Artificial duplication of the R67 dihydrofolate reductase gene to create protein asymmetry. Effects on protein activity and folding

R67 dihydrofolate reductase (DHFR), encoded by an R plasmid, provides resistance to the antibacterial drug trimethoprim. This enzyme does not exhibit any structural or sequence homologies with chromosomal DHFR. A recent crystal structure of tetrameric R67 DHFR (D. Matthews, X. Nguyen-huu, and N. Narayana, personal communication) shows a single pore traversing the length of the molecule. Numerous physical and kinetic experiments suggest the pore is the active site. Since the center of the pore possesses exact 222 symmetry, mutagenesis of residues designed to explore substrate binding will probably also affect cofactor binding. As a first step in breaking this inevitable symmetry in R67 DHFR, the gene has been duplicated. The protein product, R67 DHFRdouble, is twice the molecular mass of native R67 DHFR and is fully active with kcat = 1.2 s-1, Km(NADPH) = 2.7 microM and Km(dihydrofolate) = 6.3 microM. Equilibrium unfolding studies in guanidine-HCl indicate R67 DHFRdouble is more stable than native R67 DHFR at physically reasonable protein concentrations. Microcalorimetry studies show native R67 DHFR undergoes fully reversible thermal unfolding. Unfolding can be described by a two-state process since a ratio of delta Hcalorimetric to delta Hvan't Hoff equals 0.96. In contrast, thermal unfolding of R67 DHFRdouble is not fully reversible and possesses an oligomerization component introduced by the gene duplication event.

How do mutations at phenylalanine-153 and isoleucine-155 partially suppress the effects of the aspartate-27-->serine mutation in Escherichia coli dihydrofolate reductase?

Several second-site suppressors of the D27S lesion in Escherichia coli dihydrofolate reductase (DHFR) have been identified. The activity of the primary mutant, D27S DHRF, was found to be greatly decreased at pH 7.0, consistent with aspartic acid-27 being critically involved in proton donation during catalysis. Partial suppressors of the D27S mutation have been selected by their ability to confer an increased resistance to trimethoprim upon host E. coli; the suppressors have been identified as F153S or I155N substitutions. D27S+F153S and D27S+I155N DHFRs display 2-3-fold increases in kcat over D27S DHFR values, but only the F153S mutation decreases the Km for dihydrofolate by a factor of 2. Neither double mutant approaches wild-type DHFR activity. Unexpectedly, Phe153 and Ile155 occur on the surface of the protein and are approximately 8 and 14 A distant from the active site. Ile155 is a member of a beta-bulge. A previously identified suppressing mutation, F137S, occurs nearby and is also a member of the same beta-bulge [Howell et al. (1990) Biochemistry 29, 8561-8569]. Clustering of these three second-site mutations indicates this area of the structure may be important in protein function. Conformational changes due to the presence of these suppressing mutations are likely as the F153S and I155N mutations do not affect hydride-transfer rates upon introduction in wild-type DHFR and alterations in circular dichroism spectra are associated with the double-mutant DHFRs.

Titration of histidine 62 in R67 dihydrofolate reductase is linked to a tetramer<-->two-dimer equilibrium

R67 dihydrofolate reductase (DHFR) is an R-plasmid encoded protein that confers clinical resistance to the antibacterial drug trimethoprim. To determine whether an acidic titration in kinetic pH profiles is related to titration of histidines 62, 162, 262, and 362, the stability of tetrameric R67 DHFR has been monitored as a function of pH. For the pH range 5-8, tetrameric R67 DHFR reversibly dissociates into dimers, as monitored by ultracentrifugation and molecular sieving techniques. From the crystal structures of dimeric and tetrameric R67 DHFR [Matthews et al. (1986) Biochemistry 25, 4194-4204] (Narayana, Matthews, and Xuong, personal communication), symmetry-related histidines 62, 162, 262, and 362 occur at the two dimer-dimer interfaces and protonation of these residues could destabilize tetrameric R67 DHFR. Ionization of these histidines was confirmed by monitoring the chemical shifts of the C2 proton in NMR experiments, and best fits of an incomplete titration curve yield a pKa of 6.77. Since tryptophans 38, 138, 238, and 338 also occur at the dimer-dimer interfaces, fluorescence additionally monitors the tetramer-two dimers equilibrium. When fluorescence was monitored over the pH range 5-8, a protein concentration dependence of fluorescence was observed and global fitting of three titration curves yielded Kd = 9.72 nM and pKa = 6.84 for the linked reactions: [formula: see text] Modification of H62, H162, H262, and H362 by diethyl pyrocarbonate stabilizes dimeric R67 DHFR and causes a 200-600-fold decrease in catalytic efficiency. Decreased catalytic activity in dimeric R67 DHFR is presumably due to loss of the putative single active site pore found in tetrameric R67 DHFR.

