Mechanisms of monovalent cation action in enzyme catalysis: the tryptophan synthase alpha-, beta-, and alpha beta-reactions

The alpha-subunit of the tryptophan synthase bienzyme complex catalyzes the formation of indole from the cleavage of 3-indolyl-D-glyceraldehyde 3'-phosphate, while the beta-subunit utilizes L-serine and the indole produced at the alpha-site to form tryptophan. The replacement reaction catalyzed by the beta-subunit requires pyridoxal 5'-phosphate (PLP) as a cofactor. The beta-reaction occurs in two stages: in stage I, the first substrate, L-Ser, reacts with the enzyme-bound PLP cofactor to form an equilibrating mixture of the L-Ser Schiff base, E(Aex1), and the alpha-aminoacrylate Schiff base intermediate, E(A-A); in stage II, this intermediate reacts with the second substrate, indole, to form tryptophan. Monovalent cations (MVCs) are effectors of these processes [Woehl, E., and Dunn, M. F. (1995) Biochemistry 34, 9466-9476]. Herein, detailed kinetic dissections of stage II are described in the absence and in the presence of MVCs. The analyses presented complement the results of the preceding paper [Woehl, E., and Dunn, M. F. (1999) Biochemistry 38, XXXX-XXXX], which examines stage I, and confirm that the chemical and conformational processes in stage I establish the presence of two slowly interconverting conformations of E(A-A) that exhibit different reactivities in stage II. The pattern of kinetic isotope effects on the overall activity of the beta-reaction shows an MVC-mediated change in rate-limiting steps. In the absence of MVCs, the reaction of E(A-A) with indole becomes the rate-limiting step. In the presence of Na+ or K+, the conversion of E(Aex1) to E(A-A) is rate limiting, whereas some third process not subject to an isotope effect becomes rate determining for the NH4+-activated enzyme. The combined results from the preceding paper and from this study define the MVC effects, both for the reaction catalyzed by the beta-subunit and for the allosteric communication between the alpha- and beta-sites. Partial reaction-coordinate free energy diagrams and simulation studies of MVC effects on the proposed mechanism of the beta-reaction are presented.

Comparison of the allosteric properties of the Co(II)- and Zn(II)-substituted insulin hexamers

The positive and negative cooperativity and apparent half-site reactivity of the Co(II)-substituted insulin hexamer are well-described by a three-state allosteric model involving ligand-mediated interconversions between the three states: T3T3' right harpoon over left harpoon T3o R3o right harpoon over left harpoon R3R3' [Bloom, C. R., Heymann, R., Kaarsholm, N. C., and Dunn, M. F. (1997) Biochemistry 36, 12746-12758]. Because of the low affinity of the T state for ligands, this model is defined by four parameters: LoA and LoB, the allosteric constants for the T3T3' to T3o R3o and the T3o R3o to R3R3' transitions, respectively, and the two dissociation constants for ligand binding to T3o R3o and to R3R3'. The d-d electronic transitions of the Co(II)-substituted hexamer give optical signatures of the T to R transition which can be quantified, but the "spectroscopically silent" character of Zn(II) has made previous attempts to describe the Zn(II) species difficult. This work shows that the T to R state conformational transitions of the Zn(II) hexamer can be easily quantified using the chromophore 4-hydroxy-3-nitrobenzoate (4H3N). When the chromophore is bound to the HisB10 sites of the R state, the absorption spectrum of 4H3N is red-shifted, exhibiting strong absorbance and CD signals, whereas 4H3N does not bind to the T state. Hence, 4H3N can be employed as a sensitive indicator of conformation under conditions that do not significantly disturb the T to R state equilibrium. Using 4H3N as an indicator, these studies show that both LoA and LoB are made less favorable by the substitution of Co(II) for Zn(II); LoA is increased by 10-fold while LoB by 35-fold, whereas the ligand affinities of the phenolic pockets are unchanged.

Tricarboxylic acid cycle and anaplerotic enzymes in rhizobia

Rhizobia are a diverse group of Gram-negative bacteria comprised of the genera Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium and Azorhizobium. A unifying characteristic of the rhizobia is their capacity to reduce (fix) atmospheric nitrogen in symbiotic association with a compatible plant host. Symbiotic nitrogen fixation requires a substantial input of energy from the rhizobial symbiont. This review focuses on recent studies of rhizobial carbon metabolism which have demonstrated the importance of a functional tricarboxylic acid (TCA) cycle in allowing rhizobia to efficiently colonize the plant host and/or develop an effective nitrogen fixing symbiosis. Several anaplerotic pathways have also been shown to maintain TCA cycle activity under specific conditions. Biochemical and physiological characterization of carbon metabolic mutants, along with the analysis of cloned genes and their corresponding gene products, have greatly advanced our understanding of the function of enzymes such as citrate synthase, oxoglutarate dehydrogenase, pyruvate carboxylase and malic enzymes. However, much remains to be learned about the control and function of these and other key metabolic enzymes in rhizobia.

