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The rheological behavior, gel properties and nanostructure of complex modified fish scales gelatin (FSG) by pectin and microbial transglutaminase (MTGase) were investigated. The findings suggested that MTGase and pectin have positive effect on the gelation point, melting point, apparent viscosity and gel properties of FSG. The highest values of gel strength and melting temperature could be observed at 0.8% (w/v) pectin. Nevertheless, at highest pectin concentration (1.6% w/v), the gel strength and melting temperature of complex modified gelatin gels decreased. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) analysis revealed that MTGase catalyzed cross-links among soluble fish scales gelatin - pectin complexes, which could be responsible for the observed increase in rheological behavior, gel strength and melting temperature of modified complex gels.

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In this study, gelatin was extracted from bighead carp (Hypophthalmichthys nobilis) scales by water bath (WB) and ultrasound bath (UB) at 60°C for 1 h, 3 h and 5 h, named WB1, WB3, WB5, UB1, UB3 and UB5, respectively. The physicochemical properties of gelatin were investigated. The result indicated that gelatin extracted from bighead carp scales had a high protein content (84.15~91.85 %) with moisture (7.111 3.65 %), low ash content (0.31~0.97 %). All extracted gelatin contained α-and β-chains as the predominant components. Gelatin extracted by UB obtained much higher yield (30.94-46.67 %) than that of WB (19.15-36.39 %). More voids and less sheets of gelatin structure were observed, when the gelatin was extracted by UB for longer time. Gelatin of UB-assisted extraction normally exhibited lower gel strength and melting points than that of WB, which may be resulted from the protein degradation reflected by the FTIR spectra and higher free amino group content. However, there was no significant difference between WB1 and UB1. Therefore, the ultrasound assisted extraction in a short time was a promising method to enhance the yield and obtain gelatin with high quality.

RGS proteins (regulators of G protein signaling) are potent accelerators of the intrinsic GTPase activity of G protein ␣ subunits (GAPs), thus controlling the response kinetics of a variety of cell signaling processes. Most RGS domains that have been studied have relatively little GTPase activating specificity especially for G proteins within the G i subfamily. Retinal RGS9 is unique in its ability to act synergistically with a downstream effector cGMP phosphodiesterase to stimulate the GTPase activity of the ␣ subunit of transducin, G␣ t . Here we report another unique property of RGS9: high specificity for G␣ t . The core (RGS) domain of RGS9 (RGS9) stimulates G␣ t GTPase activity by 10-fold and G␣ i1 GTPase activity by only 2-fold at a concentration of 10 M. Using chimeric G␣ t /G␣ i1 subunits we demonstrated that the ␣-helical domain of G␣ t imparts this specificity. The functional effects of RGS9 were well correlated with its affinity for activated G␣ subunits as measured by a change in fluorescence of a mutant G␣ t (Chi6b) selectively labeled at Cys-210. K d values for RGS9 complexes with G␣ t and G␣ i1 calculated from the direct binding and competition experiments were 185 nM and 2 M, respectively. The ␥ subunit of phosphodiesterase increases the GAP activity of RGS9. We demonstrate that this is because of the ability of P␥ to increase the affinity of RGS9 for G␣ t . A distinct, nonoverlapping pattern of RGS and P␥ interaction with G␣ t suggests a unique mechanism of effector-mediated GAP function of the RGS9.The ␣ subunits of heterotrimeric G proteins function as molecular switches that determine active and inactive states of signaling pathways. The crystal structures of G␣ t 1 and G␣ i1 in their activated, inactive, and transition state forms have revealed the nature of the molecular switches (switches I, II, and III), which are local conformational changes in the regions around the nucleotide binding pocket depending on whether GTP or GDP is bound (1-4). G protein ␣ subunits are activated by seven helical membrane receptors that catalyze the exchange of GDP for GTP by decreasing the affinity of GDP for the ␣ subunit. The lifetime of the active state of an ␣ subunit is defined by the rate of intrinsic GTPase activity that converts GTP to GDP. Therefore, termination of the signal response is dependent on G␣ GTPase activity. Purified G␣ subunits typically display slow (ϳ 4/min) GTP hydrolysis that often cannot account for the deactivation rates of G protein-controlled processes, for example phototransduction (5) and ion channel regulation (6).A large family of regulatory proteins that modulate the inactivation rate of G␣ subunits by accelerating their intrinsic GTPase activity has recently been identified (7,8). These proteins, known as regulators of G protein signaling (RGS), are encoded by at least 19 genes and have been identified in mammalian tissues based on homology to the diagnostic RGS core domain of ϳ 120 amino acid residues. It is not yet certain that all RGS proteins are GAPs (GTPase-accelerati...

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The gelling and structural properties of microbial transglutaminase (MTGase) and pectin modified fish gelatin were compared to investigate their performances on altering fish gelatin properties. Our results showed that within a certain concentration, both MTGase and pectin had positive effects on the gelation point, melting point, gel strength, textural, and swelling properties of fish gelatin. Particularly, low pectin content (0.5%, w/v) could give fish gelatin gels the highest values of gel strength, melting temperature, and hardness. Meantime, flow behavior results showed that both MTGase and pectin could increase fish gelatin viscosity without changing its fluid characteristic, but the latter gave fish gelatin higher viscosity. Both MTGase and pectin could increase the lightness of fish gelatin gels but decreases its transparency. More importantly, fluorescence and UV absorbance spectra, particle size distribution, and confocal microscopy results indicated that MTGase and pectin could change the structure of fish gelatin with the formation of large aggregates. Compared with MTGae modified fish gelatin, pectin could endow fish gelatin had similar gel strength, thermal and textural properties to pig skin gelatin.

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