Structure and function of alternative proton-relay mutants of dihydrofolate reductase

Using site-specific mutagenesis, we have constructed two mutants of Escherichia coli dihydrofolate reductase (ecDHFR) to investigate further the function of a weakly acidic side chain at position 27 in substrate protonation: Asp27-->Glu (D27E) and Asp27-->Cys (D27C). The crystal structure of D27E ecDHFR in a binary complex with methotrexate shows that the side-chain oxygen atoms of Glu27 are in almost precisely the same location as those of Asp27 in the wild-type enzyme. Kinetic evidence indicates that Glu27 can indeed function efficiently in the proton relay to dihydrofolate. Even though vertebrate DHFRs all have a glutamic acid at the structurally equivalent position, the kinetic properties of Glu27 ecDHFR more closely resemble those of wild-type bacterial DHFRs than of vertebrate DHFRs. The D27C mutation produced an enzyme still capable of relaying a proton to dihydrofolate, but with the intrinsic pKa in its pH-activity profiles shifted upward to values characteristic of the more basic thiolate group. The crystal structure of the binary complex with methotrexate reveals two unexpected features: (1) the Cys27 sulfhydryl group does not point toward the pteridine-binding site, but the side chain of this residue is instead rotated 120 degrees to interact with a tyrosine side chain projecting from a neighboring beta-strand; (2) a bound ethanol molecule occupies a cavity adjacent to methotrexate. Ethanol is a component of the crystallization medium.

Analysis of hydride transfer and cofactor fluorescence decay in mutants of dihydrofolate reductase: possible evidence for participation of enzyme molecular motions in catalysis

A remarkable correlation has been discovered between fluorescence lifetimes of bound NADPH and rates of hydride transfer among mutants of dihydrofolate reductase (DHFR) from Escherichia coli. Rates of hydride transfer from NADPH to dihydrofolate change by a factor of 1,000 for the series of mutant enzymes. Since binding constants for the initial complex between coenzyme and DHFR change by only a factor of 10, the major portion of the change in hydride transfer must be attributed to losses in transition-state stabilization. The time course of fluorescence decay for NADPH bound to DHFR is biphasic. Lifetimes ranging from 0.3 to 0.5 ns are attributed to a solvent-exposed dihydronicotinamide conformation of bound coenzyme which is presumably not active in catalysis, while decay times (tau 2) in the range of 1.3 to 2.3 ns are assigned to a more tightly bound species of NADPH in which dihydronicotinamide is sequestered from solvent. It is this slower component that is of interest. Ternary complexes with three different inhibitors, methotrexate, 5-deazafolate, and trimethoprim, were investigated, along with the holoenzyme complex; 3-acetylNADPH was also investigated. Fluorescence polarization decay, excitation polarization spectra, the temperature variation of fluorescence lifetimes, fluorescence amplitudes, and wavelength of absorbance maxima were measured. We suggest that dynamic quenching or internal conversion promotes decay of the excited state in NADPH-DHFR. When rates of hydride transfer are plotted against the fluorescence lifetime (tau 2) of tightly bound NADPH, an unusual correlation is observed. The fluorescence lifetime becomes longer as the rate of catalysis decreases for most mutants studied. However, the fluorescence lifetime is unchanged for those mutations that principally alter the binding of dihydrofolate while leaving most dihydronicotinamide interactions relatively undisturbed. The data are interpreted in terms of possible dynamic motions of a flexible loop region in DHFR which closes over both substrate and coenzyme binding sites. These motions could lead to faster rates of fluorescence decay in holoenzyme complexes and, when correlated over time, may be involved in other motions which give rise to enhanced rates of catalysis in DHFR.