Regulation of pyruvate carboxylase in Rhizobium etli

Pyruvate carboxylase (PYC) is a biotin-dependent enzyme catalyzing the anaplerotic conversion of pyruvate to oxaloacetate in Rhizobium etli strain CE3. A pyc::Tn5 mutant had severely reduced growth, or failed to grow on sugars, three-carbon organic acids or glycerol, consistent with these substrates being metabolized via pyruvate. Transconjugants expressing a pyc::beta-glucuronidase gene fusion had slightly increased apparent pyc transcription during growth on pyruvate as compared to succinate, similar to the modest carbon source dependent changes in PYC activity reported previously. Biotin supplementation of cultures growing on pyruvate dramatically increased PYC activity but not apparent pyc transcription. Bacteroids isolated from bean nodules did not contain detectable PYC activity while apparent pyc transcription occurred at a moderate level.

Binding of 2,6- and 2,7-dihydroxynaphthalene to wild-type and E-B13Q insulins: dynamic, equilibrium, and molecular modeling investigations

The binding of phenolic ligands to the insulin hexamer occurs as a cooperative allosteric process. Investigations of the allosteric mechanism from this laboratory resulted in the postulation of a model consisting of a three-state conformational equilibrium and the derivation of a mathematical expression to describe the insulin system. The proposed mechanism involves allosteric transitions among two states of high symmetry, designated T3T3' (a low affinity state) and R3R3' (a high affinity state), and a third state of lower symmetry, designated T3oR3o (a state of mixed low and high affinities). To further characterize this mechanism, we present rapid kinetic fluorescence studies, equilibrium binding isotherms, and molecular modeling investigations for the Co(II)-substituted wild-type and E-B13Q mutant hexamers. These studies show that the measured on and off rates (kon and koff) for the binding of the allosteric ligands 2,6- and 2,7-dihydroxynaphthalene provide an independent measure of the dissociation constant for binding to the T3oR3o conformation (KRo). These constants are in agreement with the value obtained by computer fitting of the equilibrium binding isotherms to the quantitative allosteric mechanism. We analyze the structural differences between the T3oR3o and R6 phenolic binding sites and predict the structures of the T3oR3o-2,6-DHN and R6-2, 6-DHN complexes by 3-D molecular modeling. Assignment of H-bonding of the first hydroxyl group to CysA6 and CysA11 has been supported by stacking interactions analogous to phenol using 1H-NMR. H-bonding of the second hydroxyl group of 2,6-DHN to the GluB13 carboxylate side chains is predicted by molecular modeling and is supported by a reduction of affinity for Ca2+, which is postulated to bind to the GluB13 side chains.

Half-site reactivity, negative cooperativity, and positive cooperativity: quantitative considerations of a plausible model

The nature of cooperative allosteric interactions has been the source of controversy since the ground-breaking studies of oxygen binding to hemoglobin. Until recently, quantitative examples of a model based on the inherent symmetry and asymmetry of oligomeric proteins have been lacking. This laboratory has used the phenolic ligand binding characteristics of the insulin hexamer to develop the first quantitative model for a symmetry-asymmetry-based cooperativity mechanism. The insulin hexamer possesses positive and negative heterotropic and homotropic interactions involving two classes of sites. In this study, we explore the effects of heterotropic interactions between these sites. We show that application of the pairwise structural asymmetry theory of Seydoux, Malhotra, and Bernhard (SMB) gives excellent agreement between the ligand binding behavior and X-ray crystal structure data. Furthermore, by comparing experimental data with computer simulations, we show that the insulin hexamer can be described by a three-state SMB model involving two positive homotropic cooperative transitions linked by a negative homotropic interaction. The first transition, T3T3' right harpoon over left harpoon T3oR3o, with allosteric constant LoA = [T3T3']/[T3oR3o] and ligand dissociation constant KRo consists of a positive cooperative change from high to low symmetry that results in "half-site reactivity". The second transition, T3oR3o right harpoon over left harpoon R3R3', with allosteric constant LoB = [T3oR3o]/[R3R3'] and ligand dissociation constant KR is a change from low to high symmetry, which is also a positive cooperative process. Treatment of the two transitions as concerted and interconnected processes allows derivation of an equation for the fraction of R-state. Using this equation, the effects of changes in the four physical parameters, LoA, LoB, KR, and KRo, on the ligand binding properties of the insulin hexamer are quantitatively described.

