Why is Aged Acetylcholinesterase So Difficult to Reactivate?

Organophosphorus agents are potent inhibitors of acetylcholinesterase. Inhibition involves successive chemical events. The first is phosphylation of the active site serine to produce a neutral adduct, which is a close structural analog of the acylation transition state. This adduct is unreactive toward spontaneous hydrolysis, but in many cases can be reactivated by nucleophilic medicinal agents, such as oximes. However, the initial phosphylation reaction may be followed by a dealkylation reaction of the incipient adduct. This reaction is called aging and produces an anionic phosphyl adduct with acetylcholinesterase that is refractory to reactivation. This review considers why the anionic aged adduct is unreactive toward nucleophiles. An alternate approach is to realkylate the aged adduct, which would render the adduct reactivatable with oxime nucleophiles. However, this approach confronts a considerable—and perhaps intractable—challenge: the aged adduct is a close analog of the deacylation transition state. Consequently, the evolutionary mechanisms that have led to transition state stabilization in acetylcholinesterase catalysis are discussed herein, as are the challenges that they present to reactivation of aged acetylcholinesterase.

Enzyme reactivation by hydrogen peroxide in heme-based tryptophan dioxygenase

An intriguing mystery about tryptophan 2,3-dioxygenase is its hydrogen peroxide-triggered enzyme reactivation from the resting ferric oxidation state to the catalytically active ferrous form. In this study, we found that such an odd Fe(III) reduction by an oxidant depends on the presence of L-Trp, which ultimately serves as the reductant for the enzyme. In the peroxide reaction with tryptophan 2,3-dioxygenase, a previously unknown catalase-like activity was detected. A ferryl species (δ = 0.055 mm/s and ΔE(Q) = 1.755 mm/s) and a protein-based free radical (g = 2.0028 and 1.72 millitesla linewidth) were characterized by Mössbauer and EPR spectroscopy, respectively. This is the first compound ES-type of ferryl intermediate from a heme-based dioxygenase characterized by EPR and Mössbauer spectroscopy. Density functional theory calculations revealed the contribution of secondary ligand sphere to the spectroscopic properties of the ferryl species. In the presence of L-Trp, the reactivation was demonstrated by enzyme assays and by various spectroscopic techniques. A Trp-Trp dimer and a monooxygenated L-Trp were both observed as the enzyme reactivation by-products by mass spectrometry. Together, these results lead to the unraveling of an over 60-year old mystery of peroxide reactivation mechanism. These results may shed light on how a metalloenzyme maintains its catalytic activity in an oxidizing environment.

Oxime-Induced Reactivation of Sarin-Inhibited AChE: A Theoretical Mechanisms Study

Oximes (especially oximate anions) are used as potential reactivators of OP-inhibited AChE due to their unique alpha-effect nucleophilic reactivity. In the present study, by applying the DFT approach at the B3LYP/6-311G(d,p) level and the Møller-Plesset perturbation theory at the MP2/6-311G(d,p) level, the formoximate-induced reactivation patterns of the sarin-AChE adduct and the corresponding reaction mechanism have been investigated. The potential energy surface along the pathway of the reactivation reaction of sarin-inhibited AChE by oxime reveals that the reaction can occur quickly due to the relatively low energy barriers. A two-step process is a major pathway proposed for the studied reactivation reaction. Through the nucleophilic attack, the oximate first binds to the sarin-AChE adduct to form a relatively stable phosphorus complex. The regeneration of the serine takes place subsequently through an elimination step, which is expected to be competitive with the nucleophilic attacking process. The polarizable continuum model (PCM) has been applied to evaluate the solvate effects on the pathway. It is concluded that the reaction energy barriers are also low enough for the reaction to easily occur in solvent. The results derived from both the gas-phase model and the aqueous solvation model suggest that the studied oximate anion is an efficient antidote reagent for sarin-inhibited AChE.

Assisting the reactivation of guanidine hydrochloride-denatured aminoacylase by hydroxypropyl cyclodextrins

Cyclodextrin is a water-soluble circular oligosaccharide with a cylinder shape characterized by exterior hydrophilic rims and an interior hydrophobic cavity, which makes it an ideal additive to prevent proteins from aggregating during refolding. In this research, three hydroxypropyl cyclodextrins (HPCDs), HP-alpha-, beta-, and gamma-CD, were used to investigate the molecular mechanism of their effects on assisting aminoacylase refolding. The aggregation and reactivation experiments suggested that at moderate concentrations, HPCDs could suppress aggregation and assist aminoacylase refolding in a concentration-dependent manner, and HP-beta-CD was the most efficient of the three HPCDs. Low concentrations of HP-alpha-CD and high concentrations of HP-gamma-CD promoted off-pathway aggregation. Spectroscopic studies indicated that the hydrophobic exposure of the unstructured species in the refolded solutions was gradually reduced by the three HPCDs with the efficiency HP-beta-CD > HP-gamma-CD > HP-alpha-CD. Furthermore, the fast phase of aminoacylase reactivation was slowed down by the addition of 75 mM HP-beta- and gamma-CD, but no significant effect was observed for HP-alpha-CD. The dissimilarity in the effects of the three HPCDs suggested that the internal cavity size played a crucial role in their antiaggregation ability. Further analysis suggested that the observations might be much more complicated than expected because of the various types of interactions between cyclodextrins and proteins in addition to their ability to bind to protein aromatic residues.

Direct evidence for the formation of a complex between 1-cysteine peroxiredoxin and glutathione S-transferase pi with activity changes in both enzymes

Glutathione S-transferase pi (GST pi) has been shown to reactivate oxidized 1-cysteine peroxiredoxin (1-Cys Prx, Prx VI, Prdx6, and AOP2). We now demonstrate that a heterodimer complex is formed between 1-Cys Prx with a C-terminal His6 tag and GST pi upon incubation of the two proteins at pH 8.0 in buffer containing 20% 1,6-hexanediol to dissociate the homodimers, followed by dialysis against buffer containing 2.5 mM glutathione (GSH) but lacking 1,6-hexanediol. The heterodimer can be purified by chromatography on nickel-nitriloacetic acid agarose in the presence of GSH. N-Terminal sequencing showed that equimolar amounts of the two proteins are present in the isolated complex. In the heterodimer, 1-Cys Prx is fully active toward either H2O2 or phospholipid hydroperoxide, while the GST pi activity is approximately 25% of that of the GST pi homodimer. In contrast, the 1-Cys Prx homodimer lacks peroxidase activity even in the presence of free GSH. The heterodimer is also formed in the presence of S-methylglutathione, but no 1-Cys Prx activity is found under these conditions. The yield of heterodimer is decreased in the absence of 1,6-hexanediol or GSH. Rapid glutathionylation of 1-Cys Prx in the heterodimer is detected by immunoblotting. Subsequently, a disulfide-linked dimer is observed on SDS-PAGE, and the free cysteine content is decreased by 2 per heterodimer. The involvement of particular binding sites in heterodimer formation was tested by site-directed mutagenesis of the two proteins. For 1-Cys Prx, neither Cys47 nor Ser32 is required for heterodimer formation but Cys47 is essential for 1-Cys Prx activation. For GST pi, Cys47 and Tyr7 (at or near the GSH-binding site) are needed for heterodimer formation but three other cysteines are not. We conclude that reactivation of oxidized 1-Cys Prx by GST pi occurs by heterodimerization of 1-Cys Prx and GST pi harboring bound GSH, followed by glutathionylation of 1-Cys Prx and then formation of an intersubunit disulfide. Finally, the GSH-mediated reduction of the disulfide regenerates the reduced active-site sulfhydryl of 1-Cys Prx.

