Inactivation before significant conformational change during denaturation of papain by guanidine hydrochloride

Reversible Inhibition of Iron Oxide Nanozyme by Guanidine Chloride

Nanozymes have been widely applied in bio-assays in the field of biotechnology and biomedicines. However, the physicochemical basis of nanozyme catalytic activity remains elusive. To test whether nanozymes exhibit an inactivation effect similar to that of natural enzymes, we used guanidine chloride (GuHCl) to disturb the iron oxide nanozyme (IONzyme) and observed that GuHCl induced IONzyme aggregation and that the peroxidase-like activity of IONzyme significantly decreased in the presence of GuHCl. However, the aggregation appeared to be unrelated to the quick process of inactivation, as GuHCl acted as a reversible inhibitor of IONzyme instead of a solo denaturant. Inhibition kinetic analysis showed that GuHCl binds to IONzyme competitively with H₂O₂ but non-competitively with tetramethylbenzidine. In addition, electron spin resonance spectroscopy showed that increasing GuHCl level of GuHCl induced a correlated pattern of changes in the activity and the state of the unpaired electrons of the IONzymes. This result indicates that GuHCl probably directly interacts with the iron atoms of IONzyme and affects the electron density of iron, which may then induce IONzyme inactivation. These findings not only contribute to understanding the essence of nanozyme catalytic activity but also suggest a practically feasible method to regulate the catalytic activity of IONzyme.

Kinetic Analysis of Guanidine Hydrochloride Inactivation of β-Galactosidase in the Presence of Galactose

Inactivation of purified β-Galactosidase was done with GdnHCl in the absence and presence of varying [galactose] at 50°C and at pH 4.5. Lineweaver-Burk plots of initial velocity data, in the presence and absence of guanidine hydrochloride (GdnHCl) and galactose, were used to determine the relevant K_m and V_max values, with p-nitrophenyl β-D-galactopyranoside (pNPG) as substrate, S. Plots of ln([P]_∞ − [P]_t) against time in the presence of GdnHCl yielded the inactivation rate constant, A. Plots of A versus [S] at different galactose concentrations were straight lines that became increasingly less steep as the [galactose] increased, showing that A was dependent on [S]. Slopes and intercepts of the 1/[P]_∞ versus 1/[S] yielded k₊₀ and k'₊₀, the microscopic rate constants for the free enzyme and the enzyme-substrate complex, respectively. Plots of k₊₀ and k'₊₀ versus [galactose] showed that galactose protected the free enzyme as well as the enzyme-substrate complex (only at the lowest and highest [galactose]) against GdnHCl inactivation. In the absence of galactose, GdnHCl exhibited some degree of non-competitive inhibition. In the presence of GdnHCl, galactose exhibited competitive inhibition at the lower [galactose] of 5 mM which changed to non-competitive as the [galactose] increased. The implications of our findings are further discussed.

Biochemical and spectroscopic characterization of almond and cashew nut seed 11S legumins, amandin and anacardein

Native, undenatured amandin and anacardein secondary structures were estimated to be, respectively, 56.4 and 49% β-sheet, 14 and 23.7% α-helix, and 29.6 and 27.4% random coil. Circular dichroic (CD) and fluorescence spectroscopy were used to assess structural changes in amandin and anacardein subjected to denaturing treatments that included heat (100 °C, 5 min), guanidium HCl (GuHCl), urea, sodium dodecyl sulfate (SDS), and reducing agent, 2% v/v β-mercaptoethanol (βME) + heat. Mouse monoclonal antibodies (mAbs) 4C10 and 4F10 directed against amandin and 1F5 and 4C3 directed against anacardein were used to assess the influence of denaturing treatments on the immunoreactivity of amandin and anacardein. Among the denaturing treatments investigated, SDS and β-ME caused a significant reduction in the immunoreactivity of amandin and anacardein when probed with mAb 4C10 and 4C3, respectively.

