Suppression effect of guanidine hydrochloride on α-cyclodextrin-assisted refolding of denatured α-amylase

Liver glycogen fragility in the presence of hydrogen-bond breakers

Glycogen, a complex branched glucose polymer, is responsible for sugar storage in blood glucose homeostasis. It comprises small β particles bound together into composite α particles. In diabetic livers, α particles are fragile, breaking apart into smaller particles in dimethyl sulfoxide, DMSO; they are however stable in glycogen from healthy animals. We postulate that the bond between β particles in α particles involves hydrogen bonding. Liver-glycogen fragility in normal and db/db mice (an animal model for diabetes) is compared using various hydrogen-bond breakers (DMSO, guanidine and urea) at different temperatures. The results showed different degrees of α-particle disruption. Disrupted glycogen showed changes in the mid-infra-red spectrum that are related to hydrogen bonds. While glycogen α-particles are only fragile under harsh, non-physiological conditions, these results nevertheless imply that the bonding between β particles in α particles is different in diabetic livers compared to healthy, and is probably associated with hydrogen bonding.

Allosteric regulation of α-amylase induced by ligands binding

The conformational changes in α-amylase induced by different ligands, including metal ions, substrates, and aromatic compounds in liquor production, were systematically studied using spectroscopy. Fluorescence acrylamide quenching analysis showed that the interaction with active metal cations (K⁺, Na⁺, and Ca²⁺) led to higher exposure of the active sites in α-amylase. In contrast, interactions with substrates (soluble starch, amylose, amylopectin, wheat starch, and dextrin) reduced the degree of exposure of active sites, and the conformation of the enzyme became more rigid and compact. Although the interaction with inhibitory metal cations (Mg²⁺, Zn²⁺) and aromatic compounds generated in the brewing process (guaiacol, eugenol, thymol, and vanillin) increased the exposure of active site with a relatively low amplitude, it reduced the enzymatic activity. This finding may be due to the overall structure of the enzyme becoming looser. Structural stability showed that the active cations and substrates increased the stability of the secondary structure of the α-amylase backbone, whereas the inhibitory cations and aromatic compounds reduced the stability of the backbone but increased the compact of domain A and B. Enzymatic assays and molecular docking experiments strongly supported these conclusions. The experimental results may provide a valuable reference for controlling related conditions and improving production efficiency.

Aggregation-induced emission spectral shift as a measure of local concentration of a pH-activatable rhodamine-based smart probe

Generating activatable probes that report about molecular vicinity through contact-based mechanisms such as aggregation can be very convenient. Specifically, such probes change a particular spectral property only at the intended biologically relevant target. Xanthene derivatives, for example rhodamines, are able to form aggregates. It is typical to examine aggregation by absorption spectroscopy but for microscopy applications utilizing fluorescent probes it is very important to perform characterization by measuring fluorescence spectra. First we show that excitation spectra of aqueous solutions of rhodamine 6G can be very informative about the aggregation features. Next we establish the dependence of the fluorescence emission spectral maximum shift on the dimer concentration. The obtained information helped us confirm the possibility of aggregation of a recently designed and synthesized rhodamine 6G-based pH-activatable fluorescent probe and to study its pH and concentration dependence. The size of the aggregation-induced emission spectral shift at specific position on the sample can be measured by fluorescence microspectroscopy, which at particular pH allows estimation of the local concentration of the observed probe at microscopic level. Therefore, we show that besides aggregation-caused quenching and aggregation-induced emission also aggregation-induced emission spectral shift can be a useful photophysical phenomenon.

Study of cosolvent-induced alpha-chymotrypsin fibrillogenesis: does protein surface hydrophobicity trigger early stages of aggregation reaction?

The misfolding of specific proteins is often associated with their assembly into fibrillar aggregates, commonly termed amyloid fibrils. Despite the many efforts expended to characterize amyloid formation in vitro, there is no deep knowledge about the environment (in which aggregation occurs) as well as mechanism of this type of protein aggregation. Alpha-chymotrypsin was recently driven toward amyloid aggregation by the addition of intermediate concentrations of trifluoroethanol. In the present study, approaches such as turbidimetric, thermodynamic, intrinsic fluorescence and quenching studies as well as chemical modification have been successfully used to elucidate the underlying role of hydrophobic interactions (involved in early stages of amyloid formation) in alpha-chymotrypsin-based experimental system.