Structural basis for tumor pyruvate kinase M2 allosteric regulation and catalysis

Four isozymes of pyruvate kinase are differentially expressed in human tissue. Human pyruvate kinase isozyme M2 (hPKM2) is expressed in early fetal tissues and is progressively replaced by the other three isozymes, M1, R, and L, immediately after birth. In most cancer cells, hPKM2 is once again expressed to promote tumor cell proliferation. Because of its almost ubiquitous presence in cancer cells, hPKM2 has been designated as tumor specific PK-M2, and its presence in human plasma is currently being used as a molecular marker for the diagnosis of various cancers. The X-ray structure of human hPKM2 complexed with Mg(2+), K(+), the inhibitor oxalate, and the allosteric activator fructose 1,6-bisphosphate (FBP) has been determined to a resolution of 2.82 A. The active site of hPKM2 is in a partially closed conformation most likely resulting from a ligand-induced domain closure promoted by the binding of FBP. In all four subunits of the enzyme tetramer, a conserved water molecule is observed on the 2-si face of the prospective enolate and supports the hypothesis that a proton-relay system is acting as the proton donor of the reaction (1). Significant structural differences among the human M2, rabbit muscle M1, and the human R isozymes are observed, especially in the orientation of the FBP-activating loop, which is in a closed conformation when FBP is bound. The structural differences observed between the PK isozymes could potentially be exploited as unique structural templates for the design of allosteric drugs against the disease states associated with the various PK isozymes, especially cancer and nonspherocytic hemolytic anemia.

Catalytic Cycling in β-Phosphoglucomutase: A Kinetic and Structural Analysis,

Lactococcus lactis beta-phosphoglucomutase (beta-PGM) catalyzes the interconversion of beta-d-glucose 1-phosphate (beta-G1P) and beta-d-glucose 6-phosphate (G6P), forming beta-d-glucose 1,6-(bis)phosphate (beta-G16P) as an intermediate. Beta-PGM conserves the core domain catalytic scaffold of the phosphatase branch of the HAD (haloalkanoic acid dehalogenase) enzyme superfamily, yet it has evolved to function as a mutase rather than as a phosphatase. This work was carried out to identify the structural basis underlying this diversification of function. In this paper, we examine beta-PGM activation by the Mg(2+) cofactor, beta-PGM activation by Asp8 phosphorylation, and the role of cap domain closure in substrate discrimination. First, the 1.90 A resolution X-ray crystal structure of the Mg(2+)-beta-PGM complex is examined in the context of previously reported structures of the Mg(2+)-alpha-d-galactose-1-phosphate-beta-PGM, Mg(2+)-phospho-beta-PGM, and Mg(2+)-beta-glucose-6-phosphate-1-phosphorane-beta-PGM complexes to identify conformational changes that occur during catalytic turnover. The essential role of Asp8 in nucleophilic catalysis was confirmed by demonstrating that the D8A and D8E mutants are devoid of catalytic activity. Comparison of the ligands to Mg(2+) in the different complexes shows that a single Mg(2+) coordination site must alternatively accommodate water, phosphate, and the phosphorane intermediate during catalytic turnover. Limited involvement of the HAD family metal-binding loop in Mg(2+) anchoring in beta-PGM is consistent with the relatively loose binding indicated by the large K(m) for Mg(2+) activation (270 +/- 20 microM) and with the retention of activity found in the E169A/D170A double loop mutant. Comparison of the relative positions of cap and core domains in the different complexes indicated that interaction of cap domain Arg49 with the "nontransferring" phosphoryl group of the substrate ligand might stabilize the cap-closed conformation, as required for active site desolvation and alignment of Asp10 for acid-base catalysis. Kinetic analyses of the specificity of beta-PGM toward phosphoryl group donors and the specificity of phospho-beta-PGM toward phosphoryl group acceptors were carried out. The results support a substrate induced-fit mechanism of beta-PGM catalysis, which allows phosphomutase activity to dominate over the intrinsic phosphatase activity. Last, we present evidence that the autophosphorylation of beta-PGM by the substrate beta-G1P accounts for the origin of phospho-beta-PGM in the cell.

Inactivation of Bacterial dd-Peptidase by β-Sultams

N-Acyl-beta-sultams are time-dependent, irreversible active site-directed inhibitors of Streptomyces R61 DD-peptidase. The rate of inactivation is first order with respect to beta-sultam concentration, and the second-order rate constants show a dependence on pH similar to that for the hydrolysis of a substrate. Inactivation is due to the formation of a stable 1:1 enzyme-inhibitor complex as a result of the active site serine being sulfonylated by the beta-sultam as shown by ESI-MS analysis and by X-ray crystallography. A striking feature of the sulfonyl enzyme is that the inhibitor is not bound to the oxyanion hole but interacts extensively with the "roof" of the active site where the Arg 285 is located.

Artificial chaperone-assisted refolding of chemically denatured alpha-amylase

It is now well established that alpha-cyclodextrin (alpha-CD) is a valuable folding agent in refolding processes of several denatured enzyme solutions. The refolding of Gu-HCl denatured alpha-amylase in the dilution-additive mode revealed that alpha-CD enhanced the refolding yield by 20-30% depending upon alpha-CD concentration. However, the refolding efficiency of the Gu-HCl denatured alpha-amylase through the artificial chaperone-assisted method indicated that alpha-CD enhanced the activity recovery of denatured alpha-amylase by almost 50% and also increased the reactivation rate constant relative to the unassisted control sample. The higher refolding efficiency should be due to different mechanism played by alpha-CD in this technique. In addition, our data indicated that higher refolding yields are obtained when the residual Gu-HCl concentration is low in the refolding environment and when the capture agent is removed not in a stepwise manner from the protein-detergent complexes in the stripping step of the whole process. Collectively, the results of this investigation expand the range of procedural variations used to refold different denatured proteins through artificial chaperone-assisted method.