Kinetic analysis of urea-inactivation of beta-galactosidase in the presence of galactose

The effect of galactose on the inactivation of purified beta-galactosidase from the black bean, Kestingiella geocarpa, in 5 M urea at 50 degrees C and at pH 4.5, was determined. Lineweaver-Burk plots of initial velocity data in the presence and absence of urea and galactose were used to determine the relevant K(m) and V(max) values, with p-nitrophenyl beta-D-galactopyranoside (PNPG) as substrate, S. The inactivation data were analysed using the Tsou equation and plots. Plots of ln([P](infinity) - [P](t) ) against time in the presence of urea yielded the inactivation rate constant, A. Plots of A vs [S] at different galactose concentrations were zero order showing that A was independent of [S]. Plots of [P](infinity) vs [S] were used to determine the mode of inhibition of the enzyme by galactose, and slopes and intercepts of the 1/[P](infinity) vs. 1/[S] yielded k(+0) and k '(+0), the microscopic rate constants for the free enzyme and the enzyme-substrate complex, respectively. Plots of k(+0) and k '(+0) vs. galactose concentrations showed that galactose protected the free enzyme and not the enzyme-substrate complex against urea inactivation via a noncompetitive mechanism at low galactose concentrations and a competitive pattern of inhibition at high galactose concentrations. The implication of the different modes of inhibition in protecting the free enzyme was discussed.

Structure and denaturation of 4-chlorobenzoyl coenzyme A dehalogenase from Arthrobacter sp. strain TM-1

The secondary structure of the trimeric protein 4-chlorobenzoyl coenzyme A dehalogenase from Arthrobacter sp. strain TM-1, the second of three enzymes involved in the dechlorination of 4-chlorobenzoate to form 4-hydroxybenzoate, has been examined. E(mM) for the enzyme was 12.59. Analysis by circular dichroism spectrometry in the far uv indicated that 4-chlorobenzoyl coenzyme A dehalogenase was composed mostly of alpha-helix (56%) with lesser amounts of random coil (21%), beta-turn (13%) and beta-sheet (9%). These data are in close agreement with a computational prediction of secondary structure from the primary amino acid sequence, which indicated 55.8% alpha-helix, 33.7% random coil and 10.5% beta-sheet; the enzyme is, therefore, similar to the 4-chlorobenzoyl coenzyme A dehalogenase from Pseudomonas sp. CBS-3. The three-dimensional structure, including that of the presumed active site, predicted by computational analysis, is also closely similar to that of the Pseudomonas dehalogenase. Study of the stability and physicochemical properties revealed that at room temperature, the enzyme was stable for 24 h but was completely inactivated by heating to 60 degrees C for 5 min; thereafter by cooling at 1 degrees C min(-1) to 45 degrees C, 20.6% of the activity could be recovered. Mildly acidic (pH 5.2) or alkaline (pH 10.1) conditions caused complete inactivation, but activity was fully recovered on returning the enzyme to pH 7.4. Circular dichroism studies also indicated that secondary structure was little altered by heating to 60 degrees C, or by changing the pH from 7.4 to 6.0 or 9.2. Complete, irreversible destruction of, and maximal decrease in the fluorescence yield of the protein at 330-350 nm were brought about by 4.5 M urea or 1.1 M guanidinium chloride. Evidence was obtained to support the hypothetical three-dimensional model, that residues W140 and W167 are buried in a non-polar environment, whereas W182 appears at or close to the surface of the protein. At least one of the enzymes of the dehalogenase system (the combined 4-chlorobenzoate:CoA ligase, the dehalogenase and 4-hydroxybenzoyl coenzyme A thioesterase) appears to be capable of association with the cell membrane.

Inactivation kinetics of guanidinium chloride on Penaeus vannamei beta-N-acetyl-D-glucosaminidase and the relationship of enzyme activity and its conformation

The effects of guanidinium chloride (GuHCl) on the activity of Penaeus vannamei beta-N-acetyl-D-glucosaminidase (NAGase) have been studied. The results show that GuHCl, at appropriate concentrations, can lead to reversible inactivation of the enzyme, and the IC50 is estimated to be 0.6 M. Changes of activity and conformation of the enzyme in different concentrations of GuHCl have been studied by measuring the fluorescence spectra and its relative activity after denaturation. The fluorescence intensity of the enzyme decreases distinctly with increasing GuHCl concentrations, and the emission peaks appear red-shifted (from 339.4 to 360 nm). Changes in the conformation and catalytic activity of the enzyme are compared. The extent of inactivation is greater than that of conformational changes, indicating that the active site of the enzyme is more flexible than the whole enzyme molecule. The kinetics of inactivation has been studied using the kinetic method of the substrate reaction. The rate constants of inactivation have been determined. The value of k(+0) is larger than that of k'(+0) which suggests that the enzyme is protected by substrate to a certain extent during guanidine denaturation.