Carboxylate ions are strong allosteric ligands for the HisB10 sites of the R-state insulin hexamer

The insulin hexamer is an allosteric protein which displays positive and negative cooperativity and half-site reactivity that is modulated by strong homotropic and heterotropic ligand binding interactions at two different loci. These loci consist of phenolic pockets situated on the dimer-dimer interfaces of T-R and R-R subunit pairs and of anion sites comprising the HisB10 metal ion sites of the R3 units of the T3R3 and R6 states. In this study, we show that suitably tailored organic carboxylates are strong allosteric effectors with relatively high affinities for the R-state HisB10 metal sites. Methods of quantifying the relative affinities of ligands for these sites in both Co(II)- and Zn(II)-substituted insulin hexamers are presented. These analyses show that, in addition to the electron density on the ion, the carboxylate affinity is influenced by polar, nonpolar, and hydrophobic interactions between substituents on the carboxylate and the amphipathic protein surface of the narrow tunnel which controls ligand access to the metal ion. Since the binding of anions to the HisB10 site makes a critically important contribution to the stability of the T3R3 and R6 forms of the insulin hexamer, the design of high-affinity ligands with a carboxylate donor for coordination to the metal ion provides an opportunity for constructing insulin formulations with improved pharmaceutical properties.

Mechanisms of stabilization of the insulin hexamer through allosteric ligand interactions

The insulin hexamer is an allosteric protein capable of undergoing transitions between three conformational states: T6, T3R3, and R6. These transitions are mediated by the binding of phenolic compounds to the R-state subunits, which provide positive homotropic effects, and by the coordination of anions to the bound metal ions, which act as heterotropic effectors. Since the insulin monomer is far more susceptible than the hexamer to thermal, mechanical, and chemical degradation, insulin-dependent diabetic patients rely on pharmaceutical preparations of the Zn-insulin hexamer, which act as stable forms of the biologically active monomeric insulin. In this study, the chromophoric chelator 2,2',2"-terpyridine (terpy) has been used as a kinetic probe of insulin hexamer stability to measure the effect of homotropic and heterotropic effectors on the dissociation kinetics of the Zn2+- and Co2+-insulin hexamer complexes. We show that the reaction between terpy and the R-state-bound metal ion is limited by the T3R3 <==> T6 or R6 <==> T3R3 conformational transition steps and the dissociation of one anionic ligand, or one anionic ligand and three phenolic ligand molecules, respectively, for T3R3 and R6. Consequently, because the activation energies of these steps are dominated by the ground-state stabilization energy of the R-state species, the kinetic stabilization of the insulin hexamer toward terpy-induced dissociation is linked to the thermodynamic stabilization of the hexamer. The mass action effect of anion binding and, foremost, of phenolic ligand binding provides the major mechanism of stabilization, resulting in the tightening of the tertiary and quaternary hexamer structures. Using this kinetic method, we show that the R6 conformation of Zn-insulin in the presence of Cl- ion and resorcinol is > 1.5 million-fold more stable than the T3 units of T6 and T3R3 and > 70,000-fold more stable than the R3 unit of T3R3. Furthermore, the stabilization effect is correlated with the affinity of the ligands: the tighter the binding, the slower the reaction between terpy and R-state-bound metal ion. These concepts provide a new basis for the pharmaceutical improvement of the physicochemical stability of formulations both for native insulin and for fast-acting monomeric insulin analogues through ligand-mediated allosteric interactions.