The Interaction of Human Tryptase-β with Small Molecule Inhibitors Provides New Insights into the Unusual Functional Instability and Quaternary Structure of the Protease

Human tryptase-beta (HTbeta) is a serine protease with an atypical tetrameric structure and an unusual dependence on heparin binding or high salt for functional and structural stability. In the absence of heparin and at physiological salt, pH, and temperature, HTbeta rapidly loses activity by a reversible process that we have called spontaneous inactivation. The role of tetramer dissociation in this process is controversial. Using small irreversible or competitive inhibitors of HTbeta as stabilizing ligands, we were able to examine tetramer stability under inactivating (decay) conditions in the absence of heparin and to define further the process of spontaneous inactivation. Size exclusion chromatography showed that interaction with inhibitors stabilized the tetramer. Using sedimentation equilibrium, spontaneously inactivated HTbeta (si-HTbeta) was shown to be a destabilized tetramer that dissociates upon dilution and which in the presence of a competitive inhibitor re-formed a stable tetramer. Addition of inhibitors to si-HTbeta rescued catalytic activity as was shown after inhibitor displacement. At high concentrations of si-HTbeta (4-5 microM), the binding of inhibitor alone provided sufficient free energy for complete reactivation and tetramer stabilization, whereas at low si-HTbeta concentration (0.1 microM) where the destabilized tetramer would be mostly dissociated, reactivation required more free energy which was provided by the binding of both an inhibitor and heparin. The results demonstrate that HTbeta is a tetramer in the absence of heparin and that tetramer dissociation is a consequence of and not a prerequisite for inactivation. Heparin binding likely stabilizes the tetramer by favoring a functionally active conformation with stable intersubunit contacts, rather than by simply cross-linking active monomers.

Cofactor regeneration at the lab scale

Progress made in lab-scale applications of various coenzyme regeneration systems over the last two decades has mainly focused on the applications of NAD+/NADH- and NADP+/NADPH-dependent oxidoreductase reactions. In situ regeneration systems for these reactions, as well as whole cell, enzymatic, electro-enzymatic, chemical, and photochemical reactions are presented, including details about their efficiency and novelty. The progress of enzyme reaction engineering is also reported.

Identification of a reactivating factor for adenosylcobalamin-dependent ethanolamine ammonia lyase

The holoenzyme of adenosylcobalamin-dependent ethanolamine ammonia lyase undergoes suicidal inactivation during catalysis as well as inactivation in the absence of substrate. The inactivation involves the irreversible cleavage of the Co-C bond of the coenzyme. We found that the inactivated holoenzyme undergoes rapid and continuous reactivation in the presence of ATP, Mg2+, and free adenosylcobalamin in permeabilized cells (in situ), homogenate, and cell extracts of Escherichia coli. The reactivation was observed in the permeabilized E. coli cells carrying a plasmid containing the E. coli eut operon as well. From coexpression experiments, it was demonstrated that the eutA gene, adjacent to the 5' end of ethanolamine ammonia lyase genes (eutBC), is essential for reactivation. It encodes a polypeptide consisting of 467 amino acid residues with predicted molecular weight of 49,599. No evidence was obtained that shows the presence of the auxiliary protein(s) potentiating the reactivation or associating with EutA. It was demonstrated with purified recombinant EutA that both the suicidally inactivated and O2-inactivated holoethanolamine ammonia lyase underwent rapid reactivation in vitro by EutA in the presence of adenosylcobalamin, ATP, and Mg2+. The inactive enzyme-cyanocobalamin complex was also activated in situ and in vitro by EutA under the same conditions. Thus, it was concluded that EutA is the only component of the reactivating factor for ethanolamine ammonia lyase and that reactivation and activation occur through the exchange of modified coenzyme for free intact adenosylcobalamin.

Site-Specific Effects of Zinc on the Activity of Family II Pyrophosphatase

Family II pyrophosphatases (PPases), recently found in bacteria and archaebacteria, are Mn(2+)-containing metalloenzymes with two metal-binding subsites (M1 and M2) in the active site. These PPases can use a number of other divalent metal ions as the cofactor but are inactive with Zn(2+), which is known to be a good cofactor for family I PPases. We report here that the Mg(2+)-bound form of the family II PPase from Streptococcus gordonii is nearly instantly activated by incubation with equimolar Zn(2+), but the activity thereafter decays on a time scale of minutes. The activation of the Mn(2+)-form by Zn(2+) was slower but persisted for hours, whereas activation was not observed with the Ca(2+)- and apo-forms. The bound Zn(2+) could be removed from PPase by prolonged EDTA treatment, with a complete recovery of activity. On the basis of the effect of Zn(2+) on PPase dimerization, the Zn(2+) binding constant appeared to be as low as 10(-12) M for S. gordonii PPase. Similar effects of Zn(2+) and EDTA were observed with the Mg(2+)- and apo-forms of Streptococcus mutans and Bacillus subtilis PPases. The effects of Zn(2+) on the apo- and Mg(2+)-forms of HQ97 and DE15 B. subtilis PPase variants (modified M2 subsite) but not of HQ9 variant (modified M1 subsite) were similar to that for the Mn(2+)-form of wild-type PPase. These findings can be explained by assuming that (a) the PPase tightly binds Mg(2+) and Mn(2+) at the M2 subsite; (b) the activation of the corresponding holoenzymes by Zn(2+) results from its binding to the M1 subsite; and (c) the subsequent inactivation of Mg(2+)-PPase results from Zn(2+) migration to the M2 subsite. The inability of Zn(2+) to activate apo-PPase suggests that Zn(2+) binds more tightly to M2 than to M1, allowing direct binding to M2. Zn(2+) is thus an efficient cofactor at subsite M1 but not at subsite M2.

Solid-phase refolding of cyclodextrin glycosyltransferase adsorbed on cation-exchange resin

Expression with a fusion partner is now a popular scheme to produce a protein of interest because it provides a generic tool for expression and purification. In our previous study, a strong polycationic tail has been harnessed for an efficient purification scheme. Here, the same polycation tail attached to a protein of interest is shown to hold versatility for a solid-phase refolding method that utilizes a charged adsorbent as a supporting material. Cyclodextrin glycosyltransferase (CGTase) fused with 10 lysine residues at the C-terminus (CGTK10ase) retains the ability to bind to a cation exchanger even in a urea-denatured state. When the denatured and adsorbed CGTK10ase is induced to refold, the bound CGTK10ase aggregates little even at a g/L range. The renatured CGTK10ase can also be simply recovered from the solid support by adding high concentration of NaCl. The CGTK10ase refolded on a solid support retains specific enzyme activity virtually identical to that of the native CGTK10ase. Several factors that are important in improving the refolding efficiency are explored. Experimental results indicate that nonspecific electrostatic interactions between the charge of the ion exchanger and the local charge of CGTase other than the polycationic tag should be reduced to obtain higher refolding yield. The solid-phase refolding method utilizing a strong polycationic tag resulted in a remarkable increase in the refolding performance. Taken together with the previous report in which a series of polycations were explored for efficient purification, expression of a target protein fused with a strong polycation provides a straightforward protein preparation scheme.