Unfolding and inactivation of Ampullarium crossean beta-glucosidase during denaturation by guanidine hydrochloride

Changes of activity and conformation of Ampullarium crossean beta-glucosidase in different concentrations of guanidine hydrochloride (GuHCl) have been studied by measuring the fluorescence spectra and its relative activity after denaturation. The fluorescence intensity of the enzyme decreased distinctly with increasing guanidine concentrations, the emission peaks appeared red shifted (from 338.4 to 350.8 nm), whereas a new fluorescence emission peak appeared near 310 nm. Changes in the conformation and catalytic activity of the enzyme were compared. A corresponding rapid decrease in catalytic activity of the enzyme was also observed. The extent of inactivation was greater than that of conformational changes, indicating that the active site of the enzyme is more flexible than the whole enzyme molecule. k(+0)>k(+0)' also showed that the enzyme was protected by substrate to a certain extent during guanidine denaturation.

Acid and Chemical Induced Conformational Changes of Ervatamin B. Presence of Partially Structured Multiple Intermediates

The structural and functional aspects of ervatamin B were studied in solution. Ervatamin B belongs to the alpha + beta class of proteins. The intrinsic fluorescence emission maximum of the enzyme was at 350 nm under neutral conditions, and at 355 nm under denaturing conditions. Between pH 1.0- 2.5 the enzyme exists in a partially unfolded state with minimum or no tertiary structure, and no proteolytic activity. At still lower pH, the enzyme regains substantial secondary structure, which is predominantly a beta-sheet conformation and shows a strong binding to 8-anilino-1- napthalene-sulfonic acid (ANS). In the presence of salt, the enzyme attains a similar state directly from the native state. Under neutral conditions, the enzyme was stable in urea, while the guanidine hydrochloride (GuHCl) induced equilibrium unfolding was cooperative. The GuHCl induced unfolding transition curves at pH 3.0 and 4.0 were non-coincidental, indicating the presence of intermediates in the unfolding pathway. This was substantiated by strong ANS binding that was observed at low concentrations of GuHCl at both pH 3.0 and 4.0. The urea induced transition curves at pH 3.0 were, however, coincidental, but non-cooperative. This indicates that the different structural units of the enzyme unfold in steps through intermediates. This observation is further supported by two emission maxima in ANS binding assay during urea denaturation. Hence, denaturant induced equilibrium unfolding pathway of ervatamin B, which differs from the acid induced unfolding pathway, is not a simple two-state transition but involves intermediates which probably accumulate at different stages of protein folding and hence adds a new dimension to the unfolding pathway of plant proteases of the papain superfamily.

Structural characterization of a highly stable cysteine protease ervatamin C

Ervatamin C, a novel cysteine protease, belongs to alpha + beta class of proteins, probably with two domains, and retains both secondary and tertiary structures along with biological activity over a wide range of pH (2-12). Under neutral conditions, GuHCl and temperature-induced unfolding was cooperative with high transition midpoints and shows no structural changes in the presence of urea reflecting a remarkable stability. The fluorescence emission maximum at 350 nm suffers a blue shift of 4-5 nm upon lowering the pH and a red shift of 5 nm under denatured conditions. Unfolding transition curves at pH 2.0 are non-coincidental indicating the presence of intermediates in the unfolding pathway. At extremely low pH, the enzyme loses all the tertiary structure and proteolytic activity but retains a predominant secondary structure and a strong binding to ANS. GuHCl-induced unfolding of the enzyme in this intermediate state is noncooperative and indicates sequential unfolding of the domains.

