in the amino acids around the active site can affect the catalytic properties in a useful fashion, but not beyond what can be achieved by chemical modification (see Section 6.9.2) or by random mutation and selection. 

Analyses of the structure and modifications of the collagen molecule/chain require solubilization and fragmentation of the protein to smaller peptides. In principle, two methods can be used—non-enzymatic (chemical) or enzymatic treatment. The chemical method is cleavage by CNBr. Cyanogen bromide splits the protein molecule at specific locations—at the methionines (in this case toward the C-terminal end). In the collagen molecule, methionine is a relatively rare amino acid (some 10-20 amino acids per collagen molecule). The small number of methionine residues leads to a rather limited number of cleavage products (CNBr peptides). The profile of CNBr peptides is typical. 

At variance, a more classical chemical environment was found for the pla-tinum(II) center in the cisplatin-hen egg white lysozyme (HEWL) species (Figure 9.6B), where platinum was found to bind to the Ns of His 15 of HEWL, two ammonia molecules in the cisplatin and another one possibly to be loosely bound water molecule. No more significant modifications of the electron density map of the protein surface were observed, ruling out the presence of additional binding site, for example, the two methionine residues (Met-12... 

Chemical modification of amino acid side chain functionalities will also serve to cleave specific peptide bonds selectively. Chemical cleavage of a polypeptide chain exploits the unique reactivity of chemically modified side chains of particular amino acids in the labilization of adjacent peptide bonds by neighbouring group participation (68). The residues investigated so far for this purpose have been methionine, cysteine and the aromatic amino acids including tryptophan (438-440, 443). 

Despite the extensive use to which photochemical oxidations of proteins have been put very little attention has been given to identifying the end products of the photoreactions. Apart from methionine which is converted to methionine sulfoxide and more slowly to its sulfone, the fate of the other amino acid residues of irradiated proteins remains largely unknown. On the basis of limited chemical studies, it is clear, however, that photoreactions usually lead to a mixture of products. The multiplicity of products formed by photooxidation of enzymes will undoubtedly limit the utility of the technique. There are in fact numerous examples from work dealing with the chemical modification of enzymes which indicate that modification of a particular residue with different chemical reagents leads to enzyme derivatives with different biological and physico-chemical properties. 

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