Ligand perturbation effects on a pseudotetrahedral Co(II)(His)3-ligand site. A magnetic circular dichroism study of the Co(II)-substituted insulin hexamer

Magnetic circular dichroism (MCD) spectra of a series of adducts formed by the Co(II)-substituted R-state insulin hexamer are reported. The His-B10 residues in this hexamer form tris imidazole chelates in which pseudotetrahedral Co(II) centers are completed by an exogenous fourth ligand. This study investigates how the MCD signatures of the Co(II) center in this unit are influenced by the chemical and steric characteristics of the fourth ligand. The spectra obtained for the adducts formed with halides, pseudohalides, trichloroacetate, nitrate, imidazole, and 1-methylimidazole appear to be representative of near tetrahedral Co(II) geometries. With bulkier aromatic ligands, more structured spectra indicative of highly distorted Co(II) geometries are obtained. The MCD spectrum of the phenolate adduct is very similar to those of Co(II)-carbonic anhydrase (alkaline form) and Co(II)-beta-lactamase. The MCD spectrum of the Co(II)-R6-CN- adduct is very similar to the CN- adduct of Co(II)-carbonic anhydrase. The close similarity of the Co(II)-R6-pentafluorophenolate and Co(II)-R6-phenolate spectra demonstrates that the Co(II)-carbonic anhydrase-like spectral profile is preserved despite a substantial perturbation in the electron withdrawing nature of the coordinated phenolate oxygen atom. We conclude that this type of spectrum must arise from a specific Co(II) coordination geometry common to each of the Co(II) sites in the Co(II)-R6-phenolate, Co(II)-R6-pentafluorophenolate, Co(II)-beta-lactamase, and the alkaline Co(II)-carbonic anhydrase species. These spectroscopic results are consistent with a trigonally distorted tetrahedral Co(II) geometry (C3v), an interpretation supported by the pseudotetrahedral Zn(II)(His)3(phenolate) center identified in a Zn(II)-R6 crystal structure (Smith, G. D., and Dodson, G. G. (1992) Biopolymers 32, 441-445).

Protein architecture, dynamics and allostery in tryptophan synthase channeling

The alpha 2 beta 2 form of the tryptophan synthase bienzyme complex catalyses the last two steps in the synthesis of L-tryptophan, consecutive processes that depend on the channeling of the common metabolite, indole, between the sites of the alpha- and beta-subunits through a 25 A-long tunnel. The channeling of indole and the coupling of the activities of the two sites are controlled by allosteric signals derived from covalent transformations at the beta-site that switch the enzyme between an open, low-activity state, to which ligands bind, and a closed, high-activity state, which prevents the escape of indole.

Spectroscopic evidence for preexisting T- and R-state insulin hexamer conformations

The insulin hexamer is an allosteric protein exhibiting both positive and negative cooperative homotropic interactions and positive cooperative heterotropic interactions (C. R. Bloom et al., J. Mol. Biol. 245, 324-330, 1995). In this study, detailed spectroscopic analyses of the UV/Vis absorbance spectra of the Co(II)-substituted human insulin hexamer and the 1H NMR spectra of the Zn(II)-substituted hexamer have been carried out under a variety of ligation conditions to test the applicability of the sequential (KNF) and the half-site reactivity (SMB) models for allostery. Through spectral decomposition of the characteristic d-->d transitions of the octahedral Co(II)-T-state and tetrahedral Co(II)-R-state species, and analysis of the 1H NMR spectra of T- and R-state species, these studies establish the presence of preexisting T- and R-state protein conformations in the absence of ligands for the phenolic pockets. The demonstration of preexisting R-state species with unoccupied sites is incompatible with the principles upon which the KNF model is based. However, the SMB model requires preexisting T- and R-states. This feature, and the symmetry constraints of the SMB model make it appropriate for describing the allosteric properties of the insulin hexamer.

Substitution of pyridoxal 5'-phosphate in the O-acetylserine sulfhydrylase from Salmonella typhimurium by cofactor analogs provides a test of the mechanism proposed for formation of the alpha-aminoacrylate intermediate

O-Acetylserine sulfhydrylase (OASS) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes the final step in the de novo synthesis of L-cysteine in Salmonella typhimurium. Complementary cofactor mutagenesis in which the active site PLP is substituted with cofactor analogs is used to test the mechanism proposed for the OASS. Data obtained with the pyridoxal 5'-deoxymethylenephosphonate-substituted enzyme suggest that the binding of OAS as it forms the external Schiff base is such that the acetate side chain is properly positioned for elimination (orthogonal to the developing alpha,beta-double bond) only about 1% of the time. Data support the assignment of an enzyme group with a pK of 6.7 that interacts with the acetyl side chain, maintaining it orthogonal to the developing alpha,beta-double bond. Similar studies of the 2'-methylpyridoxal 5'-phosphate-substituted enzyme suggest that, although the mechanism is identical to that catalyzed by native OASS, the reaction coordinate for alpha-proton abstraction may be decreased compared with that observed for the native enzyme.