Comparison of conformational changes and inactivation of soybean lipoxygenase-1 during urea denaturation

The unfolding and inactivation of soybean lipoxygenase-1 during urea denaturation has been compared. Equilibrium study indicates that inactivation of the enzyme occurs at low urea concentrations before significant conformational change of the molecule as a whole. In the presence of 6.0 M urea, the unfolding of soybean lipoxygenase-1, as monitored by fluorescence intensity, is a triphasic process, while the inactivation of the enzyme shows single-phase kinetics. The rate constant of inactivation is consistent with that of the fast conformational change of the enzyme. The results suggest that active sites of lipoxygenase-1 containing iron cofactor are situated in a limited region of the enzyme molecule that is more fragile to denaturants than the protein as a whole. The kinetic theory of substrate reactions catalyzed by unstable enzymes (Duggleby (1986) J. Theor. Biol. 123, 67-80) has been applied to study the effect of substrate on enzyme inactivation. On the basis of the kinetic equation of substrate reaction in the presence of urea, inactivation rate constants for the free enzyme and enzyme-substrate complex have been determined. The substrate, linoleic acid, has no effect on inactivation of the ferric form of lipoxygenase-1.

The inactivation kinetics of papain by guanidine hydrochloride: a re-analysis

The kinetic theory of the substrate reaction during modification of enzyme activity has been applied to study the inactivation kinetics of enzymes by denaturant. However, an important problem related to the determination of the inactivation rate constants has not been considered in a previous publication (Xiao, et al., Biochim. Biophys. Acta, 1164 (1993) 54-60). In most denaturation experiments, the high concentrations of denaturants may greatly affect the kinetic behavior of the system to preclude the use of the kinetic parameters determined in the absence of denaturant. In the present study, the kinetic equation of substrate reaction in presence of denaturant has been derived. A re-examination of the effect of high concentrations of guanidine hydrochloride on the inactivation of papain, taking into consideration the effect of high concentrations of guanidine hydrochloride on the Michaelis constant, showed that, for papain, the substrate gives no protection on its inactivation. It is the purpose of the present communication to stress the importance of observing the effect of the denaturant on the kinetic parameters for kinetic analysis of enzyme inactivation by denaturants generally.

Purification and characterization of papaya glutamine cyclotransferase, a plant enzyme highly resistant to chemical, acid and thermal denaturation

Papaya glutamine cyclotransferase (PQC), present in the laticiferous cells of the tropical species Carica papaya, was purified near to homogeneity. Starting from the soluble fraction of the collected plant latex, a combination of ion-exchange chromatography on SP-Sepharose Fast Flow, hydrophobic interaction chromatography on Fractogel TSK Butyl-650 and affinity chromatography on immobilized trypsin provided a purification factor of 279 with an overall yield of 80%. In the course of the purification procedure, the two solvent accessible thiol functions located on the hydrophobic surface of the enzyme were converted into their S-methylthioderivatives. Papaya QC, a glycoprotein with a molecular mass of 33000 Da, contains a unique and highly basic polypeptide chain devoid of disulfide bridges as well as of covalently attached phosphate groups. Its absorption spectrum is dominated by the chromophores tyrosine which, nonetheless, do not contribute to the fluorescence emission of the plant enzyme. With a lambdamax of emission at 338 nm and a moderate susceptibility to be quenched by acrylamide, most of the tryptophyl residues of papaya QC appear to be sterically shielded by surrounding protein atoms. Fluorescence can thus be used to monitor unfolding of this enzyme. Preliminary experiments show that papaya QC is exceptionally resistant to chemical (guanidinium hydrochloride), acid and thermal denaturation. At first sight also, this enzyme exhibits high resistance to proteolysis by the papaya cysteine proteinases, yet present in great excess (around 100 mol of proteinases per mol of PQC) in the plant latex. Altogether, these results awaken much curiosity and interest to further investigate how the structure of this plant enzyme is specified.

