Effect of CaCl2 on denaturation and aggregation of silver carp myosin during setting
Introduction
The gel formation of fish proteins involves partial denaturation of protein, followed by irreversible aggregation, which results in a three dimensional network (Liu et al., 2010). Myosin is abundant in muscle protein and plays an essential role in gel formation in fish products. The gelation properties of myosin depend strongly on pH, temperature (Liu, Zhao, Xiong, Xie, & Qin, 2008) and divalent cations, such as calcium, magnesium and zinc (Arfat & Benjakul, 2013). Surimi gel strength can be enhanced by preincubation at a low temperature (⩽40 °C). This preincubation process is referred to as setting or suwari (Arfat and Benjakul, 2012, Sato et al., 2001). It has been generally accepted that the setting procedure is induced by endogenous transglutaminase (endo-TGase) (Yin & Park, 2014). The endo-TGase activity of surimi can be activated by calcium compounds to catalyze acyl transfer reaction between the γ-carboxyl amide carboxamide groups of glutamine and the ε-amino groups of lysine, forming stronger gel networks via ε-(γ-glutamyl)lysine cross-links (Benjakul et al., 2004, Shi et al., 2014). CaCl2 has been widely applied to improve the gel strength of surimi products (Ding et al., 2011, Ramı́rez et al., 2003). However, Ca2+ is also known as a destabilizing salt in the Hofmeister series. It may reduce the free energy required to transfer the non-polar groups into water, weaken the intramolecular hydrophobic interaction and promote the unfolding of proteins (Lertwittayanon, Benjakul, Maqsood, & Encarnacion, 2013). In this unfolding process, some functional groups, such as hydrophobic amino acid residues and sulfhydryl groups, are exposed, and the cross-linking of proteins is enhanced via hydrophobic interactions and disulfide bonds (Hemung and Yongsawatdigul, 2005, Yongsawatdigul and Sinsuwan, 2007). Additionally, when pH is far from the isoelectric point (pI) of the protein, Ca2+ may induce protein cross-linking via the salt bridges among negatively charged carboxyl groups (Arfat & Benjakul, 2012).
Silver carp (Hypophthalmichthys molitrix) is one of the most intensively cultured warm freshwater fish species in China, due to its fast growth rate, high production and low cost of cultivation (Liu, Zhao, Xiong, & Zhang, 2009). However, silver carp does not present good gel formation (Barrera, Ramı́rez, González-Cabriales, & Vázquez, 2002). In our previous study, the gel strength of silver carp surimi was improved by the addition of CaCl2, to enhance the non-disulfide covalent bonds (Liu, Xiong, & Xie, 2006). However, the effect of CaCl2 on denaturation and aggregation of silver carp myosin during setting remains to be elucidated.
The aim of this study was to investigate the effect of CaCl2 on conformation and chemical interactions of silver carp myosin, as well as the relationships between heat-induced aggregation and rheological properties during setting at 40 °C, by circular dichroism spectroscopy, surface hydrophobicity (S0-ANS), total sulfhydryl (SH) content, zeta potential, turbidity, z-average diameter (dz), and dynamic rheological analyses.
Section snippets
Materials
Live silver carp (1.5–2 kg) were obtained from the market of Huazhong Agricultural University and taken to the laboratory in a plastic bag without water within 20 min. The live fish were killed by blunt force trauma to the head and scaled, eviscerated, and washed with tap water. The dorsal muscle was separated manually from skin and bone, and kept at 4 °C for less than 1 h for myosin preparation. All chemicals used were of analytical grade.
Preparation of myosin
Myosin was prepared from the fresh silver carp as described
Effect of CaCl2 on α-helical content of silver carp myosin
Fig. 1 shows the α-helical content of myosin at various CaCl2 concentrations during setting at 40 °C. When incubated at 40 °C for 60 min, the α-helical content of the myosin without CaCl2 decreased from 77.5% to 62.4%, indicating the occurrence of the unfolding of the myosin. This might be related to the instability of hydrogen bonds, which resulted in a double helix-to-single coil transformation (Liu et al., 2008). In the presence of CaCl2, the α-helical content exhibited a similar pattern to
Conclusions
During setting at 40 °C, both CaCl2 and heating induced conformational changes of silver carp myosin, and CaCl2 induced the unfolding of myosin molecules, as evidenced by the loss of α-helical content. S0-ANS increased and incubation time (for S0-ANS to reach the maximum) was reduced with increasing CaCl2 concentration. Total SH content also decreased with increasing CaCl2 concentration, suggesting that CaCl2 promoted the formation of disulfide bonds, myosin denaturation and the subsequent
Acknowledgments
Authors gratefully acknowledge the National Natural Science Foundation of China (31201391), and the China Agriculture Research System (CARS-46-23) for the financial support.
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