Methionine Residues In Peptides Research Articles

Acyl Azolium-Photoredox-Enabled Synthesis of β-Keto Sulfides.

α-Heteroatom functionalization is a key strategy for C-C bond formation in organic synthesis, as exemplified by the addition of a nucleophile to electrophilic functional groups, such as iminium ions; oxocarbenium ions; and their sulfur analogues, sulfenium ions. We envisioned a photoredox-enabled radical Pummerer-type reaction realized through the single-electron oxidation of a sulfide. Following this oxidative event, α-deprotonation would afford α-thio radicals that participate in radical-radical coupling reactions with azolium-bound ketyl radicals, thereby accessing a commonly proposed mechanistic intermediate of the radical-radical coupling en route to functionalized additive Pummerer products. This system provides a complementary synthetic approach to highly functionalized sulfurous products, including modification of methionine residues in peptides, and beckons further exploration in C-C bond formations previously limited in the standard two-electron process.

ChemInform Abstract: A Long Story of Sensitized One‐Electron Photo‐Oxidation of Methionine

AbstractReview: [29 refs.

A Long Story of Sensitized One‐Electron Photo‐oxidation of Methionine

AbstractThe one‐electron oxidation of methionine residues in peptides and proteins has been studied for more than 20 years thanks to the time‐resolved techniques of pulse radiolysis and flash photolysis. Methionine oxidation causes highly damaging effects associated with aging processes and disease states such as Alzheimer’s or prion disease and cerebral amyloid angiopathy. In this Minireview, we focus on the photoinduced processes that help in the understanding of the primary processes of methionine residue oxidation. The photosensitizers were mostly derivatives of carboxybenzophenone. The methionine radical cations are stabilized by two‐center three‐electron bonds of various natures. However, despite numerous studies, data about the final product are still missing.

Methionine sulfoxide reductase A is a stereospecific methionine oxidase

Methionine sulfoxide reductase A (MsrA) catalyzes the reduction of methionine sulfoxide to methionine and is specific for the S epimer of methionine sulfoxide. The enzyme participates in defense against oxidative stresses by reducing methionine sulfoxide residues in proteins back to methionine. Because oxidation of methionine residues is reversible, this covalent modification could also function as a mechanism for cellular regulation, provided there exists a stereospecific methionine oxidase. We show that MsrA itself is a stereospecific methionine oxidase, producing S-methionine sulfoxide as its product. MsrA catalyzes its own autooxidation as well as oxidation of free methionine and methionine residues in peptides and proteins. When functioning as a reductase, MsrA fully reverses the oxidations which it catalyzes.

Interplay of terminal amino group and coordinating side chains in directing regioselective cleavage of natural peptides and proteins with palladium(II) complexes.

Palladium(II) ions anchored to side chains of histidine and methionine residues in peptides and proteins in weakly acidic aqueous solutions promote hydrolytic cleavage of proximate amide bonds in the backbone. In this study, we determine how attachment of Pd(II) ions to histidine and methionine anchors and also to the terminal amino group in six natural peptides (chains A and B of insulin, segment 11-14 of angiotensinogen, pentagastrin, angiotensin II, and segment 3-8 of angiotensin II) and two proteins (ubiquitin and cytochrome c) affects regioselectivity and rate of backbone cleavage. These Pd(II)-promoted reactions follow a clear pattern of regioselectivity, directed by the anchoring side chains. When the Pd(II) reagent is nonspecifically anchored to the terminal amino group, the ligating site that is present in almost all proteins, the cleavage is fortunately absent. When the reagent is anchored to a residue in positions 1, 2, or 3, cleavage is absent, because the terminal amino group and deprotonated amide nitrogen atom(s) interposed between it and the anchor "lock" the Pd(II) ion in hydrolytically inactive chelate complexes. When the reagent is anchored to residues in positions beyond 3, the second amide bond upstream from the anchor is regioselectively cleaved in all cases when the anchor was "isolated," that is, flanked by noncoordinating side chains. Segment 3-8 of angiotensin II undergoes additional cleavage, which we explain by determining the rate constants for the cleavage, identifying the rate-limiting displacement of ethylenediamine ligand from the Pd(II) ion, and detecting several intermediates. Experiments with cytochrome c demonstrate that the number of cleavage sites can be controlled by adjusting the mole ratio of the Pd(II) reagent to the substrate. Our inorganic peptidases are useful for biochemical applications because their regioselectivity and reactivity set them apart from proteolytic enzymes and organic chemical reagents.

Identification of oxidized methionine residues in peptides containing two methionine residues by derivatization and matrix-assisted laser desorption/ionization mass spectrometry.