Insight into the mechanism of the B12-independent glycerol dehydratase from Clostridium butyricum: preliminary biochemical and structural characterization

The molecular characterization of a B12-independent glycerol dehydratase from Clostridium butyricum has recently been reported [Raynaud, C., et al. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 5010-5015]. In this work, we have further characterized this system by biochemical and crystallographic methods. Both the glycerol dehydratase (GD) and the GD-activating enzyme (GD-AE) could be purified to homogeneity under aerobic conditions. In this form, both the GD and GD-AE were inactive. A reconstitution procedure, similar to what has been reported for pyruvate formate lyase activating enzyme (PFL-AE), was employed to reconstitute the activity of the GD-AE. Subsequently, the reconstituted GD-AE could be used to reactivate the GD under strictly anaerobic conditions. We also report here the crystal structure of the inactive GD in the native (2.5 A resolution, Rcryst = 17%, Rfree = 20%), glycerol-bound (1.8 A resolution, Rcryst = 21%, Rfree = 24%), and 1,2-propanediol-bound (2.4 A resolution, Rcryst = 20%, Rfree = 24%) forms. The overall fold of the GD monomer was similar to what has been observed for pyruvate formate lyase (PFL) and anaerobic ribonucleotide reductase (ARNR), consisting of a 10-stranded beta/alpha barrel motif. Clear density was observed for both substrates, and a mechanism for the dehydration reaction is presented. This mechanism clearly supports a concerted pathway for migration of the OH group through a cyclic transition state that is stabilized by partial protonation of the migrating OH group. Finally, despite poor alignment (rmsd approximately 6.8 A) of the 10 core strands that comprise the barrel structure of the GD and PFL, the C-terminal domains of both proteins align well (rmsd approximately 0.7 A) and have structural properties consistent with this being the docking site for the activating enzyme. A single point mutation within this domain, at a strictly conserved arginine residue (R782K) in the GD, resulted in formation of a tight protein-protein complex between the GD and the GD-AE in vivo, thereby supporting this hypothesis.

Refolding of a recombinant full-length non-structural (NS3) protein from hepatitis C virus by chromatographic procedures

The non-structural protein 3 (NS3) of Hepatitis C virus (HCV) was expressed as inactive aggregates in Escherichia coli. The protein was refolded by chromatographic techniques of which ion exchange chromatography was best for crude samples and gel filtration best for partially purified samples. Immobilized metal ion affinity chromatography showed intermediate performance. Gradient procedures enhanced the recovery of active NS3 protein.

The mipafox-inhibited catalytic domain of human neuropathy target esterase ages by reversible proton loss

Aging of organophosphorus (OP)-compound-inhibited neuropathy target esterase (NTE) is the critical event that initiates OP-compound-induced delayed neurotoxicity (OPIDN). Aging has classically been considered to involve side-group loss from phosphylated NTE, rendering the enzyme refractory to reactivation. N,N'-Diisopropylphosphorodiamidofluoridate (mipafox, MIP)-inhibited NTE has been thought to age quickly; however, it can be reactivated under acidic conditions. The present study was undertaken to determine whether MIP-inhibited human recombinant NTE esterase domain (NEST) ages classically by isopropylamine loss. Diisopropylphosphorofluoridate (DFP), the oxygen analogue of MIP, was used for comparison. Kinetic values for DFP against NEST were as follows: k(i) = 17 200 +/- 180 M(-1) min(-1); reactivation t(1/2) approximately 90 min at pH 8.0 and approximately 60 min at pH 5.2; k(4) = 0.108 +/- 0.041 min(-1) at pH 8.0 and 0.181 +/- 0.034 min(-1) at pH 5.2. Kinetic values for MIP against NEST were as follows: k(i) = 1880 +/- 61 M(-1) min(-1); reactivation t(1/2) = 0 min at pH 8.0 and approximately 60 min at pH 5.2; aging was complete at all time points tested at pH 8.0, but no aging occurred at pH 5.2. Mass spectrometry revealed a mass shift of 123.0 +/- 0.6 Da for the active site peptide peak of aged DFP-inhibited NEST, corresponding to a monoisopropyl phosphate adduct. In contrast, the analogous mass shift for aged MIP-inhibited NEST was 162.8 +/- 0.6 Da, corresponding to the intact N,N'-diisopropylphosphorodiamido adduct. Thus, MIP-inhibited NEST does not age by isopropylamine loss. However, because kinetically aged MIP-inhibited NEST yields an intact adduct capable of reversible deprotonation, aging could occur by proton loss. Indeed, MIP-inhibited NEST does not age at pH 5.2 but ages immediately and completely at pH 8.0. Therefore, we conclude that the MIP-NEST conjugate ages by deprotonation rather than classical side-group loss.

Recovery of the oxidative activity of caged bovine haemoglobin after UV photolysis

Caging of bovine haemoglobin with increasing amounts of 1-(2-nitrophenyl)ethyl (NPE) and uncaging after a 366 nm irradiation was examined. Caged and photolysed conjugates were characterised by enzymatic assay of the ABTS oxidation, UV/Vis absorbance, and electrospray mass ionisation. Modification of haemoglobin with 50, 75, and 100 equivalents of 1-(2-nitrophenyl)diazoethane led to a progressive decrease of enzymatic activity. Photolysis at 366 nm during 5, 15, and 30 min induced the recovery of a part of the enzymatic activity. ESI analyses showed that a reversible binding of up to 6 NPE groups per alpha-chain and that the removal of most of the photolabile groups occurred rapidly after 5 min of illumination at 366 nm and reached near completion after 15 min. A variable alteration of haemoglobin after labelling could explain that the complete removal of NPE groups did not restore its full oxidative activity.

Calcineurin hydrolysis of para-nitrophenyl phosphorothioate

para-Nitrophenyl phosphorothioate (pNPT) was hydrolyzed by calcineurin at initial rates slightly, but comparable to rates for para-nitrophenyl phosphate (pNPP). Kinetic characterization yielded higher estimates for both Km and Vmax compared to pNPP. Metal ion activation of phosphorothioate hydrolysis was more promiscuous. Unlike the hydrolysis of with pNPP, Ca2+, Mg2+, and Ba2+ activated calcineurin as well as Mn2+.

Implications of the role of reactive cystein in arginine kinase: reactivation kinetics of 5,5′-dithiobis-(2-nitrobenzoic acid)-modified arginine kinase reactivated by dithiothreitol

The reduction of 5,5'-dithiobis-(2-nitrobenzoic acid)-modified arginine kinase by dithiothreitol has been investigated using the kinetic theory of the substrate reaction during modification of enzyme activity. The results show that the modified arginine kinase can be fully reactivated by an excess concentration of dithiothreitol in a monophasic kinetic course. The presence of ATP or the transition-state analog markedly slows the apparent reactivation rate constant, while arginine shows no effect. The results of ultraviolet (UV) difference and intrinsic fluorescence spectra indicate that the substrate arginine-ADP-Mg2+ can induce conformational changes of the modified enzyme but adding NO3- cannot induce further changes that occur with the native enzyme. The reactive cysteines' location and role in the catalysis of arginine kinase are discussed. It is suggested that the cysteine may be located in the hinge region of the two domains of arginine kinase. The reactive cysteine of arginine kinase may play an important role not in the binding to the transition-state analog but in the conformational changes caused by the transition-state analog.