Investigation of the functional role of tryptophan-22 in Escherichia coli dihydrofolate reductase by site-directed mutagenesis

We have applied site-directed mutagenesis methods to change the conserved tryptophan-22 in the substrate binding site of Escherichia coli dihydrofolate reductase to phenylalanine (W22F) and histidine (W22H). The crystal structure of the W22F mutant in a binary complex with the inhibitor methotrexate has been refined at 1.9-A resolution. The W22F difference Fourier map and least-squares refinement show that structural effects of the mutation are confined to the immediate vicinity of position 22 and include an unanticipated 0.4-A movement of the methionine-20 side chain. A conserved bound water-403, suspected to play a role in the protonation of substrate DHF, has not been displaced by the mutation despite the loss of a hydrogen bond with tryptophan-22. Steady-state kinetics, stopped-flow kinetics, and primary isotope effects indicate that both mutations increase the rate of product tetrahydrofolate release, the rate-limiting step in the case of the wild-type enzyme, while slowing the rate of hydride transfer to the point where it now becomes at least partially rate determining. Steady-state kinetics show that below pH 6.8, kcat is elevated by up to 5-fold in the W22F mutant as compared with the wild-type enzyme, although kcat/Km(dihydrofolate) is lower throughout the observed pH range. For the W22H mutant, both kcat and kcat/Km(dihydrofolate) are substantially lower than the corresponding wild-type values. While both mutations weaken dihydrofolate binding, cofactor NADPH binding is not significantly altered. Fitting of the kinetic pH profiles to a general protonation scheme suggests that the proton affinity of dihydrofolate may be enhanced upon binding to the enzyme. We suggest that the function of tryptophan-22 may be to properly position the side chain of methionine-20 with respect to N5 of the substrate dihydrofolate.

Construction of a synthetic gene for an R-plasmid-encoded dihydrofolate reductase and studies on the role of the N-terminus in the protein

R67 dihydrofolate reductase (DHFR) is a novel protein that provides clinical resistance to the antibacterial drug trimethoprim. The crystal structure of a dimeric form of R67 DHFR indicates the first 16 amino acids are disordered [Matthews et al. (1986) Biochemistry 25, 4194-4204]. To investigate whether these amino acids are necessary for protein function, the first 16 N-terminal residues have been cleaved off by chymotrypsin. The truncated protein is fully active with kcat = 1.3 s-1, Km(NADPH) = 3.0 microM, and Km(dihydrofolate) = 5.8 microM. This result suggests the functional core of the protein resides in the beta-barrel structure defined by residues 27-78. To study this protein further, synthetic genes coding for full-length and truncated R67 DHFRs were constructed. Surprisingly, the gene coding for truncated R67 DHFR does not produce protein in vivo or confer trimethoprim resistance upon Escherichia coli. Therefore, the relative stabilities of native and truncated R67 DHFR were investigated by equilibrium unfolding studies. Unfolding of dimeric native R67 DHFR is protein concentration dependent and can be described by a two-state model involving native dimer and unfolded monomer. Using absorbance, fluorescence, and circular dichroism techniques, an average delta GH2O of 13.9 kcal mol-1 is found for native R67 DHFR. In contrast, an average delta GH2O of 11.3 kcal mol-1 is observed for truncated R67 DHFR. These results indicate native R67 DHFR is 2.6 kcal mol-1 more stable than truncated protein. This stability difference may be part of the reason why protein from the truncated gene is not found in vivo in E. coli.

