Characterisation of the tryptophan synthase alpha subunit in maize

BACKGROUND

In bacteria, such as Salmonella typhimurium, tryptophan is synthesized from indole-3-glycerole phosphate (IGP) by a tryptophan synthase alphabetabetaalpha heterotetramer. Plants have evolved multiple alpha (TSA) and beta (TSB) homologs, which have probably diverged in biological function and their ability of subunit interaction. There is some evidence for a tryptophan synthase (TS) complex in Arabidopsis. On the other hand maize (Zea mays) expresses the TSA-homologs BX1 and IGL that efficiently cleave IGP, independent of interaction with TSB.

RESULTS

In order to clarify, how tryptophan is synthesized in maize, two TSA homologs, hitherto uncharacterized ZmTSA and ZmTSAlike, were functionally analyzed. ZmTSA is localized in plastids, the major site of tryptophan biosynthesis in plants. It catalyzes the tryptophan synthase alpha-reaction (cleavage of IGP), and forms a tryptophan synthase complex with ZmTSB1 in vitro. The catalytic efficiency of the alpha-reaction is strongly enhanced upon complex formation. A 160 kD tryptophan synthase complex was partially purified from maize leaves and ZmTSA was identified as native alpha-subunit of this complex by mass spectrometry. ZmTSAlike, for which no in vitro activity was detected, is localized in the cytosol. ZmTSAlike, BX1, and IGL were not detectable in the native tryptophan synthase complex in leaves.

CONCLUSION

It was demonstrated in vivo and in vitro that maize forms a tryptophan synthase complex and ZmTSA functions as alpha-subunit in this complex.

Long-range side-chain-main-chain interactions play crucial roles in stabilizing the (betaalpha)8 barrel motif of the alpha subunit of tryptophan synthase

The role of hither-to-fore unrecognized long-range hydrogen bonds between main-chain amide hydrogens and polar side chains on the stability of a well-studied (betaalpha)8, TIM barrel protein, the alpha subunit of tryptophan synthase (alphaTS), was probed by mutational analysis. The F19-D46 and I97-D124 hydrogen bonds link the N terminus of a beta-strand with the C terminus of the succeeding antiparallel alpha-helix, and the A103-D130 hydrogen bond links the N terminus of an alpha-helix with the C terminus of the succeeding antiparallel beta-strand, forming clamps for the respective betaalpha or alphabeta hairpins. The individual replacement of these aspartic acid side chains with alanine leads to what appear to be closely related partially folded structures with significantly reduced far-UV CD ellipticity and thermodynamic stability. Comparisons with the effects of eliminating another main-chain-side-chain hydrogen bond, G26-S33, and two electrostatic side-chain-side-chain hydrogen bonds, D38-H92 and D112-H146, all in the same N-terminal folding unit of alphaTS, demonstrated a unique role for the clamp interactions in stabilizing the native barrel conformation. Because neither the asparagine nor glutamic acid variant at position 46 can completely reproduce the spectroscopic, thermodynamic, or kinetic folding properties of aspartic acid, both size and charge are crucial to its unique role in the clamp hydrogen bond. Kinetic studies suggest that the three clamp hydrogen bonds act in concert to stabilize the transition state leading to the fully folded TIM barrel motif.

A tightly packed hydrophobic cluster directs the formation of an off-pathway sub-millisecond folding intermediate in the alpha subunit of tryptophan synthase, a TIM barrel protein

Protein misfolding is now recognized as playing a crucial role in both normal and pathogenic folding reactions. An interesting example of misfolding at the earliest state of a natural folding reaction is provided by the alpha-subunit of tryptophan synthase, a (beta/alpha)(8) TIM barrel protein. The molecular basis for the formation of this off-pathway misfolded intermediate, I(BP), and a subsequent on-pathway intermediate, I1, was probed by mutational analysis of 20 branched aliphatic side-chains distributed throughout the sequence. The elimination of I(BP) and the substantial destabilization of I1 by replacement of a selective set of the isoleucine, leucine or valine residues (ILV) with alanine in a large ILV cluster external-to-the-barrel and spanning the N and C termini (cluster 2) implies tight-packing at most sites in both intermediates. Differential effects on I(BP) and I1 for replacements in alpha3, beta4 and alpha8 at the boundaries of cluster 2 suggest that their incorporation into I1 but not I(BP) reflects non-native folds at the edges of the crucial (beta/alpha)(1-2)beta(3) core in I(BP). The retention of I(BP) and the smaller and consistent destabilization of both I(BP) and I1 by similar replacements in an internal-to-the-barrel ILV cluster (cluster 1) and a second external-to-the-barrel ILV cluster (cluster 3) imply molten globule-like packing. The tight packing inferred, in part, for I(BP) or for all of I1 in cluster 2, but not in clusters 1 and 3, may reflect the larger size of cluster 2 and/or the enhanced number of isoleucine, leucine and valine self-contacts in and between contiguous elements of secondary structure. Tightly packed ILV-dominated hydrophobic clusters could serve as an important driving force for the earliest events in the folding and misfolding of the TIM barrel and other members of the (beta/alpha)(n) class of proteins.

