Nutrition,structure And Protein Composition Of Tilapia-soybean Protein Co-precipitates

The binary protein systems featuring superior nutritional value and functional properties have attracted great attention.At present,with high-quality animal proteins and plant proteins as the main base,the binary protein systems of complementary advantages have been proposed.In particular,existing research and application focuses on milk protein and soy protein,the binary protein systems of aquatic proteins has rarely been reported on.In this study,tilapia protein isolate?TPI?,five tilapia-soybean protein co-precipitates(TSPC_3:1,TSPC_2:1,TSPC_1:1,TSPC_1:2,and TSPC_1:3)and soybean protein isolate?SPI?were prepared by mixing tilapia meat and soybean meal at different mass ratios.Compared with TPI and SPI,the superiors of TSPC were analyzed by determining nutrition quality,structure characteristic and protein composition.Moreover,this study also provide useful information for the assessment of food protein quality.The major results are as follow:1.The crude protein contents of TPI,TSPC,and SPI were both higher than 90%.The basic ingredients contents,most of amino acid composition,and digestibility values of TSPC were between those of TPI and SPI.SDS-PAGE revealed that TSPC contained protein subunits from TPI and SPI.Compared with TPI and SPI,TSPC_2:1and TSPC_1:1contained the highest levels of tryptophan,aspartic acid,glycine,histidine and arginine relative to TPI and SPI,respectively,and TSPC_1:1showed the highest amino acid score?AAS?of tryptophan and histidine.Compared with the other proportions of TSPC,the AAS of threonine,valine,methionine,isoleucine,leucine,lysine,tryptophan,and histidine in TSPC_1:1was both higher than that of SPI.Interestingly,compared five different proportions of TSPC,the protein digestibility corrected amino acid score?PDCAAS?of TSPC_1:1was 0.86,indicating the greatest nutritional value.These results indicated that preparing tilapia-soybean protein co-precipitates by alkali solution-isoelectric point precipitation is an effective method to balance the nutritional quality of tilapia protein and soybean protein,and also an effective strategy for developing tilapia-soybean binary protein systems.2.The water solubility and thermal stability of TSPC were both higher than TPI.At a neutral p H,TSPC_1:3displayed the water solubility and denaturation temperature of 81.9%and 90.30°C,respectively.Ultraviolet-visible spectra and circular dichroism spectra shows that the local structure of TSPC were different from TPI and SPI.Fourier transform infrared spectra shows that the structure of TPI and SPI were both presented in TSPC.These results indicate that the secondary structures of TSPC were predominantly?-helix and?-sheet,and the five TSPC had structural variability.TSPC_1:1was unique compared to the other TSPC.In addition,the correlation results revealed that there was a strong correlation between amino acid and secondary structure.Methionine had robust effect on?-helix?R=0.975,p<0.01?,and phenylalanine had robust effect on?-sheet?R=0.913,p<0.01?.The correlation analysis also revealed that?-helix had a positively effect on protein digestibility?R=0.954,p<0.01?.Notably,there was a good correlation between the water solubility and secondary structure of TSPC,in which the correlation coefficients of?-helix and?-sheet were-0.964?p<0.01?and 0.743,respectively.In conclusion,TSPC could be used in food industry.Moreover,the secondary structure of food proteins closely corrected to amino acid composition,digestibility and water solubility,and?-helix and?-sheet were the main factors.3.A total of 2222,5092 and 5878 proteins were identified by proteomics in TPI,TSPC_1:1and SPI,respectively.Among them,989 proteins were unique to TSPC_1:1.Compared with TPI and SPI,the results of Gene Ontology?GO?analysis shows that three functions of cell aggregation,other organism part and other organism specifically expressed in TSPC_1:1.Meanwhile,according to the Kyoto Encyclopedia of Genes and Genomes?KEGG?pathway analysis,compared with TPI and SPI,989 KEGG pathways were unique to TSPC_1:1.These results indicate that TSPC_1:1could participate in more biochemical functions and metabolic pathways.In addition,the result of analysis the unique various types of N-glycan biosynthesis pathway in TSPC_1:1indicate that the Glycos?transf?1 domain-containing protein and alpha-1,2-Mannosidase belong to the unique 989 proteins of TSPC_1:1.In conclusion,these results showed that TSPC_1:1,TPI and SPI have significant differences in protein composition,GO function annotation,and KEGG pathway.TSPC_1:1could participate in more biological functions and deserve further application in food industry.