A second-site mutation at phenylalanine-137 that increases catalytic efficiency in the mutant aspartate-27----serine Escherichia coli dihydrofolate reductase

The adaptability of Escherichia coli dihydrofolate reductase (DHFR) is being explored by identifying second-site mutations that can partially suppress the deleterious effect associated with removal of the active-site proton donor aspartic acid-27. The Asp27----serine mutant DHFR (D27S) was previously characterized and the catalytic activity found to be greatly decreased at pH 7.0 [Howell et al. (1986) Science 231, 1123-1128]. Using resistance to trimethoprim (a DHFR inhibitor) in a genetic selection procedure, we have isolated a double-mutant DHFR gene containing Asp27----Ser and Phe137----Ser mutations (D27S+F137S). The presence of the F137S mutation increases kcat approximately 3-fold and decreases Km(DHF) approximately 2-fold over D27S DHFR values. The overall effect on kcat/Km(DHF) is a 7-fold increase. The D27S+F137S double-mutant DHFR is still 500-fold less active than wild-type DHFR at pH 7. Surprisingly, Phe137 is approximately 15 A from residue 27 in the active site and is part of a beta-bulge. We propose the F137S mutation likely causes its catalytic effect by slightly altering the conformation of D27S DHFR. This supposition is supported by the observation that the F137S mutation does not have the same kinetic effect when introduced into the wild-type and D27S DHFRs, by the altered distribution of two conformers of free enzyme [see Dunn et al. (1990)] and by a preliminary difference Fourier map comparing the D27S and D27S+F137S DHFR crystal structures.

Dihydrofolate reductase from Escherichia coli: probing the role of aspartate-27 and phenylalanine-137 in enzyme conformation and the binding of NADPH

In the absence of ligands, dihydrofolate reductase from Escherichia coli exists in at least two interconvertible conformations, only one of which binds NADPH with high affinity. This equilibrium is pH dependent, involving an ionizable group of the enzyme (pK approximately 5.5), and the proportion of the NADPH-binding conformer increases from 42% at pH 5 to 65% at pH 8. The role of specific amino acids in enzyme conformation has been investigated by studying the kinetics of NADPH binding to three dihydrofolate reductase mutants: (i) a mutant in which Asp-27, a residue that is directly involved in the binding of folates and antifolates but not NADPH, has been replaced by a serine, (ii) a mutant in which Phe-137 on the exterior of the molecule and distant from the binding sites has been replaced by a serine, and (iii) a mutant in which both Asp-27 and Phe-137 have been replaced by serines. Mutation of the Asp-27 residue reduces the affinity for NADPH by approximately 7-fold. Kinetic measurements have suggested that this is due mainly to an increase in the rate of dissociation of the initial complex and a slight shift in the enzyme equilibrium to favor the nonbinding conformation. The pH dependence of the conformer equilibrium is also shifted by approximately one pH unit to higher pH (pK approximately 6.5). In addition, the pH profile suggests the involvement of a second ionizable group having a pK of about 8 since, above pH 7, the proportion of the NADPH-binding form decreases.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of aspartate 27 of dihydrofolate reductase from Escherichia coli in interconversion of active and inactive enzyme conformers and binding of NADPH

The apoenzyme of wild-type (WT) dihydrofolate reductase (DHRF) from Escherichia coli exists in two conformational states, Et and Ew, which differ in affinity for NADPH and in kinetic competence. Dissociation constants for the binary complex of NADPH with the two conformers differ by over 100-fold (KDt = 0.17 microM, KDw = 22 microM). Rate constants governing the interconversion of conformers are small (t1/2 for Ew----Et = 71 s), and since Ew is not catalytically competent, this conversion is accompanied by an increase in catalytic velocity. The equilibrium proportion of Et in the absence of ligands is 63%, but binding of NADPH greatly increases this proportion, and t1/2 for conversion of Ew.NADPH to Et.NADPH is 30 s. This conformational equilibrium has also been examined in mutant enzyme in which aspartate 27 is replaced by asparagine (D27N E. coli DHFR). Although ASp27 is an active site residue, it does not interact directly with bound NADPH, and in the mutant the rate constant for NADPH binding to Et is unchanged as are the dissociation constants for NADPH complexes with Et or Ew. However, for mutant apoenzyme, the proportion of Et is decreased to 18% in the absence of ligands so that the overall KD for NADPH is increased (0.15 microM for WT E. coli DHFR, 0.68 microM for D27N E. coli DHFR). The lower proportion of Et is due to a decreased rate for Ew----Et (t1/2 = 221 s) and an increased rate for Et----Ew (t1/2 = 50 s versus 120 s for WT E. coli DHFR).