Comparison of inactivation and unfolding of yeast alcohol dehydrogenase during thermal denaturation

It has been reported that inactivation occurs before noticeable conformational change can be detected during denaturation of creatine kinase (ATP:creatine N-phosphotransferase, EC 2.7.3.2) and other enzymes by guanidinium chloride or urea. It has therefore been suggested that enzyme active sites may display more conformational flexibility than the enzyme molecules as a whole. The present paper compares the inactivation and unfolding of yeast alcohol dehydrogenase during thermal denaturation. Under identical conditions, inactivation takes place before noticeable conformational changes. Kinetics of unfolding can be resolved into two phases. For a given temperature, the fast phase rates are about one order of magnitude slower than the inactivation rates of the free enzyme and approximately the same magnitude as the inactivation rates of enzyme-substrate complexes. This is general accord with the suggestion made previously by Tsou, indicating that the active sites of metal enzymes are situated in a region more flexible than the molecules as a whole.

Kinetics of the thermal inactivation of alkaline phosphatase from green crab (Scylla serrata)

The kinetics of thermal inactivation of alkaline phosphatase from green crab (Scylla Serrata) has been studied using the kinetic method relating to the substrate reaction during irreversible inhibition of enzyme activity previously described by Tsou. The results show that the thermal inactivation of the enzyme is an irreversible reaction. Comparison of the microscopic rate constants for thermal inactivation of free enzyme and the enzyme-substrate complex shows that the presence of substrate has a certain protective effect against thermal inactivation.

Comparison of inactivation and conformational changes of aminoacylase during denaturation in lithium dodecylsulphate solutions

The denaturation of aminoacylase in LDS solutions of different concentrations has been studied by following the changes in the ultraviolet absorbance, circular dichroism and intrinsic fluorescence. The results obtained show that the denaturation of the enzyme results in negative peaks at 287 and 295 nm in the denatured minus native enzyme difference spectrum. The fluorescence emission intensify of the enzyme decreases with no red shift of emission maximum in LDS solutions of increasing concentrations. In the LDS concentration regions employed in present study, no marked changes of secondary structure of the enzyme have been observed by following the changes in far ultraviolet CD spectra. The inactivation of this enzyme has been followed and compared with the unfolding observed during denaturation in LDS solutions. A marked inactivation is already evident at low LDS concentrations before signification conformational changes can be detected by ultraviolet absorbance and fluorescence changes. The inactivation rate constants of free enzyme and substrate-enzyme complex were determined by the kinetics method of the substrate reaction in the presence of inactivator previously described by Tsou. At the same LDS concentrations, the inactivation rate constants of the enzyme are a order of magnitude faster than the rate constants of conformational changes at least. The above results show that the active sites of metal enzyme containing Zn2+ are also situated in a limited and flexible region of the enzyme molecule that is more fragile to denaturants than the protein as a whole.

Human placental estradiol 17 beta-dehydrogenase: structural and catalytic changes during urea denaturation

The denaturation behavior of human placental estradiol 17 beta-dehydrogenase (EC 1.1.1.62) in urea was studied by following changes in enzyme activity, conformation and oligomeric state. Results showed that the native --> unfolded transition follows a complex pattern, in which changes in both secondary and tertiary structure are simultaneous with changes in the aggregation state of enzyme. At relatively low urea (< 3 M), a major conformational transition, as monitored by CD and fluorescence measurements, is concomitant with an expanded state of the enzyme that coincides with its inactivation and the formation of polymeric species. Protein structural changes were also monitored by using the hydrophobic probe 1-anilinonaphthalene-8-sulfonic acid. The combined data suggest the existence of a molten globule state of dimeric enzyme promoted by low urea concentrations. Dilution of urea at this stage results in a full recovery of the enzymatic activity as well as of the native dimeric structure. Between 3 and 5 M urea estradiol 17 beta-dehydrogenase exists as a mixture of high molecular mass species which may be resolved by electrophoresis. In this range of urea concentration, only minor conformational changes were detected, although inactivation becomes to be irreversible. Above 5 M urea a second conformational transition takes place. Electrophoretic analysis of cross-linked samples revealed this stage results in the complete dissociation of enzyme toward unfolded monomer. It is concluded that the inactivation and unfolding of estradiol 17 beta-dehydrogenase during denaturation by urea occurs with the formation of intermediate species with different stability in which a molten globule-like state appears to be involved. The irreversibility of the process above urea 3 M is explained as the inability of aggregated enzyme to dissociate into native dimers.

