Oxidative peptidolysis in model systems and food: mechanisms and reaction products

• Applicant: Professor Dr. Michael Hellwig
• Subject Area: Food Chemistry
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 298974803

Food constituents (proteins, lipids, vitamins) are subject to oxidative degradation reactions during processing and storage. These reactions strongly influence food quality. Oxidation reactions in the side chains of proteinogenic amino acids have already been described in detail. Moreover, there are also non-enzymatic oxidation-induced peptidolytic reactions at the protein backbone (oxidative peptidolysis). The most important among these reactions is alpha-amidation, leading to the cleavage of peptides with the formation of peptide amides and keto acyl peptides. The role of oxidative peptidolysis in the chemistry of protein oxidation and especially food protein oxidation is yet unclear.Based on works of the applicant, methionine oxidation is associated with oxidative peptidolysis under conditions relevant for food processing. Thus, protein-bound methionine should be capable of transferring oxidative damage from the side-chain to the protein backbone and to promote peptide backbone cleavage in a sequence-dependent manner. It is therefore the principle objective of this project to examine the influence of amino acid side chains on backbone cleavage reactions in peptides and proteins. Tetrapeptides containing methionine will be incubated in the presence of pertinent oxidants and autoxidizing carbohydrates (e.g., ascorbic acid) and lipids under conditions relevant in food processing. Relationships between structure and properties (e.g., sequence dependence) will be deduced from the product patterns, which will be measured by chromatographic means (HPLC-UV, HPLC-MS/MS, GC-MS). The correlation of these structure-property relationships with changes in the electrochemical behavior will be of particular importance in this project. Electroanalytical methods (cyclic voltammetry, chronoamperometry) and spectroelectrochemical methods, also combined with ESR spectroscopy in situ will be applied. This will permit conclusions on the mechanism of methionine-associated alpha-amidation, for example if and how oxidative damage can be transferred from the methionine side chain to the backbone or to neighboring amino acid side chains.Amidation reactions are expected to proceed also in food with the concomitant formation of keto acyl peptides as process-induced secondary food constituents that have been overlooked up to now. Therefore, it is a further objective of this project to synthesize defined methionine-derived keto acyl peptides for the first time and to enable their analysis in complex mixtures. They will be characterized with respect to their contribution to protein carbonylation and to the formation of secondary products of protein oxidation. Finally, oxidative peptidolysis will be investigated on food-relevant proteins (whey proteins, patatin) and under the conditions of food preparation and correlated with the extent of methionine oxidation.

• DFG Programme: Research Grants