Unfolding and inactivation of fatty acid synthase from chicken liver during urea denaturation

Cellulase hydrolysis of rice straw and inactivation of endoglucanase in urea solution

In order to optimize the cellulase (from Aspergillus glaucus) hydrolysis of pretreated rice straw, the effects of varying enzyme concentration, temperature, and pH were studied. The best experimental conditions found to degrade the pretreated rice straws were 24 h of incubation at 55 °C and pH 5.0, with an enzyme concentration of 48 mg/L. Urea is one of the important nitrogen sources used in fungi culture, but it is also a denaturant. The model of denaturation of endoglucanase (EG) in urea solutions was established. The denaturation was a slow, reversible reaction. Determination of microscopic rate constants showed k(+0) > k'(+0), indicating that EG was protected by the substrate to a certain extent during denaturation. Comparison with the results from fluorescence emission spectroscopy revealed that the inactivation of EG occurred before the marked conformational changes could be detected.

The role of disulfide bonds in the conformational stability and catalytic activity of phytase

Previous studies have predicted five disulfide bonds in Aspergillus niger phytase (phy A). To investigate the role of disulfide bonds, intrinsic fluorescence spectra, far-ultraviolet circular dichroism (CD) spectra, and an enzyme activity assay were used to compare the differences of catalytic activity and conformational stability of phytase during denaturation in urea in the presence and absence of dithiothreitol (DTT). In the presence of 2 mM DTT, the inactivation and unfolding were greatly enhanced at the same concentration of denaturant. The fluorescence emission maximum red shift and decreases of ellipticity at 222 nm were in accord with the changes of catalytic activity. The kinetics of the unfolding courses were a biphasic process consisting of two first-order reactions in the absence of DTT and a monophasic process of a first-order reaction in the presence of DTT. The results suggested that the loss of enzymatic activity was most likely because of a conformational change, and that disulfide bonds played an important role in three-dimensional structure and catalytic activity.

Effect of chaotropic agents on reversible unfolding of a soybean (Glycine max) seed acid phosphatase

In this work we examined the effect of urea and guanidinium chloride on the structural stability of a single isoform of soybean seed acid phosphatase, based on the intensity of tryptophan fluorescence as a function of denaturant concentration. The free energy of unfolding, DeltaGu, was calculated at 25 degrees C as a function of the concentrations of both chaotropic agents; the conformational stability, DeltaG (H2O), was determined to be 2.48 kcal mol(-1). Center of mass, determined from analysis of fluorescence data, was used as a parameter to assess conformational changes. Our results indicate that complete enzyme inactivation occurred before full enzyme unfolding in both cases, and suggest that there are differences between the conformational flexibility of the active-site and that of the macromolecule as a whole.

Kinetic inactivation study of mushroom tyrosinase: intermediate detection by denaturants

The unfolding and inhibition study of mushroom tyrosinase have been studied in the presence of different denaturants such as sodium dodecyl sulfate (SDS), guanidine hydrochloride (GdnHCl), and urea. The kinetic two-phase rate constants were commonly measured from semilogarithmic plots of the activity versus time, which resolved into two straight lines, indicating that the inactivation process consisted of fast and slow phases as a first-order reaction. This result also implied that transient partially folded intermediate existed during tyrosinase unfolding pathway. Mushroom tyrosinase had different behaviors to denaturants in regard with: noncooperative binding manner by SDS while cooperative interactions by GdnHCl and urea; in equilibrium state, SDS-micelle never completely inactivated enzyme activity while GdnHCl has single step denaturation and urea induced a typical transition-like process. Various kinetic parameters for each denaturant were calculated and the possible unfolding pathway scheme was discussed.

Inhibitive effect of zinc ion on fatty acid synthase from chicken liver

Fatty acid synthase (FAS; acyl-CoA:malonyl-CoA C-acyltransferase [decarboxylating, oxoacyl- and enoyl-reducing and thioester-hydrolyzing], EC 2.3.1.85) is an important enzyme participating in energy metabolism in vivo which is related to adiposis and cancer [Cancer Lett. 167 (1) (2001) 99; Nat. Med. 8 (4) (2002) 335]. Tests of fast- and slow-binding inhibitions showed that fatty acid synthase of chicken liver is rapidly and irreversibly inactivated by low Zn(2+) concentrations. Electrophoresis and FPLC results showed that FAS cross-links occurred in the presence of high Zn(2+) concentrations (>4 microM) which may be another reason that FAS lost its activity. The modification velocity of FAS by DTNB decreased with increasing Zn(2+) concentration, which confirmed that Zn(2+) interacted with SH groups. Substrate protective experiments and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that all three substrates tested had some protective effects on FAS in the presence of Zn(2+), and malonyl-CoA was the most effective of the three substrates. In the presence of malonyl-CoA, the activity loss of FAS decreased sharply and almost no cross-link was observed in SDS-PAGE. This suggests that the phosphopantetheine SH group is the critical group in the cross-link and inhibition of FAS in the presence of Zn(2+).