Antioxidant Peptide Sequences

In different studies, antioxidant peptides have been identified and the antioxidant properties of these antioxidant peptides have been found to vary. For example, in the study with combinatorial tripeptides, six different antioxidant assays of the libraries demonstrated that various aspects related to structure-activity correlations affected antioxidant activity (Ohashi et al. 2015). The antioxidant effect of the N-terminal and central amino acid of the tripeptides was found to be greater than the amino acid at the C-terminal (Li et al. 2011). Two tripeptides containing Tyr have been shown to exhibit higher clearance activity than two tripeptides containing His against the hydrophilic ABTS and AAPH radical. This result shows that the amino acid residues of Tyr and Trp play an important role in expressing radical scavenging activities, namely antioxidant activity. In addition, tripeptides other than Cys amino acid-containing peptides have been shown to show poor cleansing activity against the hydro- phobic DPPH radical, i.e. their antioxidant properties have been found to be lower than those peptides.

Tripeptides generally show high correlations with linoleic acid peroxidation, ORAC and ABTS assays, antioxidant activity, and low correlations with peroxyni- trite (PN) scavenge activity, FRAP, and DPPH assays. The highest correlations were observed during ORAC and ABTS analyses. Concomitant use of these assays has the potential as an effective approach to screening and determining antioxidant activity after antioxidant peptide isolation (Ohashi et al. 2015). Tripeptides related to Pro-His-His and tripeptides contains either two Tyr or two His amino acid residues has been identified as an active core of the antioxidant peptides. On the other hand, when two Tyr amino acid-containing tripeptides and two His amino acid-containing tripeptides were compared it is found that Tyr amino acid-containing tripeptides showed higher activities in the peroxidation of the linoleic acid. Also, Cys amino acid-containing tripeptides showed a high PN scavenging activity. On the other hand, powerful synergistic effects with phenolic (e.g. butylated hydroxyanisole and б-tocopherol) antioxidants were showed on Tyr-His-Tyr tripeptides. Replacing Pro- Her-Her’s N-terminal or C-terminal to other amino acid residues has been shown to affect the activity for being antioxidant. These results showed that the amino acid sequence, composition, and size is important for the antioxidant feature of the peptides. In addition, this study is proof that it is responsible for the inhibition of oxidative reactions initiated by different amino acid residues and peptide sequences, different types of free radicals, or pro-oxidants. This inhibition can occur in different situations. For instance, metal ions, as well as in different molecular conditions, e.g. lipid, aqueous, or emulsion systems, different pH situations, and the presence of different compounds in food matrices or biological systems, etc. (Huang, Ou, and Prior 2005; Ohashi et al. 2015). In a study, it is demonstrated that amino active hydrogen atoms and carboxyl active hydrogen do have an important antioxidant activity. In consideration of these peptides, Gly-Tyr-Gly, Gly-Tyr-Tyr-Gly, and Tyr-Gly-Gly-Tyr antioxidant activity, the peptides that contain tyrosine amino acid showed different activities according to the activity on the properties of ROS and/or RNS. However, the antioxidant activity against the hypochlorite ion was not affected by the number and the position of tyrosine amino acid residues. Also, the number of tyrosine amino acid residues altered antioxidant activity against the peroxyl radical, but when it comes to the position of tyrosine amino acid it did not have any great importance. Besides the position of the tyrosine, amino acid was only an important factor in radical scavenging activity on PN. This suggests that antioxidant peptides may be designed according to the properties of amino acid side chain groups (Matsui et al. 2018). Antioxidant peptides are critical for future destiny of health science because oxidative stress arises because of the imbalance between the production of ROS and antioxidative defenses and this can cause problems as is explained earlier. It has been shown that there is a link between oxidative stress and many diseases such as, diabetes, cancer, Alzheimer’s, joint inflammation, and schizophrenia. Oxidative stress is known to be reduced by consuming antioxidant-rich foods. Antioxidants can slow down or prevent this oxidation (Chakrabarti, Jahandideh, and Wu 2014; Li and Yu 2015; Pisoschi and Pop 2015; Liu et al. 2016). Peptides in foodstuffs show antioxidant activity (Nongonierma et al. 2015; Safitri, Herawati, and Hsu 2017). This antioxidant activity depends on the correct positioning of amino acids in the protein sequence (Unal, §ener, and Cemek 2018). For example, in studies conducted to prevent linoleic acid oxidation, peptides containing proline at the N-terminal have a more effective antioxidant role than those containing proline at the C-terminal and a higher metal-chelate level at the C-terminal than those containing histidine residues at the C-terminal (Unal, §ener, and Cemek 2018). Also, antioxidant peptides can be produced either with animal or plant sources.

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