Role of aspartate 27 in the binding of methotrexate to dihydrofolate reductase from Escherichia coli

Dihydrofolate reductase from wild-type Escherichia coli (WT-ECDHFR) and from a mutant enzyme in which aspartate 27 is replaced by asparagine have been compared with respect to the binding of the inhibitor methotrexate (MTX). Although the Asp27----Asn substitution causes only small changes in the association rate constants (kon) for the formation of binary and ternary (with NADPH) complexes, the dissociation rate constants for these complexes (koff) are increased for the mutant enzyme by factors of about 5- and 100-fold, respectively, at pH 7.65. In binding experiments, the initial MTX binary and ternary complexes of the mutant enzyme were found to undergo relatively rapid isomerization (kobs approximately 17 and 145 s-1, respectively). Although such rapid isomerization of complexes of WT-ECDHFR could not be detected in binding experiments, evidence of a slow isomerization (k = 4 x 10(-3) s-1) of the ternary WT-ECDHFR.MTX.NADPH complex was obtained from progress of inhibition experiments. This slow isomerization increases binding of MTX to WT-ECDHFR only 2.4-fold (much less than previously estimated). From presently available data, we could not determine the contribution of the rapid isomerization of complexes to the binding of MTX to the mutant enzyme. The Asp27----Asn substitution increases the overall dissociation constant (KD) 9-fold for the binary complex and 85-fold for the ternary complex. When it is also taken into account that a proton ultimately derived from the solvent must be added to MTX bound to the WT enzyme, but not to MTX bound to the mutant enzyme, these increases in KD for the mutant enzyme correspond to decreases in binding energy for MTX of 3.9 and 5.2 kcal/mol at pH 7.65 for the binary and ternary complexes, respectively.

Construction of a dihydrofolate reductase-deficient mutant of Escherichia coli by gene replacement

The dihydrofolate reductase (fol) gene in Escherichia coli has been deleted and replaced by a selectable marker. Verification of the delta fol::kan strain has been accomplished using genetic and biochemical criteria, including Southern analysis of the chromosomal DNA. The delta fol::kan mutation is stable in E. coli K549 [thyA polA12 (Ts)] and can be successfully transduced to other E. coli strains providing they have mutations in their thymidylate synthetase (thyA) genes. A preliminary investigation of the relationship between fol and thyA gene expression suggests that a Fol- cell (i.e., a dihydrofolate reductase deficiency phenotype) is not viable unless thymidylate synthetase activity is concurrently eliminated. This observation indicates that either the nonproductive accumulation of dihydrofolate or the depletion of tetrahydrofolate cofactor pools is lethal in a Fol- ThyA+ strain. Strains containing the thyA delta fol::kan lesions require the presence of Fol end products for growth, and these lesions typically increase the doubling time of the strain by a factor of 2.5 in rich medium.

Construction of an altered proton donation mechanism in Escherichia coli dihydrofolate reductase

We have explored the substrate protonation mechanism of Escherichia coli dihydrofolate reductase by changing the location of the proton donor. A double mutant was constructed in which the proton donor of the wild-type enzyme, aspartic acid-27, has been changed to serine and simultaneously an alternative proton donor, glutamic acid, has replaced threonine at position 113. The active site of the resulting variant enzyme molecule should therefore somewhat resemble that proposed for the R67 plasmid-encoded dihydrofolate reductase [Matthews, D. A., Smith, S. L., Baccanari, D. P., Burchall, J. J., Oatley, S. J., & Kraut, J. (1986) Biochemistry 25, 4194]. At pH 7, the double-mutant enzyme has a 3-fold greater kcat and an unchanged Km(dihydrofolate) as compared with the single-mutant Asp-27----Ser enzyme described previously [Howell, E. E., Villafranca, J. E., Warren, M. S., Oatley, S. J., & Kraut, J. (1986) Science (Washington, D.C.) 231, 1123]. Additionally, its activity vs pH profiles together with observed deuterium isotope effects, suggest that catalysis depends on an acidic group with a pKa of 8. It is concluded that the dihydropteridine ring of a bound substrate molecule can indeed be protonated by a glutamic acid side chain at position 113 (instead of an aspartic acid side chain at position 27), but with greatly decreased efficiency: at pH 7, the double mutant still has a 25-fold lower kcat (1.2 s-1) and a 2900-fold lower kcat/km(dihydrofolate) (8.6 X 10(3) s-1 M-1) than the wild-type enzyme.

