| Myofibrillar protein(MP),the main components of muscles,has many advantages,such as great amino acid composition,high digestibility and low sensitization.In terms of structure,MP is natural flexible chain and rich in active groups on side chain,which is good raw material for building functional colloids.It also has great potential in developing new muscle protein thickening fluid food,which could meet the personalized needs of special people for product nutrition and texture.However,the low solubility of MP in water or low ionic strength solution limits the development and utilization of MP in low salt fluid food,which is a key processing bottleneck.Enzymatic deamidation is a green technology of protein modification with mild control,specific substrate,less side chain reaction and no chemical addition,which is usually used to improve the solubility of foodborne protein.Protein-glutaminase(PG)used in this study could specifically catalyze glutamine residues on polypeptides or protein side chains,and convert polar uncharged residues into glutamate residues with negatively charge,so the charge distribution and structure of substrate protein would change significantly.As a result of that PG deamidation could catalyze glutamine to produce carboxylic acid groups,the electrostatic repulsion between protein molecules would be significantly enhanced,thus leading to a significant increasing in the water solubility of MP.In order to reveal the mechanism of enhancing solubility,myosin is further studied to explore the effects of deamidation on the filament assembly under the condition of low ionic strength.This study could provide a theoretical basis for the targeted design of muscle proteins.The main contents and conclusions of this study are as follows:1.Chicken breast myofibrillar protein(CMP)was deamidated by PG for different time(0,4,8,12,16 h)could achieve different degree of deamidation(DD).After deamidation for 16 h,DD was 6.54% and solubility of CMPs increased from 18.84% to 63.99%.The zeta-potential of CMPs also increased,and its absolute value was almost higher than 30 m V.In addition,deamidation did not cause the loss of amino acid nutritional value and protein proteolysis,and the α-helix structure content increased to 52.2% gradually with the degree of deamidation,which enhanced the flexibility of CMPs structure.The structure of CMPs was expanded by deamidation,and the tryptophan residues and hydrophobic groups buried in the CMPs were exposed,and the surface hydrophobicity was also increased.It could be observed by atomic force microscopy and transmission electron microscopy that the microstructure of CMPs was in a highly ordered state when it was not treated.With the prolongation of deamidation time,the fiber structure of CMPs gradually dissociated and formed monomers such as myosin and actin or oligomers.2.PG deamidation could effectively inhibit the filament assembly behavior of myosin in low ionic strength solution and reduce the particle size of myosin filament.Transmission electron microscopy images showed that the untreated myosin in low ionic strength solution presented a filammy state with a length of 705.38 nm and a width of 68.72 nm,suggesting that it was an assembly filament.With the prolongation of deamidation time,the content of myosin filaments decreased continuously,and the length and width of the assembly of single crude filaments decreased significantly,which were about 218 nm and 12 nm,respectively.The appearance of myosin solution was gradually clear and transparent.Dynamic in situ monitoring of the quartz crystal microbalance with dissipation showed that the adsorption mass of the myosin monomer on the sensor was significantly reduced after deamidation,indicating that the ability of the myosin monomer to assemble the filament was significantly reduced.By simulating the surface charge distribution of myosin,it was found that the untreated myosin had positive and negative charge clusters on rod tail.Deamidation could change the distribution of positive and negative charge clusters,and presumably affect the regular assembly of myosin dimer,thus inhibiting the formation of myosin filament.In general,PG catalyzed the deamination of glutamine residues of polypeptides or protein side chains specifically,converted the polar uncharged residues into negatively charged glutamate residues,and induced the generation of carboxylic acid groups,enhancing the electrostatic repulsion between protein molecules.This electrostatic repulsion inhibited myosin assembly by inhibiting rod-rod electrostatic interactions in the myosin tail,thereby promoting protein-water interactions and improving the solubility of CMPs. |
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