Conformational changes in the alpha-subunit coupled to binding of the beta 2-subunit of tryptophan synthase from Escherichia coli: crystal structure of the tryptophan synthase alpha-subunit alone

When the tryptophan synthase alpha- and beta(2)-subunits combine to form the alpha(2)beta(2)-complex, the enzymatic activity of each subunit is stimulated by 1-2 orders of magnitude. To elucidate the structural basis of this mutual activation, it is necessary to determine the structures of the alpha- and beta-subunits alone and together with the alpha(2)beta(2)-complex. The crystal structures of the tryptophan synthase alpha(2)beta(2)-complex from Salmonella typhimurium (Stalpha(2)beta(2)-complex) have already been reported. However, the structures of the subunit alone from mesophiles have not yet been determined. The structure of the tryptophan synthase alpha-subunit alone from Escherichia coli (Ecalpha-subunit) was determined by an X-ray crystallographic analysis at 2.3 A, which is the first report on the subunits alone from the mesophiles. The biggest difference between the structures of the Ecalpha-subunit alone and the alpha-subunit in the Stalpha(2)beta(2)-complex (Stalpha-subunit) was as follows. Helix 2' in the Stalpha-subunit, including an active site residue (Asp60), was changed to a flexible loop in the Ecalpha-subunit alone. The conversion of the helix to a loop resulted in the collapse of the correct active site conformation. This region is also an important part for the mutual activation in the Stalpha(2)beta(2)-complex and interaction with the beta-subunit. These results suggest that the formation of helix 2'that is essential for the stimulation of the enzymatic activity of the alpha-subunit is constructed by the induced-fit mode involved in conformational changes upon interaction between the alpha- and beta-subunits. This also confirms the prediction of the conformational changes based on the thermodynamic analysis for the association between the alpha- and beta-subunits.

Biochemical analysis of a thermostable tryptophan synthase from a hyperthermophilic archaeon

Pyridoxal 5'-phosphate-dependent tryptophan synthase catalyzes the last two reactions of tryptophan biosynthesis, and is comprised of two distinct subunits, alpha and beta. TktrpA and TktrpB, which encode the alpha subunit and beta subunit of tryptophan synthase from a hyperthermophilic archaeon, Thermococcus kodakaraensis KOD1, were independently expressed in Escherichia coli and their protein products were purified. Tryptophan synthase complex (Tk-TS complex), obtained by heat treatment of a mixture of the cell-free extracts containing each subunit, was also purified. Gel-filtration chromatography revealed that Tk-TrpA was a monomer (alpha), Tk-TrpB was a dimer (beta2), and Tk-TS complex was a tetramer (alpha2 beta2). The Tk-TS complex catalyzed the overall alphabeta reaction with a specific activity of 110 micromol Trp per micromol active site per min under its optimal conditions (80 degrees C, pH 8.5). Individual activity of the alpha and beta reactions of the Tk-TS complex were 8.5 micromol indole per micromol active site per min (70 degrees C, pH 7.0) and 119 micromol Trp per micromol active site per min (90 degrees C, pH 7.0), respectively. The low activity of the alpha reaction of the Tk-TS complex indicated that turnover of the beta reaction, namely the consumption of indole, was necessary for efficient progression of the alpha reaction. The alpha and beta reaction activities of independently purified Tk-TrpA and Tk-TrpB were 10-fold lower than the respective activities detected from the Tk-TS complex, indicating that during heat treatment, each subunit was necessary for the other to obtain a proper conformation for high enzyme activity. Tk-TrpA showed only trace activities at all temperatures examined (40-85 degrees C). Tk-TrpB also displayed low levels of activity at temperatures below 70 degrees C. However, Tk-TrpB activity increased at temperatures above 70 degrees C, and eventually at 100 degrees C, reached an equivalent level of activity with the beta reaction activity of Tk-TS complex. Taking into account the results of circular dichroism analyses of the three enzymes, a model is proposed which explains the relationship between structure and activity of the alpha and beta subunits with changes in temperature. This is the first report of an archaeal tryptophan synthase, and the first biochemical analysis of a thermostable tryptophan synthase at high temperature.

Identifying the structural boundaries of independent folding domains in the alpha subunit of tryptophan synthase, a beta/alpha barrel protein

Two equilibrium intermediates have previously been observed in the urea denaturation of the alpha subunit of tryptophan synthase (alphaTS) from Escherichia coli, an eight-stranded beta/alpha barrel protein. In the current study, a series of amino-terminal fragments were characterized to probe the elementary folding units that may be in part responsible for this complex behavior. Stop-codon mutagenesis was used to produce eight fragments ranging in size from 105-214 residues and containing incremental elements of secondary structure. Equilibrium studies by circular dichroism indicate that all of these fragments are capable of adopting secondary structure. All except for the shortest fragment fold cooperatively. The addition of the fourth, sixth, and eighth beta-strands leads to distinct increases in structure, cooperativity, and/or stability, suggesting that folding involves the modular assembly of betaalphabeta supersecondary structural elements. One-dimensional NMR titrations at high concentrations of urea, probing the environment around His92, were also performed to test for the presence of residual structure in the fragments. All fragments that contained the first four betaalpha units of structure exhibited a cooperative unfolding transition at high concentrations of urea with significant but reduced stability relative to the full-length protein. These results suggest that the residual structure in alphaTS requires the participation of hydrophobic residues in multiple beta-strands that span the entire sequence.

The hyperthermostable indoleglycerol phosphate synthase from Thermotoga maritima is destabilized by mutational disruption of two solvent-exposed salt bridges

The recombinantly expressed protein indoleglycerol phosphate synthase from the hyperthermophilic bacterium Thermotoga maritima (tIGPS) was purified and characterized with respect to oligomerization state, catalytic properties and thermostability. This enzyme from the biosynthetic pathway of tryptophan is a monomer in solution. In contrast to IGPS from the hyperthermophilic archaeon Sulfolobus solfataricus, tIGPS shows high catalytic activity at room temperature and only weak product inhibition. In order to test the hypothesis that salt bridges in a critical context contribute to the high thermostability of tIGPS, two solvent-exposed salt bridges were selected, based on its three-dimensional structure, for individual disruption by site-directed mutagenesis. The first salt bridge fixes the N terminus to the core of the protein, and the second serves as a clamp between helices alpha1 and alpha8, which are widely separated in sequence but adjacent in the (betaalpha)8-barrel. Kinetics of irreversible heat inactivation reveal that the salt bridge crosslinking helices alpha1 and alpha8 stabilizes tIGPS more strongly than that tethering the N terminus.

