Abstract
Oxidation of methionine (Met) is an important reaction that plays a key role in protein modifications during oxidative stress and aging. The first steps of Met oxidation involve the creation of very reactive and short-lived transients. Application of complementary time-resolved radiation and photochemical techniques (pulse radiolysis and laser flash photolysis together with time-resolved CIDNP and ESR techniques) allowed comparing in detail the one-electron oxidation mechanisms initiated either by ●OH radicals and other one-electron oxidants or the excited triplet state of the sensitizers e.g., 4-,3-carboxybenzophenones. The main purpose of this review is to present various factors that influence the character of the forming intermediates. They are divided into two parts: those inextricably related to the structures of molecules containing Met and those related to external factors. The former include (i) the protection of terminal amine and carboxyl groups, (ii) the location of Met in the peptide molecule, (iii) the character of neighboring amino acid other than Met, (iv) the character of the peptide chain (open vs cyclic), (v) the number of Met residues in peptide and protein, and (vi) the optical isomerism of Met residues. External factors include the type of the oxidant, pH, and concentration of Met-containing compounds in the reaction environment. Particular attention is given to the neighboring group participation, which is an essential parameter controlling one-electron oxidation of Met. Mechanistic aspects of oxidation processes by various one-electron oxidants in various structural and pH environments are summarized and discussed. The importance of these studies for understanding oxidation of Met in real biological systems is also addressed.
Highlights
Methionine (Met) is an important sulfur-containing amino acid often playing a protective role in a protein oxidation due to the fact that the thioether group is oxidized by many reactive species [1,2]
The replacement of the carboxyl group with an ester or an amide group in methionine Met(S N)+ is inhibited. This is reflected in much longer lifetime ( 1/2~1.1 does not significantly change the primary radical reactions related to the oxidation process, ms) of this radical, which was directly observed in the example of the methionine ethyl except for the fact that the decarboxylation process involving the intramolecular Met(S∴N)+
Was observed when the number of proline residues was changed from two to three). These observations were reproduced by Langevin dynamics and statistical mechanical theory showing that for peptides with zero to two Pro residues contact between sulfur atoms was controlled by the activated formation of (S∴S)+ but for the peptides with three and four proline residues was controlled by relative diffusion of the >S·+ radical cation and an unoxidized S atom [68]
Summary
Methionine (Met) is an important sulfur-containing amino acid often playing a protective role in a protein oxidation due to the fact that the thioether group is oxidized by many reactive species [1,2]. The complete and detailed description of the primary steps of the oxidation mechanisms of Met residues located in the interior of long oligopeptides and proteins is a significant and original contribution in understanding oxidation reactions in real biological systems which might be of great help in imagining new strategies in the struggle against “uncontrolled oxidative stress”. The results of these studies and conclusions drawn from them are summarized in this review
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