Inactivation and conformational changes of aminoacyclase in trifluoroethanol solutions

The inactivation and unfolding of aminoacyclase (EC 3.5.1.14) during denaturation by different concentrations of trifluoroethanol (TFE) have been studied. A marked decrease in enzyme activity was observed at low TFE concentrations. The kinetic theory of the substrate reaction during irreversible inhibition of enzyme activity described previously by Tsou [Tsou (1988), Adv. Enzymol. Related Areas Mol. Biol. 61, 381-436] was applied to study the kinetics of the inactivation course of aminoacyclase during denaturation by TFE. The inactivation rate constants for the free enzyme and substrate-enzyme complex were determined by Tsou's method. The inactivation reaction was a monophasic first-order reaction. The kinetics of the unfolding course were a biphasic process consisting of two first-order reactions. At 2% TFE concentration, the inactivation rate of the enzyme was much faster than the unfolding rate. At a higher concentration of TFE (10%), the inactivation rate was too fast to be determined by conventional methods, whereas the unfolding course remained as a biphasic process with fast and slow reactions occurring at measurable rates. The results suggest that the aminoacyclase active site containing Zn2+ ions is situated in a limited and flexible region of the enzyme molecule that is more fragile to the denaturant than the protein as a whole.

Sequential unfolding of adenylate kinase during denaturation by guanidine hydrochloride

The unfolding of adenylate kinase in GuHCl of increasing concentrations has been followed by a combination of different methods. Molecular packing was measured by size-exclusion chromatography (SEC), exposure of buried Tyr residues by second- derivative spectra, loss of secondary structure by circular dichroism in the far-ultraviolet and the decrease in surface hydrophobicity by ANS binding. The conformational changes of adenylate kinase as followed by the above methods depend differently on GuHCl concentration. The concentrations of GuHCl at which 50% changes as measured by the above four methods occur are 0.3, 0.46, 0.64 and 0.64 M, respectively. SEC measurements show that with increasing GuHCl concentrations, the process of unfolding of adenylate kinase involves two slowly interconvertible intermediate stages, I1, and I2, the last is in a more advanced state of unfolding but is still more compact than the fully unfolded state, U, as indicated by their elution volumes in the SEC profile. There is also evidence to suggest that both the intermediates I1 and I2 may contain additional intermediary components in rapid equilibrium as indicated by the gradual shift of both peaks in the SEC elution profile. A sequential mechanism is suggested for the unfolding of adenylate kinase with increasing guanidine hydrochloride concentrations.

Charge-shielding and the "paradoxical" stimulation of tubulin polymerization by guanidine hydrochloride

Low concentrations of guanidine hydrochloride (GuHCl) increase the rate (and to a lesser degree, the extent) of tubulin polymerization as assessed by light scattering. Maximum enhancement occurs at 120-160 mM GuHCl followed by decreases at higher GuHCl. The latent period is decreased, and there is a 3-4 fold reduction in the critical concentration of polymerization. Electronmicrographs reveal microtubules in the controls and an increasing fraction of total polymers present as aberrant microtubules as the GuHCl concentration is increased from 20 to 100 mM. The GuHCl effect is markedly reduced, but not abolished, in tubulin S (in which the anionic C termini of both monomers have been removed). The GuHCl-induced polymerization has an absolute requirement for GTP and taxol or DMSO, is very sensitive to podophyllotoxin inhibition, and can overcome urea-mediated inhibition of polymerization. Guanidinium analogues mimic the GuHCl effect roughly as a function of the number of potential hydrogen bonds. The anions of the guanidine salts superimpose their inhibitory action on the guanidinium cation effect according to the lyotropic series. At higher GuHCl concentrations (peak effect 500-700 mM), a different polymer (type II) is formed that is GTP and taxol independent, but whose polymerization is retarded but not prevented by podophyllotoxin. Its structure resembles the fibrillar network seen in unfolding intermediates of other proteins. We conclude that both charge and hydrogen-bonding ability are major contributors to the GuHCl-induced promotion of tubulin polymerization, and that charge-shielding is likely to be the basis for this effect.