Cryo-crystallography of a true substrate, indole-3-glycerol phosphate, bound to a mutant (alphaD60N) tryptophan synthase alpha2beta2 complex reveals the correct orientation of active site alphaGlu49

The reversible cleavage of indole-3-glycerol by the alpha-subunit of tryptophan synthase has been proposed to be catalyzed by alphaGlu49 and alphaAsp60. Although previous x-ray crystallographic structures of the tryptophan synthase alpha2beta2 complex showed an interaction between the carboxylate of alphaAsp60 and the bound inhibitor indole-3-propanol phosphate, the carboxylate of alphaGlu49 was too distant to play its proposed role. To clarify the structural and functional roles of alphaGlu49, we have determined crystal structures of a mutant (alphaD60N) alpha2beta2 complex in the presence and absence of the true substrate, indole-3-glycerol phosphate. The enzyme in the crystal cleaves indole-3-glycerol phosphate very slowly at room temperature but not under cryo-conditions of 95 K. The structure of the complex with the true substrate obtained by cryo-crystallography reveals that indole-3-glycerol phosphate and indole-3-propanol phosphate have similar binding modes but different torsion angles. Most importantly, the side chain of alphaGlu49 interacts with 3-hydroxyl group of indole-3-glycerol phosphate as proposed. The movement of the side chain of alphaGlu49 into an extended conformation upon binding the true substrate provides evidence for an induced fit mechanism. Our results demonstrate how cryo-crystallography and mutagenesis can provide insight into enzyme mechanism.

Multifunctional tryptophan-synthesizing enzyme. The molecular weight of the Euglena gracilis protein is unexpectedly low

After developing a suitable procedure to produce large amounts of Euglena gracilis as well as a reliable protocol to purify the multifunctional tryptophan-synthesizing enzyme derived from it (Schwarz, T., Bartholmes, P., and Kaufmann, M. (1995) Biotechnol. Appl. Biochem. 22, 179-190), we here describe structural and catalytic properties of the multifunctional tryptophan-synthesizing enzyme. The kinetic parameters kcat of all five activities and Km for the main substrates were determined. The relative molecular weight under denaturing conditions as judged by SDS-polyacrylamide gel electrophoresis is 136,000. Cross-linking as well as gel filtration experiments revealed that the enzyme exists as a homodimer. Neither intersubunit disulfide linkages nor glycosylations were detected. On the other hand, the polypeptide chains are blocked N-terminally. Complete tryptic digestion of the protomer, high pressure liquid chromatography separation of the resulting peptides, and N-terminal sequence analysis of homogenous peaks as judged by matrix-assisted laser/desorption ionization time-of-flight mass spectrometry was performed. Depending on the sequenced peptides, alignments to all entries of the SwissProt data base resulted in both strong sequence homologies to known Trp sequences and no similarities at all. Proteolytic digestion under native conditions using endoproteinase Glu-C uncovered one major cleavage site yielding a semistable, N-terminally blocked fragment with a molecular weight of 119,000. In addition, an increase in beta-elimination accompanied by a decrease in beta-replacement activity of the beta-reaction during proteolysis was observed.

Tryptophan synthase from Agrobacterium tumefaciens 8628: isolation and properties

Tryptophan synthase was isolated from a highly virulent strain of Agrobacterium tumefaciens 8628 (octopine type). Separation of tryptophan synthase from thermolabile protease was accomplished using fractionation with polyethylene glycol-6000 followed by ion-exchange chromatography with a pH gradient. Molecular weights of alpha- and beta-subunits are 33 and 51 kD, respectively. The tryptophan synthase is stable at 60 degrees C because of heat-tolerance beta-subunits. After heating the activity of tryptophan synthase increased up to 20 times while temperature-labile proteases lost their activities. Reaction with antibodies showed the presence of four protein bands, one of which was coeluted with nucleic acids during ion-exchange chromatography. It is suggested that the basic tryptophan synthase is encoded by trp genes in a plasmid and its role is to provide the precursor with the prokaryotic pathway of indole-3-acetic acid biosynthesis, which determines the virulence of A. tumefaciens. There is perhaps a cooperation between iaaM, iaaH, and trp genes in the plasmid during plant cell transformation.

On the role of helix 0 of the tryptophan synthetase alpha chain of Escherichia coli

The role of helix 0 of the alpha chain (TrpA) of the tryptophan synthetase alpha2beta2 multi-functional enzyme complex of Escherichia coli was examined by deleting amino-terminal residues 2-6, 2-11, or 2-19 of TrpA. Selected substitutions were also introduced at TrpA positions 2-6. The altered genes encoding these polypeptides were overexpressed from a foreign promoter on a multicopy plasmid and following insertion at their normal chromosomal location. Each deletion polypeptide was functional in vivo. However all appeared to be somewhat more labile and insoluble and less active enzymatically than wild type TrpA. The deletion polypeptides were overproduced and solubilized from cell debris by denaturation and refolding. Several were partially purified and assayed in various reactions in the presence of tryptophan synthetase beta2 (TrpB). The purified TrpADelta2-6 and TrpADelta2-11 deletion polypeptides had low activity in both the indole + serine --> tryptophan reaction and the indoleglycerol phosphate + serine --> tryptophan reaction. Poor activity in each reaction was partly due to reduced association of TrpA with TrpB. The addition of the TrpA ligands, alpha-glycerophosphate or indoleglycerol phosphate, during catalysis of the indole + serine --> tryptophan reaction increased association and activity. These findings suggest that removal of helix 0 of TrpA decreases TrpA-TrpB association as well as the activity of the TrpA active site. Alignment of the TrpA sequences from different species indicates that several lack part or all of helix 0. In some of these polypeptides, extra residues at the carboxyl end may substitute for helix 0.