Hydrogen peroxide induces the dissociation of GroEL into monomers that can facilitate the reactivation of oxidatively inactivated rhodanese

Although, several studies have been reported on the effects of oxidants on the structure and function of other molecular chaperones, no reports have been made so far for the chaperonin GroEL. The ability of GroEL to function under oxidative stress was investigated in this report by monitoring the effects of hydrogen peroxide (H(2)O(2)) on the structure and refolding activity of this protein. Using fluorescence spectroscopy and light scattering, we observed that GroEL showed increases in exposed hydrophobic sites and changes in tertiary and quaternary structure. Differential sedimentation, gel electrophoresis, and circular dichroism showed that H(2)O(2) treated GroEL underwent irreversible dissociation into monomers with partial loss of secondary structure. Relative to other proteins, GroEL was found to be highly resistant to oxidative damage. Interestingly, GroEL monomers produced under these conditions can facilitate the reactivation of H(2)O(2)-inactivated rhodanese but not urea-denatured rhodanese. Recovery of approximately 84% active rhodanese was obtained with either native or oxidized GroEL in the absence of GroES or ATP. In comparison, urea-denatured GroEL, BSA and the refolding mixture in the absence of proteins resulted in the recovery of 72, 50, and 49% rhodanese activity, respectively. Previous studies have shown that GroEL monomers can reactivate rhodanese. Here, we show that oxidized monomeric GroEL can reactivate oxidized rhodanese suggesting that GroEL retains the ability to protect proteins during oxidative stress.

Human glycine N-methyltransferase is unfolded by urea through a compact monomer state

Human recombinant glycine N-methyltransferase (GNMT) unfolding by urea was studied by enzyme activity, size-exclusion chromatography, fluorescence spectroscopy, and circular dichroism. Urea unfolding of GNMT is a two-step process. The first transition is a reversible dissociation of the GNMT tetramer to compact monomers in 1.0-3.5M urea with enzyme inactivation. The compact monomers were characterized by Stokes radius (R(s)) of 40.7A equal to that of globular proteins with the same molecular mass as GNMT monomers, absence of exposure of tryptophan residues into solvent, and presence of about 50% of secondary structure of native protein. The second step of GNMT unfolding is a reversible transition of monomers from compact to a fully unfolded state with R(s) of 50A, exposed tryptophan residues, and disrupted secondary structure in 8M urea.

Effect of thiohydroxyl compounds on tyrosinase: inactivation and reactivation study

An unusual thioether bridge (Cys-His) has been detected at the active site of mushroom tyrosinase, and the effects of thiohydroxyl compounds such as dithiothreitol (DTT) and beta-mercaptoethanol (beta-ME) on Cu2+ at the active site have been elucidated. Treatment with DTT and beta-ME on mushroom tyrosinase completely inactivated 3,4-dihydroxyphenylalanine oxidase activity in a dose-dependent manner. Sequential kinetic studies revealed that DTT and beta-ME caused different mixed-type inhibition mechanisms: the slope-parabolic competitive inhibition (Ki = 0.143 mM) by DTT and slope-hyperbolic noncompetitive inhibition (Ki = 0.0128 mM) by beta-ME, respectively. Kinetic Scatchard analysis consistently showed that mushroom tyrosinase had multiple binding sites for DTT and beta-ME with different affinities. Reactivation study of inactivated enzyme by addition of Cu2+ confirmed that DTT and beta-ME directly bound with Cu2+ at the active site. Our results may provide useful information regarding interactions of tyrosinase inhibitor for designing an effective whitening agent targeted to the tyrosinase active site.

Core Formation in Escherichia coli Bacterioferritin Requires a Functional Ferroxidase Center

Bacterioferritin from Escherichia coli is able to accumulate large quantities of iron in the form of an inorganic iron(III) mineral core. Core formation in the wild-type protein and a number of ferroxidase center variants was studied to determine key features of the core formation process and, in particular, the role played by the ferroxidase center. Core formation rates were found to be iron(II)-dependent and also depended on the amount of iron already present in the core, indicating the importance of the core surface in the mineralization reaction. Core formation was also found to be pH-dependent in terms of both rate and iron-loading characteristics, occurring with maximum efficiency at pH 6.5. Even at this optimum pH, however, the effective iron capacity was approximately 2700 per molecule, i.e., well below the theoretical limit of approximately 4500, suggesting that competing oxidation/precipitation processes have a major influence on the amount of iron accumulated. Disruption of the ferroxidase center, by site-directed mutagenesis or by chemical inhibition with zinc(II), had a profound effect on core formation. Effective iron capacities were found to be linked to iron(II) oxidation rates, and in zinc(II)-inhibited wild-type and E18A bacterioferritins core formation was severely restricted. Zinc(II) was also able, even at low stoichiometries (12-60 ions/protein), to significantly inhibit further core formation in protein already containing a substantial core, indicating the importance of the ferroxidase center throughout the core formation process. A mechanism is proposed that incorporates essential roles for the core surface and the ferroxidase center. A central feature of this mechanism is that dioxygen cannot readily gain access to the core, perhaps because the channels through the bacterioferritin coat are hydrophilic and dioxygen is nonpolar.

An amperometric bi-enzyme sensor for determination of formate using cofactor regeneration

A biosensor for detection of formate at submicromolar concentrations has been developed by co-immobilizing formate dehydrogenase (FDH, E.C. 1.2.1.2), salicylate hydroxylase (SHL, E.C. 1.14.13.1) and NAD(+) linked to polyethylene glycol (PEG-NAD(+)) in a poly(vinyl alcohol) (PVA) matrix in front of a Clark-electrode. The principle of the bi-enzyme scheme is as follows: formate dehydrogenase converts formate into carbon dioxide using PEG-NAD(+). Corresponding PEG-NADH produced is then oxidized to PEG-NAD(+) by salicylate hydroxylase using sodium salicylate and oxygen. The oxygen consumption is monitored with the Clark-electrode. The advantages of this biosensor approach are the effective re-oxidation of PEG-NADH, and the entrapment of PEG-NAD(+) resulting in avoiding the addition of expensive cofactor to the working medium for each measurement. This bi-enzyme sensor has achieved a linear range of 1-300 microM and a detection limit of 1.98 x 10(-7) M for formate (S/N=3), with the response time of 4 min. The working stability is limited to 7 days due to the inactivation of the enzymes. Only sodium salicylate was needed in milli-molar amounts.

Catalysis of protein folding by an immobilized small-molecule dithiol

The isomerization of non-native disulfide bonds often limits the rate of protein folding. Small-molecule dithiols can catalyze this process. Here, a symmetric trithiol, tris(2-mercaptoacetamidoethyl)amine, is designed on the basis of criteria known to be important for efficient catalysis of oxidative protein folding. The trithiol is synthesized and attached to two distinct solid supports via one of its three sulfhydryl groups. The resulting immobilized dithiol has an apparent disulfide E degrees ' = -208 mV, which is close to that of protein disulfide isomerase (E degrees ' = -180 mV). Incubation of the dithiol immobilized on a TentaGel resin with a protein containing non-native disulfide bonds produced only a 2-fold increase in native protein. This dithiol appeared to be inaccessible to protein. In contrast, incubation of the dithiol immobilized on styrene-glycidyl methacrylate microspheres with the non-native protein produced a 17-fold increase in native protein. This increase was 1.5-fold greater than that of a monothiol immobilized on the microspheres. Thus, the choice of both the solid support and thiol can affect catalysis of protein folding. The use of dithiol-decorated microspheres is an effective new strategy for preparative protein folding in vitro.