An engineered disulfide bond in dihydrofolate reductase

Substitution of cysteine for proline-39 in Escherichia coli dihydrofolate reductase by oligonucleotide-directed mutagenesis positions the new cysteine adjacent to already existing cysteine-85. When the mutant protein is expressed in the E. coli cytosol, the cysteine sulfur atoms are found, by X-ray crystallographic analysis, to be in van der Waals contact but not covalently bonded to one another. In vitro oxidation by dithionitrobenzoate results in formation of a disulfide bond between residues 39 and 85 with a geometry close to that of the commonly observed left-handed spiral. Comparison of 2.0-A-refined crystal structures of the oxidized (cross-linked) and reduced (un-cross-linked) forms of the mutant enzyme shows that the conformation of the enzyme molecule was not appreciably affected by formation of the disulfide bond but that details of the molecule's thermal motion were altered. The disulfide-cross-linked enzyme is at least 1.8 kcal/mol more stable with respect to unfolding, as measured by guanidine hydrochloride denaturation, than either the wild-type or the reduced (un-cross-linked) mutant enzyme. Nevertheless, the cross-linked form is not more resistant to thermal denaturation. Moreover, the appearance of intermediates in the guanidine hydrochloride denaturation profile and urea-gradient polyacrylamide gels indicates that the folding/unfolding pathway of the disulfide-cross-linked enzyme has changed significantly.

Nuclear magnetic resonance study of the state of protonation of inhibitors bound to mutant dihydrofolate reductase lacking the active-site carboxyl

13C nuclear magnetic resonance spectra have been obtained for complexes of [2-13C]methotrexate and [2-13C]trimethoprim with wild-type dihydrofolate reductase (DHFR) from Escherichia coli and with two mutant enzymes in which aspartic acid-27 is replaced by asparagine and by serine, respectively. In both the wild-type and mutated enzymes, exchange between the free inhibitor and the enzyme-complexed inhibitor is slow on the NMR time scale; hence, despite the considerably increased dissociation constants for binary complexes with the enzymes, the dissociation rate remains small relative to the frequency separation of the resonances. In all cases but one, the pKa of an inhibitor that is complexed to enzyme differs greatly from that of the free inhibitor. However, while the pKa of both inhibitors in complexes with the wild-type enzyme is elevated to above 10, the pKa of the inhibitors complexed with the Asn-27 and Ser-27 enzymes is lowered to a value below 4. Exact determinations of bound pKa values are limited by the solubility of the enzyme and the dissociation constants of the complexes. The single exception to these general conclusions is the ternary complex of the Ser-27 DHFR with trimethoprim and NADPH. In this complex, both free and enzyme-complexed trimethoprim exhibit similar pKa values (approximately equal to 7.6). However, both the exchange between free and enzyme-complexed inhibitor and the protonation of the enzyme-complexed inhibitor are slow in the NMR time scale, so that the spectra reveal three resonances corresponding to free inhibitor, to protonated enzyme-complexed inhibitor, and to unprotonated enzyme-complexed inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional role of aspartic acid-27 in dihydrofolate reductase revealed by mutagenesis

The crystal structures and enzymic properties of two mutant dihydrofolate reductases (Escherichia coli) were studied in order to clarify the functional role of an invariant carboxylic acid (aspartic acid at position 27) at the substrate binding site. One mutation, constructed by oligonucleotide-directed mutagenesis, replaces Asp27 with asparagine; the other is a primary-site revertant to Ser27. The only structural perturbations involve two internally bound water molecules. Both mutants have low but readily measurable activity, which increases rapidly with decreasing pH. The mutant enzymes were also characterized with respect to relative folate: dihydrofolate activities and kinetic deuterium isotope effects. It is concluded that Asp27 participates in protonation of the substrate but not in electrostatic stabilization of a positively charged, protonated transition state.