Unfolding properties of tryptophan-containing alpha-subunits of the Escherichia coli tryptophan synthase

The urea-induced unfolding of the Escherichia coli tryptophan synthase alpha-subunit is examined via fluorescence measurements with tryptophan-containing alpha-subunit mutants, constructed by in vitro mutagenesis. Early unfolding studies with urea and guanidine suggested that the wild type protein unfolded in a two-step process with a stable intermediate composed of a native alpha-1 folding unit (residues 1-188) and a completely unfolded alpha-2 folding unit (residues 189-268). Recently, more detailed spectroscopic and calorimetric data from the Matthews and Yutani groups indicate that such a structure for the intermediates seems unlikely. Previously, we described the introduction of Trp residues as unfolding reporter groups separately into each of the folding domains and showed that these proteins are wild type enzymatically and in their stability to urea. The unfolding behavior of these alpha-subunits, monitored by fluorescence intensity changes at the discrete emission lambda max for each, in both equilibrium and kinetic experiments, suggest that: (a) both folding units commence unfolding simultaneously (near 2 M urea); (b) the larger alpha-1 unit unfolds in a multistep process, initially yielding a partially unfolded intermediate form which subsequently appears to unfold progressively to completion; and (c) the smaller alpha-2 unit unfolds in a single step event. These results are also clearly incompatible with the early proposals on the structure of the intermediate. It is suggested here that the intermediate is heterogeneous, consisting of a stable, partially unfolded form of alpha-1 attached to either a completely folded or completely unfolded form of alpha-2. These results are consistent with and provide an added dimension to the recent description of the proposed structure of the intermediate.

Importance of residues 2-9 in the immunoreactivity, subunit interactions, and activity of the beta 2 subunit of Escherichia coli tryptophan synthase

The epitope recognized by a monoclonal antibody (mAb19) directed against the beta 2 subunit of Escherichia coli tryptophan synthase was found to be carried by residues 2-9 of the beta chain. The affinities of mAb19 for peptides of different lengths containing the 2-9 sequence were close to 0.6 x 10(9) M-1, the affinity of mAb19 for native beta 2. In view of these results, a model is proposed to account for the kinetics of appearance of the epitope during in vitro renaturation of beta 2 (Murry-Brelier, A., and Goldberg, M.E. (1988) Biochemistry 27, 7633-7640). A mutant producing beta chains lacking residues 1-9 (beta delta 1-9) was prepared. The beta delta 1-9 protein was able to fold into a heat stable homodimer resembling wild type beta 2. Isolated beta delta 1-9 had no detectable enzymatic activity. It could bind alpha chains extremely weakly and be slightly activated. In the presence of the 1-9 peptide, the beta delta 1-9 protein could bind alpha chains much more strongly and generate a 50% active enzyme. Thus, although having little role in the overall folding and stability of the protein, the 1-9 sequence of the beta chain appears strongly involved in the alpha-beta interactions and in the enzymatic activity.

Overexpression and purification of the separate tryptophan synthase alpha and beta subunits from Salmonella typhimurium

To obtain high levels of expression of the free alpha and beta subunits of tryptophan synthase from Salmonella typhimurium, we have used two plasmids (pStrpA and pStrpB) that carry the genes encoding the alpha and beta subunits, respectively. The expression of each plasmid in Escherichia coli CB149 results in overproduction of each subunit. We also report new and efficient methods for purifying the individual alpha and beta subunits. Microcrystals of the beta subunit are obtained by addition of polyethylene glycol 8000 and spermine to crude bacterial extracts. This crystallization procedure is similar to methods used previously to grow crystals of the S. typhimurium tryptophan synthase alpha 2 beta 2 complex for X-ray crystallography and to purify this complex by crystallization from bacterial extracts. The results suggest that purification by crystallization may be useful for other overexpressed enzymes and multienzymes complexes. Purification of the alpha subunit utilizes ammonium sulfate fractionation, chromatography on diethylaminoethyl-Sephacel, and high-performance liquid chromatography on a Mono Q column. The purified alpha and beta subunits are more than 95% pure by the criterion of sodium dodecyl sulfate gel electrophoresis. The procedures developed can be applied to the expression and purification of mutant forms of the separate alpha and beta subunits. The purified alpha and beta subunits provide useful materials for studies of subunit association and for investigations of other properties of the separate subunits.

