Thermal stability of the human blood serum acid alpha(1)-glycoprotein in acidic media

Physicochemical properties and microstructural behaviors of rice starch/soy proteins mixtures at different proportions

Rice starch (RS) and soy protein (SP) were mixed at various ratios and the physicochemical, rheological and tribological properties of the resulted pastes were analyzed. Microscopy and spectral techniques were applied to follow the structural changes during cooling and heating processes. Higher proportion of SP resulted in lower swelling power. According to DSC and RVA results, SP inhibited the gelatinization of starch, leading to higher pasting temperature; the peak, trough, final viscosities, set back and enthalpy changes were decreased with increasing SP concentration. All the samples presented a shear thinning behavior and the mixtures with soy proteins showed lower viscosity, yield stress and consistency coefficient than pure starch paste. The creep-recovery test showed that the addition of soy proteins resulted in more liquid-like mixtures, causing weakening of instantaneous elastic recovery. The combination of rice starch and soy proteins increased the friction, compared with their individual components, except RS/SP 9/1 at sliding speed >20 mm/s. According to the microstructure and spectra analysis, the soy protein adhered on the surface of starch granules, which might have hindered the leaching of amylopectin; and the association between starch and soy proteins mainly occurred when the starch granules were gelatinized, which could be reinforced during starch retrogradation.

Temperature-Induced Molten Globule-like State in Human α1-Acid Glycoprotein: An Infrared Spectroscopic Study†

Despite extensive investigations on thermal denaturation of alpha(1)-acid glycoprotein (AGP) using a variety of techniques, structural features of the folded-unfolded state in terms of residual secondary structures and the structural transitions involved in this process have not been fully characterized. In this study we employed FT-IR spectroscopy to investigate the thermal unfolding and reversibility of temperature-induced changes in AGP. The data revealed a fully reversible beta-sheet-rich protein which exhibits a molten globule-like state, an important protein folding intermediate. 2D-IR COS revealed the sequence of the conformational changes occurring before denaturation and confirmed the formation of this intermediate which was further supported by CD spectroscopy. On account of the similarities in the FT-IR spectra of AGP with those of porcine odorant-binding protein (OBP), homology modeling of AGP using OBP as template was performed. The resemblance of AGP and OBP 3D structures confirmed the similarities of data obtained using FT-IR spectroscopy. Overall, FT-IR spectroscopy appears to be useful for investigating the structural characteristics and stability of proteins whose 3D structures are unavailable and for assessing the molten globule-like state in small beta-sheet-rich proteins.

Structure of human alpha1-acid glycoprotein and its high-affinity binding site

Secondary and tertiary structures of human blood alpha(1)-acid glycoprotein, a member of the lipocalin family, have been studied for the first time by infrared and Raman spectroscopies. Vibrational spectroscopy confirmed details of the secondary structure and the structure content predicted by homology modeling of the protein moiety, i.e., 15% alpha-helices, 41% beta-sheets, 12% beta-turns, 8% bands, and 24% unordered structure at pH 7.4. Our model shows that the protein folds as a highly symmetrical all-beta protein dominated by a single eight-stranded antiparallel beta-sheet. Thermal dynamics in the range 20-70 degrees C followed by Raman spectroscopy and analyzed by principle component analysis revealed full reversibility of the protein motion upon heating dominated by decreasing of beta-sheets. Raman difference spectroscopy confirmed the proximity of Trp(122) to progesterone binding.