Pyruvate carboxylase from Rhizobium etli: mutant characterization, nucleotide sequence, and physiological role

Pyruvate carboxylase (PYC), a biotin-dependent enzyme which catalyzes the conversion of pyruvate to oxaloacetate, was hypothesized to play an important anaplerotic role in the growth of Rhizobium etli during serial subcultivation in minimal media containing succinate (S. Encarnación, M. Dunn, K. Willms, and J. Mora, J. Bacteriol. 177:3058-3066, 1995). R. etli and R. tropici pyc::Tn5-mob mutants were selected for their inability to grow in minimal medium with pyruvate as a sole carbon source. During serial subcultivation in minimal medium containing 30 mM succinate, the R. etli parent and pyc mutant strains exhibited similar decreases in growth rate with each subculture. Supplementation of the medium with biotin prevented the growth decrease of the parent but not the mutant strain, indicating that PYC was necessary for the growth of R. etli under these conditions. The R. tropici pyc mutant grew normally in subcultures regardless of biotin supplementation. The symbiotic phenotypes of the pyc mutants from both species were similar to those of the parent strains. The R. etli pyc was cloned, sequenced, and found to encode a 126-kDa protein of 1,154 amino acids. The deduced amino acid sequence is highly homologous to other PYC sequences, and the catalytic domains involved in carboxylation, pyruvate binding, and biotinylation are conserved. The sequence and biochemical data show that the R. etli PYC is a member of the alpha4, homotetrameric, acetyl coenzyme A-activated class of PYCs.

Evidence of a low-barrier hydrogen bond in the tryptophan synthase catalytic mechanism

In the absence of other substrates, L-Ser reacts rapidly with the tryptophan synthase alpha 2 beta 2 bienzyme from Salmonella typhimurium at pH 7.8 and 25 degrees C to give an equilibrating mixture of species dominated by comparable amounts of the L-Ser external aldimine Schiff base, E(Aex1), and the alpha-aminoacrylate Schiff base, E(A-A). The D-isomer of Ser is unreactive toward alpha 2 beta 2, and therefore, D,L-Ser can be used in place of L-Ser for investigations of catalytic mechanism. Due to the equilibrium isotope effect, when alpha-2H-D,L-Ser is substituted for alpha-1H-D,L-Ser, the position of equilibrium is shifted in favor of E(Aex1). On a much slower time scale, the 2H sample undergoes the exchange of enzyme bound 2H for the 1H of solvent water and is converted to a distribution of E(Aex1) and E(A-A) identical to that obtained with the 1H sample. This slow exchange indicates that the proton abstracted from the alpha-carbon of E(Aex1) is sequestered within a solvent-excluded site in E(A-A). Analysis of the UV/vis spectra gave an isotope effect on the equilibrium distribution of E(Aex1) and E(A-A) of KH/KD = 1.80 +/- 0.18. This large equilibrium isotope effect is the consequence of an unusual isotope fractionation factor of 0.62 for the residue which functions as the base to deprotonate and protonate the alpha-carbon proton in E(Aex1). A fractionation factor of 0.62 qualifies as evidence for the involvement of a low-barrier H-bond (LBHB) in this equilibration. Since this effect arises from abstraction of the alpha-proton from E(Aex1), the LBHB must be associated with the E(A-A) species. In contrast to weak H-bonds with energies of 3-12 kcal/mol, LBHBs are proposed to exhibit energies in the 12-24 kcal/mol range [Frey, P.A., Whitt, S.A., & Tobin, J. B. (1994) Science 264, 1927-1930]. Possible roles for this LBHB both in the chemical mechanism and in the stabilization of the closed conformation of E(A-A) are discussed.

Kinetic isotope effects as a probe of the beta-elimination reaction catalyzed by O-acetylserine sulfhydrylase