The tryptophan synthase alpha 2 beta 2 complex. Cleavage of a flexible loop in the alpha subunit alters allosteric properties

This study explores the catalytic and allosteric roles of a flexible loop in tryptophan synthase. Trypsin is known to cleave the tryptophan synthase alpha 2 beta 2 complex in an alpha subunit loop at Arg-188. Cleavage yields an active "nicked" alpha 2 beta 2 derivative. The new results provide evidence that the alpha subunit loop serves two important roles: substrate binding and communicating the effects of substrate binding to the beta subunit. A role for the loop in substrate binding is supported by our finding that addition of a substrate analogue of the alpha subunit, alpha-glycerol 3-phosphate, decreases the rate of cleavage by trypsin. An allosteric role for the loop is supported by the finding although the native alpha 2 beta 2 complex is strongly inhibited by alpha-glycerol 3-phosphate, the nicked alpha 2 beta 2 complex is desensitized to this inhibition. The time course of proteolysis in the presence and absence of alpha-glycerol 3-phosphate is followed by sodium dodecyl sulfate-gel electrophoresis and by assays of activity in the presence and absence of alpha-glycerol 3-phosphate. We use spectroscopic measurements of the pyridoxal phosphate-L-tryptophan intermediates at the active site of the beta subunit to determine the affinity of the native and nicked enzymes for L-tryptophan and alpha-glycerol 3-phosphate. Although cleavage alters the equilibrium distribution of intermediates and reduces the affinity for alpha-glycerol 3-phosphate, it has little effect on the affinity for amino acids bound to the beta subunit. We conclude that the loop in the alpha subunit is important for ligand binding and for communicating the effects of ligand binding from the alpha subunit to the beta subunit in the alpha 2 beta 2 complex.

Microspectrophotometric studies on single crystals of the tryptophan synthase alpha 2 beta 2 complex demonstrate formation of enzyme-substrate intermediates

Microspectrophotometry of single crystals of the tryptophan synthase alpha 2 beta 2 complex from Salmonella typhimurium is used to compare the catalytic and regulatory properties of the enzyme in the soluble and crystalline states. Polarized absorption spectra demonstrate that chromophoric intermediates are formed between pyridoxal phosphate at the active site of the beta subunit and added substrates, substrate analogs, and reaction intermediate analogs. Although the crystalline and soluble forms of the enzyme produce some of the same enzyme-substrate intermediates, including Schiff base and quinonoid intermediates, in some cases the equilibrium distribution of these intermediates differs in the two states of the enzyme. Ligands which bind to the active site of the alpha subunit alter the distribution of intermediates formed at the active site of the beta subunit in both the crystalline and soluble states. The three-dimensional structures of the tryptophan synthase alpha 2 beta 2 complex and of a derivative with indole-3-propanol phosphate bound at the active site of the alpha subunit have recently been reported (Hyde, C. C., Ahmed, S. A., Padlan, E. A., Miles, E. W., and Davies, D. R. (1988) J. Biol. Chem. 264, 17857-17871). Our present findings help to establish experimental conditions for selecting defined intermediates for future x-ray crystallographic analysis of the alpha 2 beta 2 complex with ligands bound at the active sites of both alpha and beta subunits. These crystallographic studies should explain how catalysis occurs at the active site of the beta subunit and how the binding of a ligand to one active site affects the binding of a ligand to the other active site which is 25 A away.

Tryptophan synthase alpha subunit glutamic acid 49 is essential for activity. Studies with 19 mutants at position 49

We have obtained a complete set of 20 variants of the alpha subunit of tryptophan synthase of Escherichia coli at position 49 in order to extend our previous studies on the effects of single amino acid replacements at position 49 on structure and function. Thirteen mutant alpha subunits have been newly constructed by site-directed mutagenesis using oligonucleotides. Six mutants were available from previous studies. We find that the wild type and all of the mutant alpha subunits form alpha 2 beta 2 complexes with the beta 2 subunit of tryptophan synthase with similar association constants and similarly stimulate the activity of the beta 2 subunit in the synthesis of L-tryptophan from L-serine and indole. Thus none of the changes at position 49 produces a change in the conformation of the alpha subunit which significantly interferes with normal subunit interaction. However, the 19 mutant alpha 2 beta 2 complexes are completely devoid of activity in reactions normally catalyzed by the active site of the alpha subunit. This is the first time that these several activities have been measured with a series of highly purified alpha subunits altered by mutation at a single site. Our finding that the mutant in which glutamic acid 49 is substituted by aspartic acid is totally devoid of alpha activity is especially significant and is strong evidence that glutamic acid 49 is an essential catalytic base in the reaction catalyzed by the alpha subunit. This result is consistent with the results of previous genetic studies, with evolutionary comparisons using sequence analysis, and with recent results from x-ray crystallography of the alpha 2 beta 2 complex of tryptophan synthase from Salmonella typhimurium.

Microcrystals of tryptophan synthase alpha 2 beta 2 complex from Salmonella typhimurium are catalytically active

An improved and efficient method has been developed for the purification of the tryptophan synthase alpha 2 beta 2 complex (EC 4.2.1.20) from Salmonella typhimurium containing a multicopy plasmid. Microcrystals prepared in 12% poly(ethylene glycol) 8000 containing 2.5 mM spermine are shown by scanning electron microscopy to have the same crystal habit as the larger crystals that are being used for structural analysis by X-ray crystallography. The average dimensions of the crystals are 33 microns (length) X 9 microns (width) X 3 microns (maximum thickness). Our finding that suspensions of microcrystals are active in several reactions catalyzed by the active sites of the alpha and beta 2 subunits demonstrates that both active sites are functional in the crystal and accessible to substrates. Thus the larger crystals being used for X-ray crystallographic studies should form complexes with substrates and analogues at both active sites and should yield functionally relevant structural information. A comparison of the reaction rates of suspensions of microcrystals with those of the soluble enzyme shows that the maximum rate of the crystalline enzyme is 0.8 that of the soluble enzyme in the cleavage of indole-3-glycerol phosphate (alpha reaction), 0.3 that of the soluble enzyme in the synthesis of L-tryptophan by the beta reaction or the coupled alpha beta reaction, and 2.7 that of the soluble enzyme in the serine deaminase reaction. These small differences in rates probably reflect functional differences between the crystalline and soluble enzymes since the reaction rates of the microcrystals are calculated to be virtually free of diffusional limitation under these reaction conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection and identification of intermediates in the reaction of L-serine with Escherichia coli tryptophan synthase via rapid-scanning ultraviolet-visible spectroscopy

