Structural properties of antioxidant peptides released from sweet apricot kernel protein with regulation by ultrasound-assisted enzymolysis

The protein-rich residues of sweet apricot kernel (SAK) after oil extraction can make bioactive peptides by enzymatic hydrolysis. Ultrasound-assisted enzymolysis (UAE) will improve conventional enzymolysis, but it is unknown how to regulate the release of bioactive peptides from SAK protein (SAKP). Here, the effects of UAE on the structural properties and antioxidant activity of SAKP hydrolysates (SAKPHs) by Alcalase, Neutrase, Papain and Flavourzyme were investigated. Compared with traditional enzymolysis, the optimized ultrasound-assisted enzymolysis conditions (20% ultrasound power at 50 °C for 0.5 h) showed an improved hydrolysis of SAKP, an increased intensity of CO and C-O-Fe peaks, and a decreased molecular weight distribution of peptides, resulting in the improvement of the antioxidant activity of the peptides. The results of UPLC-ESI-MS/MS identified 132 peptides in the SAKPHs. These peptides hydrolyzed by Alcalase, Flavourzyme, Neutrase and Papain in traditional enzymatic processes showed serine, aspartic acid, lysine and arginine-type characteristics at the sequence terminus or the cleavage site. However, the identified peptides released by the four proteases during ultrasound-assisted enzymolysis showed serine; arginine, aspartic acid and leucine; lysine; arginine and aspartic acid as sequence terminus or cleavage sites. Molecular docking simulation indicates that the differential active pockets of Alcalase, Flavourzyme, Neutrase and Papain in traditional enzymolysis are Asp30-Gly102-Pro108-Ser109-Ser116-Tyr119-Gly131-Thr139-Asn144-Arg150-Lys151-Asn152-Asp153-Glu163-Asn166-Lys281-Ser324-Ser326-Pro425-Thr427, Gln91-Lys99-Ser248-Lys255-Val285-Glu287-Ala299-Cys301-Tyr325-Asn337-Gln359-Arg362, Ser49-Tyr107-Ser119-Val140-Asn152-Tyr194-Lys220-Arg221-Asn228 and Gly23-Arg111-Gln128-Asn155-Tyr208-Pro209 residues, respectively. Similarly, the correspondingly differential active pockets for ultrasound-assisted enzymolysis are Gly164-Ala172-Tyr195-Phe240-Asn383-Arg396, BMA3-Man4-Asp27-Tyr29-Glu43-Lys54-Glu61-Lys118-Ser153-Asp183-Ser186-Asp286-Asp298-Thr302-Thr305-Asp307-Ser339-Asp346, Glu167-Ser223 and Asp6-Gly20-Gys22-Arg58-Gln92-Gln114-Ala126-Lys156 residues, respectively. As a result, this investigation proves that ultrasound selectively increases cleavage sites of enzymolysis and the targeted release of antioxidant peptides from dietary proteins.