Stabilization and re-activation of trapped enzyme by immobilized heat shock protein and molecular chaperones

The potential of using immobilized Heat Shock Protein 70 (HSP 70) in combination with other molecular chaperones to ameliorate problems of enzyme denaturation was investigated. Firefly luciferase was used as a model enzyme due to its sensitivity to thermal denaturation, and the availability of a sensitive chemiluminescent assay method for determination of relative activity of this enzyme. Control experiments and development of effective combinations of HSP with other chaperones involved re-activation of enzyme in bulk solution. A combination of HSP 70, alpha-crystallin and reticulocyte lysate (RL) in bulk solution were found to re-activate soluble firefly luciferase to about 60% of the initial activity after the enzyme activity had been reduced to less than 2% by thermal denaturation. HSP 70 that was covalently immobilized onto glass surfaces was also able to re-activate denatured enzyme that was in bulk solution. Over 30% of the initial activity could be regained from heat denatured enzyme when using immobilized HSP in the presence of other chaperones. The activity of soluble enzyme decayed to negligible values in a period of days when stored at room temperature. In the presence of immobilized HSP and chaperones, activity stabilized at about 10% of the initial activity even after many weeks. The results suggest that immobilized molecular chaperones such as HSP 70 may provide some potential for stabilization and re-activation of enzymes that are trapped in thin aqueous films for applications in biosensors and reactors.

A Rho-kinase inhibitor, soluble guanylate cyclase activator and nitric oxide-releasing PDE5 inhibitor: novel approaches to erectile dysfunction

Approximately 50% of men aged over 40 suffer from male erectile dysfunction. Treatment options have widened since the launch of the phosphodiesterase type 5 (PDE5) inhibitor, sildenafil citrate (Viagra trade mark ). However, a certain portion of the patient population, such as diabetics, do not gain significant benefit from PDE5 inhibitors, possibly due to a lack of endogenous nitric oxide. Therefore, new treatment modalities based on the absence of endogenous nitric oxide have been developed. Among them are Rho-kinase inhibitors, soluble guanylate cyclase activators and nitric oxide-releasing PDE5 inhibitors. The available data concerning these compounds will be summarised and their therapeutic potential for male erectile dysfunction will be discussed.

Reversible ligand-induced dissociation of a tryptophan-shift mutant of phosphofructokinase from Bacillus stearothermophilus

The biophysical properties of a tryptophan-shifted mutant of phosphofructokinase from Bacillus stearothermophilus (BsPFK) have been examined. The mutant, designated W179Y/Y164W, has kinetic and thermodynamic properties similar to the wild-type enzyme. A 2-fold decrease in kcat is observed, and the mutant displays a 3-fold smaller K(0.5) for the substrate, fructose-6-phosphate (Fru-6-P), as compared to the wild-type enzyme. The dissociation constant for the inhibitor, phospho(enol)pyruvate (PEP), increases 2-fold, and the coupling parameter, Q(ay), decreases 2-fold. This suggests that while the mutant displays a slightly decreased affinity for PEP, PEP is still an effective inhibitor once bound. The new position of the tryptophan in W179Y/Y164W is approximately 6 A from the Fru-6-P portion of the active site. A 25% decrease in fluorescence intensity is observed upon Fru-6-P binding, and an 80% decrease in fluorescence intensity is observed with PEP binding. In addition, the intrinsic fluorescence polarization increases from 0.327 +/- 0.001 to 0.353 +/- 0.001 upon Fru-6-P binding, but decreases to 0.290 +/- 0.001 when PEP binds. Most notably, the presence of PEP induces dissociation of the tetramer. Dissociation of the tetramer into dimers occurs along the active site interface and can be monitored by the loss in activity or the loss in tryptophan fluorescence that is observed when the enzyme is titrated with PEP. Activity can be protected or recovered by incubating the enzyme with Fru-6-P. Recovery of activity is enzyme concentration dependent, and the rate constant for association is 6.2 +/- 0.3 M(-1) x s(-1). Ultracentrifugation experiments revealed that in the absence of PEP the mutant enzyme exists in an equilibrium between the dimer and tetramer forms with a dissociation constant of 11.8 +/- 0.5 microM, while in the presence of PEP the enzyme exists in equilibrium between the dimer and monomer forms with a dissociation constant of 7.5 +/- 0.02 microM. A 3.1 A crystal structure of the mutant enzyme suggests that the amino acid substitutions have not dramatically altered the tertiary structure of the enzyme. While it is clear that wild-type BsPFK exists as a tetramer under these same conditions, these results suggest that quaternary structural changes probably play an important role in allosteric communication.

Rhodanese can partially refold in its GroEL-GroES-ADP complex and can be released to give a homogeneous product

Molecular chaperones GroEL and GroES facilitate reactivation of denatured rhodanese which folds poorly unless the process is assisted. The present work tests the hypothesis that more extensively unfolded forms of rhodanese bind tighter than those forms that appear later in the folding pathway. The study of the interaction of different urea-induced forms of rhodanese with GroEL suggests that species preceding the domain folded form bind directly and productively to GroEL. Rhodanese partially folds while in the GroEL-GroES-ADP complex, but it does not significantly reach an active state. Partially folded rhodanese can be released from the GroEL-GroES-ADP complex by subdenaturing concentrations of urea as a homogeneous species that is committed to fold to the native conformation with little or no partitioning to the aggregated state. Dilution of denatured rhodanese to the same final concentration gives less active enzyme and significant aggregation. Urea denaturation studies show that active rhodanese released from complexes behaves identically to native enzyme, while spontaneously folded rhodanese has a different stability. These results are interpreted using a previously proposed model based on studies of unassisted rhodanese folding [Gorovits, B. M., McGee, W. A., and Horowitz, P. M. (1998) Biochim. Biophys. Acta 1382, 120-128. Panda, M., Gorovits, B. M., and Horowitz, P. M. (2000) J. Biol. Chem. 275, 63-70].

Drug delivery systems employing 1,6-elimination: releasable poly(ethylene glycol) conjugates of proteins

Using lysozyme as a representative protein substrate that loses its activity when PEGylation takes place on the epsilon-amino group of lysine residues, various amounts of a novel releasable PEG linker (rPEG) were conjugated to the protein. rPEG-lysozyme conjugates were relatively stable in pH 7.4 buffer for over 24 h. However, regeneration of native protein from the rPEG conjugates occurred in a predictable manner during incubation in high pH buffer or rat plasma, as demonstrated by enzymatic activity and structural characterization. The rates of regeneration were also correlated with PEG number: native lysozyme was released more rapidly from the monosubstituted conjugate than from the disubstituted conjugate, suggesting possible steric hindrance to the approach of cleaving enzymes. Recovery of normal activity and structure for the regenerated native lysozyme was shown by a variety of assays.