Rapid-scanning stopped-flow (RSSF) UV-visible spectroscopy has been used to investigate the UV-visible absorption changes (300-550 nm) that occur in the spectrum of enzyme-bound pyridoxal 5'-phosphate during the reaction of L-serine with the alpha 2 beta 2 and beta 2 forms of Escherichia coli tryptophan synthase. In agreement with previous kinetic studies [Lane, A., & Kirschner, K. (1983) Eur. J. Biochem. 129, 561-570], the reaction with alpha 2 beta 2 was found to occur in three detectable relaxations (1/tau 1 greater than 1/tau 2 greater than 1/tau 3). The RSSF data reveal that during tau 1, the internal aldimine, E(PLP), with lambda max = 412 nm (pH 7.8), undergoes rapid conversion to two transient species, one with lambda max congruent to 420 nm and one with lambda max congruent to 460 nm. These species decay in a biphasic process (1/tau 2, 1/tau 3) to a complicated final spectrum with lambda max congruent to 350 nm and with a broad envelope of absorbance extending out to approximately 525 nm. Analysis of the time-resolved spectra establishes that the spectral changes in tau 2 are nearly identical with the spectral changes in tau 3. Kinetic isotope effects due to substitution of 2H for the alpha-1H of serine were found to increase the amount of the 420-nm transient and to decrease the amount of the species with lambda max congruent to 460 nm. These findings identify the serine Schiff base (the external aldimine) as the 420 nm absorbing, highly fluorescent transient; the species with lambda max congruent to 460 nm is the delocalized carbanion (quinoidal) species derived from abstraction of the alpha proton from the external aldimine. The reaction of L-serine with beta 2 consists of two relaxations (1/tau 1 beta greater than 1/tau 2 beta) and yields a quasi-stable species with lambda max = 420 nm, in good agreement with a previous report [Miles, E. W., Hatanaka, M., & Crawford, I. P. (1968) Biochemistry 7, 2742-2753]. Analysis of the RSSF spectra indicates that the same spectral change occurs in each phase of the reaction. The similarity of the spectral changes that occur in tau 2 and tau 3 of the alpha 2 beta 2 reaction is postulated to originate from the existence of two (slowly) interconverting forms of the enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)

High-pressure dissociation of the beta 2-dimer of tryptophan synthase from Escherichia coli monitored by sucrose gradient centrifugation

The isolated beta 2-dimer of Escherichia coli tryptophan synthase exhibits reversible high-pressure deactivation and hybridization with an equilibrium transition at 690 and 870 bar for the apoenzyme and holoenzyme, respectively. To investigate the hypothetical dissociation mechanism ultracentrifugal analysis has been applied. In a conventional swing-out rotor (r(max) = 16 cm, fill-height 9 cm) a pressure gradient of 1 less than p less than 1840 bar is formed at maximum speed (40 000 rpm). Using a sucrose gradient to stabilize the particle distribution, pressure-dependent alterations of the state of association of oligomeric systems may be determined. In the present experiments ovalbumin (with a molecular mass close to the beta-monomer) has been used as a reference. The radial sedimentation velocity of the beta 2-dimer (in 5-20% sucrose, 10 degrees C) is found to decrease significantly at p approximately equal to 850 bar. From the slopes in an r-r(degrees) vs t plot the limiting values for the particle weight at the meniscus and the bottom of the tube are found to be the beta 2-dimer (M(r) = 85 800) and the beta-monomer (M(r) = 42 900), thus proving pressure-dependent dissociation. Since sucrose stabilizes the native quaternary structure, the beta 2 leads to 2 beta transition is shifted towards higher pressures compared to the dissociation in standard buffer. Conventional quench experiments in high-pressure cells in the presence of 13% (w/v) sucrose confirm the result of the sucrose gradient centrifugation with respect to the critical pressure where deactivation (and dissociation) occur.

The tryptophan synthase alpha 2 beta 2 complex: a comparison of the reactivity of amino groups in the alpha and beta 2 subunits and in the complex by differential labeling studies

The interaction of the alpha and beta 2 subunits of tryptophan synthase of Escherichia coli to form an alpha 2 beta 2 complex has been probed by differential labeling studies. In the first step the separate alpha or beta 2 subunit or the alpha 2 beta 2 complex was labeled by reductive methylation with trace amounts of [3H]HCHO in the presence of NaCNBH3. In the second step the 3H-labeled preparation was fully labeled under denaturing conditions with [14C]HCHO and NaCNBH3. Peptides containing labeled monomethyl or dimethyl amino groups were isolated after thermolytic digestion or after cyanogen bromide treatment. The 3H/14C ratio of each peptide is a measure of the relative reactivity of the amino group or groups in each peptide. The most reactive amino group in the alpha subunit, lysine-109, is strongly shielded from modification in the alpha 2 beta 2 complex. The most reactive amino group in the beta 2 subunit, the amino-terminal threonine, is not shielded from modification in the alpha 2 beta 2 complex.