Oxidation of methionine residues in peptides and proteins occurs in vivo or may be an artifact resulting from purification steps. We present a three step method for the localization of methionine sulfoxides in peptides with two methionine residues. In the first step, the N-terminus as well as other reactive side chain functions are blocked by acetylation. The resulting protected peptides are cleaved by cyanogen bromide. The cleavage does not occur at methionine sulfoxide but only at reduced methionine residues forming new amino termini. The newly formed amino group is then derivatized with a bromine containing compound in the last step of the procedure. The resulting peptide can easily be identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry using both the characteristic isotope pattern of the halogen and the metastable loss of methanesulfenic acid from oxidized residues. This procedure allows the unequivocal localization of oxidized methionines even in complex peptide mixtures.

Enhancement of Cyanogen Bromide Cleavage Yields for Methionyl-Serine and Methionyl-Threonine Peptide Bonds

Cyanogen bromide (CNBr) is a common chemical used to hydrolyze peptide bonds C-terminal to methionine residues in peptides and proteins. In most cases, the efficiency of this bond cleavage is greater than 90% except in situations where a serine or threonine residue follows methionine in the amino acid sequence. We have explored the mechanism of the methionyl-serine and methionyl-threonine CNBr cleavage inefficiencies and have developed a simple methodology to more than double cleavage yields relative to standard literature conditions. This method entails increasing the concentration of water during the cleavage reaction either by reducing the formic acid concentration or by performing the cleavage in an acidic aqueous medium. This approach provides a more desirable methodology from the perspective of enhanced yields and greater ease of handling in cases of large-scale use.

Identification of oxidized methionine in peptides.

The positive- and negative-ion collision-induced dissociation spectra of peptides containing methionine, methionine sulphoxide and methionine sulphone have been studied. Characteristic fragmentations were identified and evaluated as possible indicators for the presence of oxidized methionine residues in peptides. It was found that the elimination of CH3SOH (-64 u) from [M + H]+ is unique for peptides that contain methionine sulphoxide. Sequence ions containing the oxidized methionine undergo the same elimination, allowing unambiguous sequence determination. Methionine sulphone exhibits an analogous elimination of CH3SO2H (-80 u) from the protonated molecule, but not from sequence ions.

L-Methionine increases the rate of reaction of 5′-guanosine monophosphate with the anticancer drug cisplatin: mixed-ligand adducts and reversible methionine binding

L-Methionine (L-HMet) increased the rate of reaction of the anticancer drug cisplatin, cis-[PtCl2(NH3)2], with guanosine 5′-monophosphate (5′-GMP) at pH 7. The course of the reaction has been elucidated by 1H and [1H, 15N] NMR spectroscopy. Novel intermediates detected and characterized include cis-[Pt(5′-GMP-N7)(L-HMet-S)(NH3)2]2+ and [Pt(L-Met-S,N)(5′-GMP-N7)(NH3)]+(charges on 5′-GMP ignored), the formation of which involves ammine release. Monodentate S-bound L-HMet can co-ordinate reversibly, whereas S,N-chelated L-Met is much less reactive. Thus methionine residues in peptides and proteins could play a role in the transfer of Pt onto DNA. Comparative reactions of [Pt(en)Cl2](en = 1,2-diaminoethane) have also been investigated.

Selective oxidation and reduction of methionine residues in peptides and proteins by oxygen exchange between sulfoxide and sulfide.

Treatment of amino acids, peptides, and proteins with aqueous solution of dimethyl sulfoxide (Me2SO) and hydrochloric acid (HCl) resulted in the oxidation of methionine to methionine sulfoxide. In addition to methionine, SH groups are also oxidized, but this reaction proceeds after a lag period of 2 h. Other amino acids are not modified by aqueous Me2SO/HCl. The reaction is strongly pH-dependent. Optimal conditions are 1.0 M HCl, 0.1 M Me2SO, at 22 degrees C. The reaction exhibits pseudo-first order kinetics with Kobs = 0.23 +/- 0.015 M-1 min-1 at 22 degrees C. Incubation of methionine sulfoxide with dimethyl sulfide and HCl resulted in the conversion of methionine sulfoxide to methionine. This reaction is fast (t1/2 = 4 min at room temperature) and quantitative at relatively anhydrous condition (i.e. at H2O:concentrated HCl:dimethyl sulfide ratio of 2:20:1). Quantitative conversions of methionine sulfoxide back to methionine are obtained in peptides and proteins as well, with no observable other side reactions in amino acids and proteins. The wide applications of this selective oxidation and reduction of methionine residues are demonstrated and discussed.

Determination of the number of methionine residues in peptides by the method of partial substitution

Determination of the number of methionine residues in peptides by the method of partial substitution