Primary and alpha-secondary deuterium kinetic isotope effects have been measured for the O-acetylserine sulfhydrylase from Salmonella typhimurium using both steady-state and single-wavelength stopped-flow studies. Data suggest an asymmetric transition state for alpha-proton abstraction by the active site lysine and the elimination of the acetyl group of O-acetyl-L-serine (OAS) to form the alpha-aminoacrylate intermediate. The value of D(V/KOAS) using OAS-2-d is dependent on pH from 5.8 to 7.0 with independent values of 2.8 and 1.7 estimated at low and high pH, respectively. Thus, OAS is sticky, and a value of 1.5 is calculated for the forward commitment to catalysis, indicating that the OAS external Schiff base preferentially partitions toward the alpha-aminoacrylate intermediate compared to OAS being released from enzyme. The intrinsic primary deuterium isotope effect determined from single-wavelength stopped-flow studies of alpha-proton abstraction by the active site lysine is about 2.0. D(V/KOAS) and T(V/KOAS) were determined as 2.6 +/- 0.1 and 4.2 +/- 0.2 at pH 6.1, respectively, giving a calculated intrinsic deuterium isotope effect of 3.3 +/- 0.9, consistent with the D(V/KOAS) obtained from steady-state studies at low pH. The alpha-secondary deuterium kinetic isotope effect using OAS-3,3-d2 is 1.11 +/- 0.06 obtained by direct comparison of initial velocities and 1.2 obtained by single-wavelength stopped-flow experiments. Data can be compared to a value of 1.81 +/- 0.04 using OAS-3,3-d2 for alpha-DKeq for the first half-reaction.

beta-Site covalent reactions trigger transitions between open and closed conformations of the tryptophan synthase bienzyme complex

The tryptophan synthase bienzyme complex (alpha2beta2) from Salmonella tryphimurium catalyzes the final steps in the biosynthesis of L-Trp. To investigate the roles played by conformational change in tryptopthan synthase catalysis, the fluorophore 8-anilino-1-naphthalensulfonate (ANS) is used to identify conformational states. The binding of ANS to the alpha2beta2 bienzyme complex is accompanied by a dramatic enhancement of ANS fluorescence and a shift of the emission maximum from 520 to 482 nm. The ANS binding isotherm is biphasic and consists of a class of moderately high-affinity, noninteracting sites with a stoichiometry of 1 site/alpha beta dimeric unit (Kd' = 62 + or - 15 micrometer) and a much weaker set of non-specific interactions with K'd>1mM. Our findings show that the affinity of the enzyme for ANS is strongly decreased (> 10-fold) by interactions at two loci 30 angstroms apart: (i) the binding of the alpha-site ligands, 3-indole-D-glycerol 3'-phosphate or alpha-glycerol phosphate (GP) or (ii) reaction at the beta-subunit to form either the alpha-aminoacrylate Schiff base, E(A-A), or quinonoid species, E(Q). In contrast, formation of the L-Ser and L-Trp external aldimines E(Aex1) and E(Aex2) at the beta-site causes a 2-3 fold decrease in the affinity of the enzyme for ANS. The combination of E(A-A)or E(Q) with GP brings about almost complete displacement of ANS, indicating that these interactions drive a conformation change in alphabeta subunit pairs which prevents the binding of ANS. These results are consistent with a model which postulates that alphabeta subunit pairs undergo ligand-mediated transitions between open and closed conformations during the catalytic cycle. Consistent with the kinetic data showing that binding of alpha-site ligands increases the affinity of the beta site for L-Ser and that formation of E(A-A) activates the alpha reaction [Brzović, P. S., Ngo, K., & Dunn, M. F. (1992) Biochemistry 31, 3831-3839], while mutations in alpha subunit loops 2 and 6 prevent the ligand- mediated transition to a closed structure [Brzović, P.S., Hyde, C.C., Miles, E.W., & Dunn, M.F. (1993) Biochemistry 32, 10404-10413], we conclude that reciprocal ligand-mediated allosteric interactions between the heterologous subunits promote conformational transitions between open and closed structures in alphabeta subunit pairs which function to coordinate catalytic activities and facilitate the channeling of indole between the two catalytic sites.

Formation of the alpha-aminoacrylate immediate limits the overall reaction catalyzed by O-acetylserine sulfhydrylase

O-Acetylserine sulfhydrylase-A (OASS-A) catalyzes the final step in the synthesis of L-cysteine, viz., the beta-substitution of acetate in O-acetyl-L-serine (OAS) by sulfide via a ping-pong kinetic mechanism . Rapid-scanning stopped-flow and single-wavelength absorbance and fluorescence stopped-flow experiments were carried out to obtain information on the location and amount of limitation of rate-determining steps for the overall reaction and the individual half-reactions of OASS-A. The first half-reaction, conversion of OAS to the alpha-aminoacrylate intermediate and acetate, is rate-limiting for the overall reaction catalyzed by OASS-A. No intermeidates are detected within the second half-reaction, and thus rate constants for all steps must be > or = 1000s-1 at the lowest sulfide concentration used. Within the first half reaction, formation of the extrernal Schiff base (Kassociation = 0.2 mM-1) is observed in the first milliseconds, followed by its slower conversion to the alpha-aminocacrylate intermediate with a rate constant of 300 s-1, close to the value of 130 s-1 obtained for V/Et [Tai, C.H., Nalabolu, S.R., Jacobson, T.M., Minter D.E., & Cook, P.F. (1993) Biochemistry 32, 6433-6442]. Addition of L-cysterine ot OASS-A results in a rapid formation of the external Schiff base (Kassociation = 6.7 mM-1), followed by transient formation of the alpha-aminoacylate intermediate with a slightly lower rate (70-100 s-1) compared to OAS. The alpha-aminoacrylate intermediate decays to generate a species absorbing maximally at 418 nm, resulting from attack of the cysteine thiol to give ether in external Schiff base linkage with the active site PLP.