Fluorescence-quenching studies on a conformational transition within a domain of the beta 2 subunit of Escherichia coli tryptophan synthase

The fluorescence quenching by acrylamide of the single tryptophan residue in the beta 2 subunit of tryptophan synthase from Escherichia coli K12 is studied for different states of the protein: the native apo-enzyme and holo-enzyme, the nicked apo-protein and holo-protein and the isolated proteolytic fragment F1 corresponding to the N-terminal two thirds of beta 2. The quenching constants measured are used to estimate the accessibility of the tryptophan residue in these different forms. The results are discussed in terms of conformational transition within the F1 domain, occurring in the presence of the cofactor, pyridoxal 5'-phosphate, in the native enzyme. The proteolytic cleavage of the native enzyme is shown to render the nicked protein unable to undergo this conformational change.

The accessibility of the active site and conformation states of the beta 2 subunit of tryptophan synthase studied by fluorescence quenching

The rate of quenching of the fluorescence of pyridoxal 5'-phosphate in the active site of the beta 2 subunit of tryptophan synthase from Escherichia coli was measured to estimate the accessibility of the coenzyme to the small molecules iodide and acrylamide. The alpha subunit and the substrate L-serine substantially reduced the quenching rate. For iodide, the order of decreasing quenching was: Schiff's base of N alpha-acetyl-lysine with pyridoxal 5'-phosphate greater than holo beta 2 subunit greater than holo alpha 2 beta 2 complex approximately equal to holo beta 2 subunit + L-serine greater than holo alpha 2 beta 2 complex + L-serine. The coenzyme in the beta 2 subunit is apparently freely accessible to both iodide and acrylamide (kappa approximately equal to 2 X 10(9) M-1 s-1), but the alpha subunit and L-serine decrease the rate by factors of 2-5. Quenching of the fluorescence of the single tryptophan residue of the beta 2 subunit revealed that the apo and holo forms exist in different states, whereas the alpha subunit stabilizes a third conformation. As the alpha subunit binds to the beta 2 subunit, the tryptophan residue, which is within 2.2 nm of the active site of the beta 2 subunit, probably rotates with respect to the plane of the ring of the coenzyme, such that fluorescence energy transfer from tryptophan to pyridoxal phosphate is greatly reduced. The alpha subunit strongly protects the active-site ligand indole propanol phosphate from quenching with acrylamide, consistent with the active site being deep in a cleft in the protein. Iodide induces dissociation of the holo alpha 2 beta 2 complex [E. W. Miles & M. Moriguchi (1977) J. Biol. Chem. 252, 6594-6599]. The effect of iodide on the fluorescence properties of holo alpha 2 beta 2 complex allows us to estimate an upper limit for the dissociation constant for the alpha 2 beta 2 complex of 10(-8) M, in the absence of iodide.

Tryptophan biosynthesis in the marine luminous bacterium Vibrio harveyi

Tryptophan biosynthetic enzyme levels in wild-type Vibrio harveyi and a number of tryptophan auxotrophs of this species were coordinately regulated over a 100-fold range of specific activities. The tryptophan analog indoleacrylic acid evoked substantial derepression of the enzymes in wild-type cells. Even higher enzyme levels were attained in auxotrophs starved for tryptophan, regardless of the location of the block in the pathway. A derepressed mutant selected by resistance to 5-fluorotryptophan was found to have elevated basal levels of trp gene expression; these basal levels were increased only two- to threefold by tryptophan limitation. The taxonomic implications of these and other biochemical results support previous suggestions that the marine luminous bacteria are more closely related to enteric bacteria than to other gram-negative taxa.

Photoaffinity labeling of the indole sites on the Escherichia coli tryptophan synthase alpha-subunit

The alpha subunit of the Escherichia coli tryptophan synthase catalyzes the reversible aldolytic reaction: Indole-3-glycerol phosphate in equilibrium indole + glyceraldehyde 3-phosphate. The use of 5-azidoindole as a photoaffinity label has made the generation of a number of enzyme-substrate complexes possible, each with a given degree of saturation of the two postulated indole sites. When assayed in the reverse reaction (indole-3-glycerol phosphate synthesis), samples of alpha subunit treated at concentrations of 5-azidoindole less than or equal to 2 mM show a progressive 30-40% activation. A gradual inactivation occurs only in samples irradiated at concentrations in excess of 2 mM 5-azidoindole, and this inactivation is complete at 8-10 mM. A quantitatively similar activation occurs in the forward reaction (indole synthesis), however inactivation in this case is incomplete, with complexes treated at 8-12 mM 5-azidoindole retaining 30-40% relative activity in this reaction. When treated alpha subunits were assayed for their abilities to complement the beta 2-subunit in the reactions indole + L-serine leads to L-tryptophan + H2O and indole-3-glycerol phosphate + L-serine leads to L-tryptophan + glyceraldehyde 3-phosphate, quantitatively lesser amounts of activation followed by total inactivation are observed over a similar range of 5-azidoindole concentrations.

Purification and properties of tryptophan synthase from baker's yeast (Saccharomyces cerevisiae)

Tryptophan synthase was purified from baker's yeast. The purified enzyme exhibited one band on polyacrylamide-gel electrophoresis, had no detectable N-terminal amino acid and C-terminal alanine. The amino acid composition was close to that predicted by recent studies on the DNA sequence of the structural gene for the enzyme. Kinetic parameters for the following three activities were measured: indole-serine condensation, indolylglycerol phosphate lyase and the overall reaction of serine with 1-(indol-3-yl)glycerol 3-phosphate. The Km for indole was much lower than suggested by previous investigations, and the value of 11 microM was measured by a fluorimetric assay.