Evaluation of artificial chaperoning behavior of an insoluble cyclodextrin-rich copolymer: Solid-phase assisted refolding of carbonic anhydrase

Insoluble beta-cyclodextrin (beta-CD) copolymers have been used for the refolding of thermally and/or chemically denatured carbonic anhydrase with refolding yield of 40% using 300 mg of the copolymer/ml refolding solution containing 0.042 mg/ml protein. In an attempt to enhance the refolding yield with lower quantities of the copolymer, a new beta-CD-rich copolymer with higher beta-CD content was synthesized. Regarding the need for rapid stripping of the detergent molecules from the detergent-protein complexes formed in the capture step of the technique (artificial chaperone-assisted refolding), experimental variables (e.g. copolymer and the protein contents) were optimized to improve the refolding yields along with depressing the aggregate formation. In addition, comparative studies using different ionic detergents and the copolymer were conducted to get a more comprehensive understanding of the detergent's tail length in the stripping step of the process. Our results indicated that under the optimal developed refolding environment, the denatured CA was refolded with a yield of 75% using only 5mg of the copolymer/1.2 ml refolding solution containing 0.0286 mg/ml protein. Taking into account the recycling potential of the copolymer, the new resin, with significant cost-cutting capability, is a suitable candidate for industrial applications.

Control of aggregation in protein refolding: Cooperative effects of artificial chaperone and cold temperature

Refolding of GuHCl-denatured recombinant-human growth hormone (r-hGH) was investigated in both dilution additive and artificial chaperone assisted modes. In both techniques, it was found that CTAB is a better additive (in dilution mode) or a capturing agent (in artificial chaperone method). Neither of the two techniques was capable of complete inhibition of aggregates formed during refolding process. In dilution, using CTAB or alpha-cyclodextrin (alpha-CD) as two different additives, the aggregation was inhibited by almost 55%. However, the extent of inhibition raised to almost 82% in artificial chaperone assisted mode using CTAB as the capturing and alpha-CD as the stripping agents. Maximum inhibition of aggregation (up to 97%) was obtained when the entire process of refolding was done at 4 degrees C. However, under this temperature program, the far-UV CD and intrinsic fluorescence spectra of the refolded samples were not superimposable on their respective native spectra. The spectra superimposibilities were obtained when the refolding process was achieved under a well worked out temperature program: incubation of the sample for 3 min at 4 degrees C after initiation of the stripping step followed by overnight incubation at 22 degrees C. Based on these data, it is expected that higher activity recovery yields of recombinant proteins, particularly at relatively higher protein concentrations, could be achieved by getting a better molecular understanding of major factors responsive for aggregation and refolding pathways.

Protein refolding assisted by molecular tube based α-cyclodextrin as an artificial chaperone

In this study, we evaluated, for the first time, the application of molecular tube based alpha-cyclodextrin for improving the refolding yields of two different enzymes: carbonic anhydrase and alkaline phosphatase. Our results indicate that under the optimal developed refolding environments, the denatured carbonic anhydrase and alkaline phosphatase were refolded with a yield of 51 and 61% using 15 and 5 mg/ml of the molecular tube, respectively. Regardless of lower refolding yields compared with liquid-phase artificial chaperone assisted approach, the new technique (solid-phase artificial chaperone assisted refolding) benefits from easier and faster separation of the refolded product from the refolding environment, recycling of the stripping agent, and finally, significantly less environmental effect at the industrial levels. However, further improvements in solid-phase artificial chaperone assisted technique are needed either through synthesizing better stripping agents or by optimizing and defining better refolding environments.

Fibrinogen-β-Estradiol Binding Studied by Fluorescence Spectroscopy: Denaturation and pH Effects

Fibrinogen is a blood plasma protein that plays a crucial role in hemostasis. It is known that erythrocyte aggregation increases in the presence of fibrinogen, and that beta-estradiol decreases erythrocyte aggregation with a constant fibrinogen concentration. In this work, we have used intrinsic tryptophan fluorescence to obtain information on the conformational changes of fibrinogen upon the recently proposed interaction with beta-estradiol. To evaluate the effect on the conformational changes during fibrinogen-beta-estradiol binding, fluorescence experiments were performed using guanidine hydrochloride (0-6 M) as denaturant, at different pH values. The results obtained for pH 6.5 and 8.0 showed no effect during the binding. The main differences were observed between pH 4.2 and 7.4, in the absence and in the presence of two different denaturant concentrations (1 and 5 M). A red shift of the fluorescence emission from 344 to 354 nm is observed when denaturant concentration is above 3 M for all studied pH values. This phenomenon may be explained by the loss of compact structure of the protein in the presence of denaturant, with tryptophan residues exposure to the aqueous environment and alteration of fibrinogen-beta-estradiol binding. These results demonstrate that the binding sites of fibrinogen are strongly dependent on the conformational state of the protein.