Mechanism-based inactivation of VanX, a D-alanyl-D-alanine dipeptidase necessary for vancomycin resistance

VanX is a zinc-dependent D-Ala-D-Ala amino dipeptidase required for high-level resistance to vancomycin. The enzyme is also able to process dipeptides with bulky C-terminal amino acids [Wu, Z., Wright, G. D., and Walsh, C. T. (1995) Biochemistry 34, 2455-2463]. We took advantage of this observation to design and synthesize the dipeptide-like D-Ala-D-Gly(SPhip-CHF(2))-OH (7) as a potential mechanism-based inhibitor. VanX-mediated peptide cleavage generates a highly reactive 4-thioquinone fluoromethide which is able to covalently react with enzyme nucleophilic residues, resulting in irreversible inhibition. Inhibition of VanX by 7 was time-dependent (K(irr) = 30+/-1 microM; k(inact) = 7.3+/- 0.3 min(-1)) and active site-directed, as deduced from substrate protection experiments. Nucleophilic compounds such as sodium azide, potassium cyanide, and glutathione did not protect the enzyme from inhibition, indicating that the generated nucleophile inactivates VanX before leaving the active site. The failure to reactivate the dead enzyme by gel filtration or pH modification confirmed the covalent nature of the reaction that leads to inactivation. Inactivation was associated with the elimination of fluoride ion as deduced from (19)F NMR spectroscopy analysis and with the production of fluorinated thiophenol dimer 12. These data are consistent with suicide inactivation of VanX by dipeptide 7. The small size of the VanX active site and the presence of a number of nucleophilic side chains at the opening of the active site gorge [Bussiere, D. E., et al. (1998) Mol. Cell 2, 75-84] associated with the high observed partition ratio of 7500+/-500 suggest that the inhibitor is likely to react at the entrance of the active site cavity.

Dissociative mechanism of thermal denaturation of rabbit skeletal muscle glycogen phosphorylase b

The thermal stability of rabbit skeletal muscle glycogen phosphorylase b was characterized using enzymological inactivation studies, differential scanning calorimetry, and analytical ultracentrifugation. The results suggest that denaturation proceeds by the dissociative mechanism, i.e., it includes the step of reversible dissociation of the active dimer into inactive monomers and the following step of irreversible denaturation of the monomer. It was shown that glucose 1-phosphate (substrate), glucose (competitive inhibitor), AMP (allosteric activator), FMN, and glucose 6-phosphate (allosteric inhibitors) had a protective effect. Calorimetric study demonstrates that the cofactor of glycogen phosphorylase-pyridoxal 5'-phosphate-stabilizes the enzyme molecule. Partial reactivation of glycogen phosphorylase b preheated at 53 degrees C occurs after cooling of the enzyme solution to 30 degrees C. The fact that the rate of reactivation decreases with dilution of the enzyme solution indicates association of inactive monomers into active dimers during renaturation. The allosteric inhibitor FMN enhances the rate of phosphorylase b reactivation.

alpha-Crystallin facilitates the reactivation of hydrogen peroxide-inactivated rhodanese

It was previously shown that rhodanese, inactivated with hydrogen peroxide, could only be reactivated in the presence of a reductant or the substrate thiosulfate if these reagents were added soon after inactivation and if the oxidant was removed. Here, we report on the facilitated reactivation (75%) of hydrogen peroxide-inactivated rhodanese by the chaperone alpha-crystallin. Reactivation by the chaperone still required a reductant and thiosulfate. Without alpha-crystallin, but in the presence of the reductant and thiosulfate, the inactivated enzyme regained about 39% of its original activity. The alpha-crystallin-assisted reactivation of hydrogen peroxide-inactivated rhodanese was independent of ATP. Further, we found, that alpha-crystallin interacted transiently, but could not form a stable complex with hydrogen peroxide-inactivated rhodanese. Unlike in prior studies that involved denaturation of rhodanese through chemical or thermal means, we have clearly shown that alpha-crystallin can function as a molecular chaperone in the reactivation of an oxidatively inactivated protein.

Specificities of reactivating factors for adenosylcobalamin-dependent diol dehydratase and glycerol dehydratase

Adenosylcobalamin-dependent glycerol and diol dehydratases undergo inactivation by the physiological substrate glycerol during catalysis. In the permeabilized cells of Klebsiella pneumoniae, Klebsiella oxytoca, and recombinant Escherichia coli, glycerol-inactivated glycerol dehydratase and diol dehydratase are reactivated by their respective reactivating factors in the presence of ATP, Mg2+, and adenosylcobalamin. Both of the reactivating factors consist of two subunits. To examine the specificities of the reactivating factors, their genes or their hybrid genes were co-expressed with dehydratase genes in E. coli cells in various combinations. The reactivating factor of K. oxytoca for diol dehydratase efficiently cross-reactivated the inactivated glycerol dehydratase, whereas the reactivating factor of K. pneumoniae for glycerol dehydratase hardly cross-reactivated the inactivated diol dehydratase. Both of the two hybrid reactivating factors rapidly reactivated the inactivated glycerol dehydratase. In contrast, the hybrid reactivating factor containing the large subunit of the glycerol dehydratase reactivating factor hardly reactivated the inactivated diol dehydratase. These results indicate that the glycerol dehydratase reactivating factor is much more specific for the dehydratase partner than the diol dehydratase reactivating factor and that a large subunit of the reactivating factors principally determines the specificity for a dehydratase.

Mechanism of reactivation of coenzyme B12-dependent diol dehydratase by a molecular chaperone-like reactivating factor

The mechanism of reactivation of diol dehydratase by its reactivating factor was investigated in vitro by using enzyme. cyanocobalamin complex as a model for inactivated holoenzyme. The factor mediated the exchange of the enzyme-bound, adenine-lacking cobalamins for free, adenine-containing cobalamins through intermediate formation of apoenzyme. The factor showed extremely low but distinct ATP-hydrolyzing activity. It formed a tight complex with apoenzyme in the presence of ADP but not at all in the presence of ATP. Incubation of the enzyme.cyanocobalamin complex with the reactivating factor in the presence of ADP brought about release of the enzyme-bound cobalamin, leaving the tight apoenzyme-reactivating factor complex. Although the resulting complex was inactive even in the presence of added adenosylcobalamin, it dissociated by incubation with ATP, forming the apoenzyme, which was reconstitutable into active holoenzyme with added coenzyme. Thus, it was established that the reactivation of the inactivated holoenzyme by the factor in the presence of ATP and Mg2+ takes place in two steps: ADP-dependent cobalamin release and ATP-dependent dissociation of the apoenzyme.factor complex. ATP plays dual roles as a precursor of ADP in the first step and as an effector to change the factor into the low-affinity form for diol dehydratase. The enzyme-bound adenosylcobalamin was also susceptible to exchange with free adeninylpentylcobalamin, although to a much lesser degree. The mechanism for discrimination of adenine-containing cobalamins from adenine-lacking cobalamins was explained in terms of formation equilibrium constants of the cobalamin.enzyme.reactivating factor ternary complexes. We propose that the reactivating factor is a new type of molecular chaperone that participates in reactivation of the inactivated enzymes.

Analysis of cholinesterase inactivation and reactivation by systematic structural modification and enantiomeric selectivity

We show here with a congeneric series of Rp- and Sp-alkoxymethyl phosphonothiolates of known absolute stereochemistry that chiral selectivity in their reaction with acetylcholinesterase can be described in terms of discrete orientational and steric requirements. Stereoselectivity depends on acyl pocket dimensions, which govern leaving group orientation and a productive association of the phosphonyl oxygen in the oxyanion hole. Overall geometry is consistent with a pentavalent intermediate where the attacking serine and leaving group are at apical positions. Oxime reactivation of the phosphonylated enzyme occurs through a similar associative intermediate presumably forming an oxime phosphonate. The oximes of differing structure show distinct angles of attacking the phosphate where the attack angles and access to the phosphorus are constrained in the sterically impacted gorge. Hence, efficacy of oxime reactivation is dependent on both oxime and conjugated phosphonate structures.