Allosteric regulation of tryptophan synthase: effects of pH, temperature, and alpha-subunit ligands on the equilibrium distribution of pyridoxal 5'-phosphate-L-serine intermediates

The equilibrium distribution of catalytic intermediates formed in the reaction of L-serine with the tryptophan synthase alpha 2 beta 2-complex from Salmonella typhimurium has been investigated by absorption and fluorescence spectroscopy as a function of pH, temperature, and alpha-subunit ligands. The novel result of this study is that the equilibrium between the two major catalytic species, the external aldimine and the alpha-aminoacrylate, is modulated by the ionization of two groups with apparent pK values of 7.8 +/- 0.3 and 10.3 +/- 0.2. Protonation of these groups stabilizes the alpha-aminoacrylate Schiff base by an estimated 100-fold with respect to the external aldimine. Furthermore, the formation of the alpha-aminoacrylate from the external aldimine is an endothermic process. Temperature slightly affects the apparent pK values but remarkably influences the amplitude of the phase associated with the ionization of each group. At 20 degrees C, each phase accounts for nearly half of the titration. Since the isolated beta 2-dimer does not exhibit a pH-dependent distribution of intermediates, the alpha-beta-subunit interactions seem critical to the onset of this functional property of the beta-subunit. The modulation of intersubunit interactions by the alpha-subunit ligands DL-alpha-glycerol 3-phosphate and phosphate leads to significant changes in the pH-dependent distribution of intermediates. At saturating concentrations of either of these alpha-subunit ligands, the alpha-aminoacrylate Schiff base is the predominant species over a wide pH range while the apparent pK values of the groups that control the equilibrium are not significantly affected. The pH-dependent interconversion of catalytic intermediates here reported has not been previously detected because phosphate buffers have usually been employed in the studies of this enzyme. Our findings are discussed in the light of a model in which specific protein conformations are associated with the external aldimine and the alpha-aminoacrylate Schiff bases, the latter being stabilized by temperature, protons, and alpha-subunit ligands.

Monovalent metal ions play an essential role in catalysis and intersubunit communication in the tryptophan synthase bienzyme complex

This investigation shows that the alpha 2 beta 2 tryptophan synthase bienzyme complex from Salmonella typhimurium is subject to monovalent metal ion activation. The effects of the monovalent metal ions Na+ and K+ were investigated using rapid scanning stopped-flow (RSSF), single-wavelength stopped-flow (SWSF), and steady-state techniques. RSSF measurements of individual steps in the reaction of L-serine and indole to give L-trytophan (the beta-reaction) as well as the reaction of 3-indole-D-glycerol 3'-phosphate (IGP) with L-serine (the alpha beta-reaction) demonstrate that monovalent metal ions such as Na+ and K+ change the distribution of intermediates in both the transient and steady states. Therefore the metal ion effect alters relative ground-state energies and the relative positions of ground- and transition-state energies. The RSSF spectra and SWSF time courses show that the turnover of indole is significantly reduced in the absence of either Na+ or K+. The alpha-aminoacrylate Schiff base species, E(A-A), is in a less active state in the absence of monovalent metal ions. Na+ decreases the steady-state rate of IGP cleavage (the alpha-reaction) to about 30% of the value obtained in the absence of metal ions. Steady-state investigations show that in the absence of monovalent metal ions the alpha- and alpha beta-reactions have the same activity. Na+ binding gives a 30-fold stimulation of the alpha-reaction when the beta-site is in the E(A-A) form.(ABSTRACT TRUNCATED AT 250 WORDS)

Allosteric linkages between beta-site covalent transformations and alpha-site activation and deactivation in the tryptophan synthase bienzyme complex