Comparison of inactivation and unfolding of green crab (Scylla serrata) alkaline phosphatase during denaturation by guanidinium chloride

Green crab (Scylla serrata) alkaline phosphatase (EC 3.1.3.1) is a metalloenzyme, each active site in which contains a tight cluster of two zinc ions and one magnesium ion. Unfolding and inactivation of the enzyme during denaturation in guanidinium chloride (GuHCl) solutions of different concentrations have been compared. The kinetic theory of the substrate reaction during irreversible inhibition of enzyme activity previously described by Tsou [(1988), Adv. Enzymol. Related Areas Mol. Biol. 61, 381-436] has been applied to a study on the kinetics of the course of inactivation of the enzyme during denaturation by GuHCl. The rate constants of unfolding and inactivation have been determined. The results show that inactivation occurs before noticeable conformational change can be detected. It is suggested that the active site of green crab alkaline phosphatase containing multiple metal ions is also situated in a limited region of the enzyme molecule that is more fragile to denaturants than the protein as a whole.

Inactivation and unfolding of aminoacylase during denaturation in sodium dodecyl sulfate solutions

During denaturation by sodium dodecyl sulfate (SDS), aminoacylase shows a rapid decrease in activity with increasing concentration of the detergent to reach complete inactivation at 1.0 mM SDS. The denatured minus native-enzyme difference spectrum showed two negative peaks at 287 and 295 nm. With the increase of concentration of SDS, both negative peaks increased in magnitude to reach maximal values at 5.0 mM SDS. The fluorescence emission intensity of the enzyme decreased, whereas there was no red shift of emission maximum in SDS solutions of increasing concentration. In the SDS concentration regions employed in the present study, no marked changes of secondary structure of the enzyme have been observed by following the changes in far-ultraviolet CD spectra. The inactivation of this enzyme has been followed and compared with the unfolding observed during denaturation in SDS solutions. A marked inactivation is already evident at low SDS concentration before significant conformational changes can be detected by ultraviolet absorbance and fluorescence changes. The inactivation rate constants of free enzyme and substrate-enzyme complex were determined by the kinetics method of the substrate reaction in the presence of inactivator previously described by Tsou [Tsou (1988), Adv. Enzymol. Related Areas Mol. Biol. 61, 381-436]. It was found that substrate protects against inactivation and at the same SDS concentrations, the inactivation rate of the free enzyme is much higher than the unfolding rate. The above results show that the active sites of metal enzyme containing Zn2+ are also situated in a limited and flexible region of the enzyme molecule that is more fragile to denaturants than the protein as a whole.

Comparison of inactivation and conformational changes of aminoacylase during guanidinium chloride denaturation

The inactivation and unfolding of aminoacylase (EC 3.5.1.14) during denaturation by different concentrations of guanidinium chloride (GuHCl) have been compared. A marked decrease in enzyme activity is already evident at low GuHCl concentrations before significant unfolding of the enzyme molecule, as monitored by fluorescence, ultraviolet difference absorption and CD measurement. The kinetic theory of the substrate reaction during irreversible inhibition of enzyme activity previously described by Tsou has been applied to a study on the kinetics of the course of inactivation of aminoacylase during denaturation by GuHCl. The inactivation rate constants of free enzyme and substrate-enzyme complex were determined by Tsou's method. The inactivation reaction kinetics were found to be a monophasic first-order reaction. The kinetics of the unfolding were a bisphasic process consisting of two first-order reactions. At lower GuHCl concentration (< 1.0 M), the enzyme activity was stripped at a high rate whereas its conformation was only slightly affected. At 1.0 M GuHCl, the inactivation rate of the enzyme was much faster than the unfolding rate. At higher GuHCl concentrations (> 1.0 M), the inactivation rate was too fast to be measured by conventional dynamic methods, whereas the unfolding remained as a bisphasic process with the fast reaction accruing very fast and the slow reaction occurring at a measurable rate. The results suggest that active sites of aminoacylase containing Zn2+ ions are situated in a limited region of the enzyme molecule that is more fragile to denaturants than the protein as a whole.