Directed mutagenesis of dihydrofolate reductase

Three mutations of the enzyme dihydrofolate reductase were constructed by oligonucleotide-directed mutagenesis of the cloned Escherichia coli gene. The mutations--at residue 27, aspartic acid replaced with asparagine; at residue 39, proline replaced with cysteine; and at residue 95, glycine replaced with alanine--were designed to answer questions about the relations between molecular structure and function that were raised by the x-ray crystal structures. Properties of the mutant proteins show that Asp-27 is important for catalysis and that perturbation of the local structure at a conserved cis peptide bond following Gly-95 abolishes activity. Substitution of cysteine for proline at residue 39 results in the appearance of new forms of the enzyme that correspond to various oxidation states of the cysteine. One of these forms probably represents a species cross-linked by an intrachain disulfide bridge between the cysteine at position 85 and the new cysteine at position 39.

Comparative inactivation and inhibition of the anomerase and isomerase activities of phosphoglucose isomerase

Several metabolic compounds have been found to be competitive inhibitors of the anomerase activity of phosphoglucose isomerase (EC 5.3.1.9).Ki values for erythrose 4-phosphate, 6-phosphogluconate, and fructose 1,6-bisphosphate for the anomerase reaction are 0.32 muM, 21 muM, and 84 muM respectively at 0 degree and pH 8.2. A significant difference between the fructose 1,6-bisphosphate inhibition constants for both activities was found (Ki(isomerase) = 800 muM and Ki(anomerase) = 140 muM). Also the Km values for both activities were found to be significantly different (Km(isomerase) = 140 muM and Km(anomerase) = 3.6 muM). Attempts to independently alter the anomerase to isomerase activity ratio through protein modification yielded mixed results. While several modifying reagents destroyed the catalytic activities at identical rates, inactivation by iodoacetamide or pyridoxal 5' phosphate sensitized photo-oxidation displayed differential initial effects on the two activities with the anomerase activity being the less affected. These data support the theory that an imidazole residue is catalytically important for isomerization, but less so for anomerization.

Coated tube enzyme immunoassay: factors affecting sensitivity and effects of reversible protein binding to polystyrene

Coated tube enzyme immunoassay using alkaline phosphatase conjugated to rabbit (anti-human IgG) antiserum was studied to determine conditions of maximum sensitivity. The competitive binding assay utilized showed a large increase in sensitivity with immobilized antigen levels below the levels giving rise to the maximum in the coating-antigen dilution series. The effects of reversible antigen binding to the solid phase were investigated by comparison of untreated polystyrene tubes, polystyrene tubes treated with glutaraldehyde and glass tubes activated with an aminosilane. The use of glutaraldehyde treated tubes reduced, and the use of activated glass tubes prevented the time dependent release of immobilized antigen seen with the untreated polystyrene tubes. By comparison of these solid phases, it is shown that reversible antigen immobilized in a competitive binding assay gives rise to poorer conjugate binding (three-fold), and poorer sensitivity (six-fold). A noncompetitive response was found to occur at high free antibody levels and low competing antigen concentrations. This binding behavior is moderated by the minimization of the reversible antigen immobilization.

Liver aldolase anomeric specificity

Stopped-flow kinetic studies of liver aldolase and of mixed liver-muscle aldolase catalyzed reactions of fructose 1,6-bisphosphate (FBP) have been carried out and interpreted by computer simulation. These experiments indicate no utilization or binding of the alpha anomer by the liver enzyme unlike the findings for either the muscle aldolase which binds the alpha anomer nonproductively or the yeast aldolase which catalyzes its cleavage. Both beta-fructose 1,6-bisphosphate and its acyclic keto form may serve as substrates, necessitating the spontaneous anomerization of the alpha anomer before its utilization. Thus, liver aldolase cleaves 100% of the substrate present in the millisecond time scale because of the inability to bind alpha-FBP, allowing rapid spontaneous anomerization. This result fulfills earlier predictions of the differing specificities and substrate binding properties for aldolases from yeast, muscle, and liver.