A reactivating factor for coenzyme B12-dependent diol dehydratase

Adenosylcobalamin-dependent diol dehydratase of Klebsiella oxytoca undergoes suicide inactivation by glycerol, a physiological substrate. The coenzyme is modified through irreversible cleavage of its cobalt-carbon bond, resulting in inactivation of the enzyme by tight binding of the modified coenzyme to the active site. Recombinant DdrA and DdrB proteins of K. oxytoca were co-purified to homogeneity from cell-free extracts of Escherichia coli overexpressing the ddrAB genes. They existed as a tight complex, i.e. a putative reactivating factor, with an apparent molecular weight of 150,000. The factor consists of equimolar amounts of the two subunits with Mr of 64,000 (A) and 14,000 (B), encoded by the ddrA and ddrB genes, respectively. Therefore, its subunit structure is most likely A2B2. The factor not only reactivated glycerol-inactivated and O2-inactivated holoenzymes but also activated enzyme-cyanocobalamin complex in the presence of free adenosylcobalamin, ATP, and Mg2+. The reactivating factor mediated ATP-dependent exchange of the enzyme-bound cyanocobalamin for free 5-adeninylpentylcobalamin in the presence of ATP and Mg2+, but the reverse was not the case. Thus, it can be concluded that the inactivated holoenzyme becomes reactivated by exchange of the enzyme-bound, adenine-lacking cobalamins for free adenosylcobalamin, an adenine-containing cobalamin.

Reversible modification of tissue-type plasminogen activator by methylphosphonate esters

In spite of their rapid aqueous hydrolysis, 4-nitrophenyl 4-X-phenacyl methylphosphonates (X = H, (PMN) CH3, CH3O, Cl and NO2) inactivate many serine proteases of the pancreatic and blood coagulation systems efficiently. The rate constants, K/Ki, for the inactivation of tissue-type plasminogen activator enzyme (t-PA) are 470-750 M-1 S-1 with PMN, 4-CH3-PMN, and 4-CH3O-PMN in pH 7.8, 0.05 M Tris buffer at 7.0 +/- 0.5 degrees C, but t-PA cannot be inhibited with the 4-Cl and NO2 derivatives due to rapid competing hydrolysis. Enzyme activity returns from each enzyme-adduct at a characteristic rate, due to a self-catalyzed intramolecular reactivation process. The rate constants for spontaneous reactivation of t-PA from the adducts formed with the three inhibitors are K = 0.25-12.3 x 10(-2) min-1 at pH 7.4 and 25.0 +/- 0.1 degrees C and pH-dependent with an apparent pK approximately 8.3. The recovery of t-PA activity from the adducts in 40% human plasma buffered at pH 7.4 is the same or twice that in plain buffer. The presence of fibrin has a slight effect on inactivation but not on reactivation. The modulation of enzyme activity by reversible generation of the phosphonylated adducts has potential for medical application.

Oxime-induced reactivation of carboxylesterase inhibited by organophosphorus compounds

A structure-activity analysis of the ability of oximes to reactivate rat plasma carboxylesterase (CaE) that was inhibited by organophosphorus (OP) compounds revealed that uncharged oximes, such as 2,3-butanedione monoxime (diacetylmonoxime) or monoisonitrosoacetone, were better reactivators than cationic oximes. Cationic oximes that are excellent reactivators of OP-inhibited acetylcholinesterase, such as pyridine-2-aldoxime or the bis-pyridine aldoximes, HI-6 and TMB-4, produced poor reactivation of OP-inhibited CaE. The best uncharged reactivator was 2,3-butanedione monoxime, which produced complete reactivation at 0.3 mM in 2 h of CaE that was inhibited by phosphinates, alkoxy-containing phosphates, and alkoxy-containing phosphonates. Complete reactivation of CaE could be achieved even after inhibition by phosphonates with highly branched alkoxy groups, such as sarin and soman, that undergo rapid aging with acetylcholinesterase. CaE that was inhibited by phosphonates or phosphates that contained aryloxy groups were reactivated to a lesser extent. The cause of this decreased reactivation appears to be an oxime-induced aging reaction that competes with the reactivation reaction. This oxime-induced aging reaction is accelerated by electron-withdrawing substituents on the aryloxy groups of phosphonates and by the presence of multiple aryloxy groups on phosphates. Thus, reactivation and aging of OP-inhibited CaE differ from the same processes for OP-inhibited acetylcholinesterase in both their oxime specificity and inhibitor specificity and, presumably, in their underlying mechanisms.

Prediction of an inter-residue interaction in the chaperonin GroEL from multiple sequence alignment is confirmed by double-mutant cycle analysis

A search for co-ordinated amino acid changes in the hsp60 family of chaperonins suggested that cysteine residues at positions 137 and 518 in the Escherichia coli chaperonin GroEL may interact with each other. In order to determine whether this interaction indeed exists we constructed a double-mutant cycle comprising wild-type GroEL, the single mutants Cys137-->Ser and Cys518-->Ser and the corresponding double mutant. The effects of the two mutations on the function of GroEL, in assisting the refolding of a non-folded protein substrate (rhodanese), are shown to be non-additive. It is also shown that ADP by itself specifically destabilizes the Cys518-->Ser mutant GroEL particle with this effect being suppressed in the double mutant. The observed pattern of co-ordinated mutations in the hsp60 family of chaperonins is thus shown to reflect a real interaction, though most likely indirect, between Cys137 and Cys518 in GroEL. Our study demonstrates that patterns of co-ordinated mutations combined with double-mutant cycle analysis can provide structural information on interactions in a protein without an available three-dimensional structure at atomic resolution.

Chemical modification of Pseudomonas fluorescens malonyl-CoA synthetase by diethylpyrocarbonate: kinetic evidence for an essential histidyl residue on alpha subunit

Malonyl-CoA synthetase from Pseudomonas fluorescens was inactivated by diethylpyrocarbonate (DEP) with the second-order rate constant of 775 M-1 min-1 at pH 7.0, 25 degrees C, showing a concomitant increase in absorbance at 242 nm due to the formation of N-carbethoxyhistidyl derivatives. The inactivated enzyme at low concentration of DEP (less than 0.2 mM) could be completely reactivated by hydroxylamine but not completely reactivated at high concentration (greater than 0.5 mM), indicating that there may be another functional group modified by DEP. Complete inactivation of malonyl-CoA synthetase required the modification of seven residues per molecule of enzyme; however, only one is calculated to be essential for enzyme activity by a statistical analysis of the residual enzyme activity. pH dependence of inactivation indicated the involvement of a residue with a pK alpha of 6.7, which is closely related to that of histidyl residue of proteins. When alpha subunit treated with DEP was mixed with beta subunits complex, the enzyme activity completely disappeared, whereas when beta subunit complex treated with the reagent was mixed with alpha subunit, the activity remained. Inactivation of the enzyme by the reagent was protected by the presence of malonate and ATP. These results indicate that a catalytically essential histidyl residue is located at or near the malonate and ATP binding region on alpha subunit of the enzyme.