This work examines two aspects of the catalytic mechanism and allosteric regulation of the tryptophan synthase bienzyme complex from Salmonella typhimurium: (a) the chemical mechanism by which indole and other nucleophiles react with the enzyme-bound alpha-aminoacrylate Schiff base intermediate, E(A-A), to form quinonoidal intermediates, E(Q), and (b) the effects of covalent transformations at the beta-site on the catalytic activity of the alpha-site. Transient kinetic studies in combination with alpha-secondary deuterium isotope effects are undertaken to determine the mechanism of nucleophile addition to E(A-A). These studies establish that nucleophilic attack is best described by a two-step reaction sequence consisting of a binding step that is followed by Michael addition to the conjugated double bond of E(A-A). Analysis of isotope effects suggests that the transition state for indole addition gives an E(A-A) beta-carbon that resembles an sp3 center, while the stronger nucleophiles, indoline and beta-mercaptoethanol, have transition states that appear to more closely resemble an sp2 beta-carbon. The effects of beta-site covalent transformations on alpha-site catalysis were studied using quasi-stable beta-site intermediates and the alpha-site substrate analogue 3-[6-nitroindole]-D-glycerol 3'-phosphate (6-nitro-IGP). It was found that the cleavage of 6-nitro-IGP is strongly activated by the formation of E(A-A) and various E(Q) species at the beta-site but not by external aldimine species. Therefore, we conclude that the conversion of the L-Ser external aldimine to E(A-A) is the beta-site process which activates the alpha-site, while conversion of E(Q) to the L-Trp external aldimine triggers deactivation of the alpha-site. These findings are discussed within the context of allosteric regulation of substrate channeling in tryptophan synthase catalysis.

Ligand binding to wild-type and E-B13Q mutant insulins: a three-state allosteric model system showing half-site reactivity

By using ultra-violet and visible absorbance in conjunction with high field 1H-nuclear magnetic resonance spectroscopy, the insulin hexamer has been shown to undergo two allosteric transitions in solution involving three allosteric states (T6<-->T3 R3<-->R6). A simple mathematical model consisting of four variables has been derived that quantitatively describes the complex homotropic and heterotropic interactions that modulate these allosteric transitions. The mutation of one residue, Glu-B13 to Gln, results in an unexpected change in the T3R3 to R6 equilibrium by a factor of 10(7).

Structural asymmetry and half-site reactivity in the T to R allosteric transition of the insulin hexamer

The zinc-insulin hexamer, the storage form of insulin in the pancreas, is an allosteric protein capable of undergoing transitions between three distinct conformational states, designated T6, T3R3, and R6, on the basis of their ligand binding properties, allosteric behavior, and pseudo point symmetries [Kaarsholm, N. C., Ko, H.-C., & Dunn, M. F. (1989) Biochemistry 28, 4427-4435]. The transition from the T-state to the R-state involves a coil-to-helix transition in residues 1-8 of the B-chain wherein the ring of PheB1 is displaced by approximately 30 A. This motion also is accompanied by small changes in the positions of A-chain residues and other B-chain residues. In this paper, one- and two-dimensional (COSY and NOESY) 1H NMR are used to characterize the ligand-induced T to R transitions of wild-type and EB13Q mutant human zinc-insulin hexamers and to make sequence-specific assignments of all resonances in the aromatic region of the R6 complex with resorcinol. The changes in the 1H NMR spectrum (at 500 and 600 MHz) that occur during the T to R transition provide specific signatures of the conformation change. Analysis of the dependence of these spectral changes for the phenol-induced transition as a function of the concentration of phenol establish (1) that the interconversion of T6 and R6 occurs via a third species assigned as T3R3 and (2) that the system shows both negative and positive cooperative allosteric behavior. One- and two-dimensional COSY and NOESY studies show that, in the absence of phenolic compounds, anions act as heterotropic effectors that shift the distribution of hexamer conformations in favor of the R-state with the order of effectiveness, SCN- > N3- >> I- >> Cl-. Analysis of one- and two-dimensional spectra indicate that with wild-type insulin, SCN- and N3- give T3R3 species, whereas the EB13Q mutant gives an R6 species. An allosteric model for the insulin T to R transition based on the structural asymmetry model [Seydoux, F., Malhotra, O. P., & Bernhard, S. A. (1974) CRC Crit. Rev. Biochem. 2, 227-257] is proposed that explains the negative and positive allosteric properties of the system, including the role of T3R3 and the action of homotropic and heterotropic effectors.