Small-angle X-ray scattering studies of tryptophan synthase from Escherichia coli and its alpha and beta 2 subunits

The alpha and beta 2 subunits of tryptophan synthase were investigated by small-angle X-ray scattering. The molecular parameters are: radius of gyration, alpha: 1.95 nm, beta 2: 3.01 nm; maximum particle diameter, alpha: 5.8 nm, beta 2: 10.5 nm; and hydrated volume, alpha: 60 nm3, beta 2 160 nm3. The shape of the alpha subunit can best be described by a circular cylinder, slightly tapered at one end. An elongated elliptical cylinder with its cross section larger in the middle than at the ends was found to be a model equivalent in scattering to the beta 2 subunits. The alpha 2 beta 2 enzyme complex was found to have a radius of gyration of 4.01 nm, a maximum length of 13.5 nm, and a hydrated volume of 270 nm3. No satisfactory fit of the scattering data was obtainable by mere apposition of the models of the alpha and beta 2 subunits. Two cylinders overlapping laterally fit the experimental data considerably better, suggesting changes in the conformation of the subunits on forming the alpha 2 beta 2 complex.

Subunit interactions of tryptophan synthase from Escherichia coli as revealed by binding studies with pyridoxal phosphate analogues

An improved purification procedure for the alpha 2 beta 2 complex of tryptophan synthase from Escherichia coli has been developed. It consists of DEAE-Sephacel chromatography, followed by hydrophobic chromatography on Sepharose CL 4B, and leads to material with a higher specific activity than reported previously. Inhibition studies, equilibrium dialysis, and spectrophotometric titration were used to study the binding both of pyridoxal phosphate analogues and of bisubstrate analogues. Pyridoxine 5'-phospate and N-phosphopyridoxyl-L-serine bind to the enzyme, but pyridoxamine 5'-phoshate and N-phosphopridoxyl-L-alanine do not. N-Phosphopyridoxyl-L-tryptophan is bound only weakly, although L-tyrptophan binds strongly to the alpha 2 holo beta 2 complex. It is likely that either differences is protonation or in geometry are responsible for the low affinity of the bisubstrate analogues in comparison to that of the external aldimines of either L-serine or L-tyrptophan with pyridoxal 5'-phosphate. As previously found with pyridoxal 5'-phosphate, pyridoxine 5'-phosphate, and N-phosphopryidoxyl-L-serine bind noncooperatively to two identical binding sites in the alpha 2 apo beta 2 complex. The same ligands bind with positive cooperatively to two binding sites in the apo beta 2 subunit. Because the analogues mimic the binding behavior of pyridoxal 5'-phosphate to both proteins, the internal aldimine of pyridoxal 5'-phosphate to the lysine amino group contributes only to the strength of that binding. The nickel apo beta 2 subunit, which is produced by limited proteolysis with trypsin, binds pyroxine 5'-phosphate noncooperatively to two identical sites. Therefore, the loop of polypeptide chain connecting the two autonomous domains of folding must be intact for enzyme activity, for the binding of the alpha subunit, and for cooperative binding of pyridoxine 5'-phosphate.

Affinity chromatography of tryptophan synthase from Escherichia coli. Systematic studies with immobilized tryptophanol phosphate

Inhibition studies and affinity chromatography indicate that derivatives of tryptophanol phosphate are suitable ligands for the affinity chromatography of tryptophan synthase. A phenyl group on the spacer arm strengthens the interaction of immobilized tryptophanol phosphate with the enzyme. The alpha 2 beta 2 complex specifically requires the presence of 0.3--0.5 M phosphate ions for binding. The alpha subunit binds in dilute Tris buffer, but its binding is also enhanced by the presence of phosphate ions. The beta 2 subunit binds unspecifically but strongly to the affinity material and to a variety of other immobilized hydrophobic ligands. Binding studies with suspensions of affinity material show that the alpha subunit interacts rapidly and reversibly. Indoleglycerol phosphate and indolepropanol phosphate release bound alpha 2 beta 2 complex and alpha subunit in a competitive manner, indicating that the interaction occurs biospecifically, i.e. via the active site of alpha subunit. L-Serine is a non-competitive inhibitor of binding. These results are discussed with regard to the composite-active-site hypothesis [T. E. Creighton (1970) Eur. J. Biochem, 13, 1--10]. Both the alpha subunit and the alpha 2 beta 2 complex of tryptophan synthase from Escherichia coli can be obtained with high yields and in homogenous form by absorption to the affinity material from partially purified preparations. Elution is achieved with linear gradients either of indolepropanol phosphate or of indoleglycerol phosphate or, in the case of the complex, of L-serine. At the low concentrations of the complex found in crude extracts of wild-type E. coli cells, the unexpectedly high affinity of the beta 2 subunit for hydrophobic ligands leads to partial dissociation of the complex.

Purification and partial characterization of the B subunit of Serratia marcescens tryptophan synthetase

A trpE mutant of Serratia marcescens (E-7) was isolated, and the multimeric enzyme tryptophan synthetase (EC 4.2.1.20) was purified to homogeneity from derepressed cells. The A and B subunits were resolved, and the B subunit was partially characterized and compared with the Escherichia coli B subunit as part of a comparative evolution study of the trpB cistron of the trp operon in the Enterobacteriaceae. The S. marcescens B subunit is a dimer (beta(2)), and its molecular weight was estimated to be 89,000. The separate subunits (beta monomers) had molecular weights of approximately 43,000. The B subunit required pyridoxal phosphate for catalytic activity and had an apparent K(m) of 9 x 10(-6) M. The N terminus of the B subunit was unavailable for reaction with terminal amine reagents (blocked), whereas carboxypeptidase digestion released a C-terminal isoleucine. Using S. marcescens B antiserum in agar immunodiffusion gave an almost complete reaction of identity between the B subunits of S. marcescens and E. coli. The antiserum was used in microcomplement fixation, allowing for a comparison of the overall antigenic surface structure of the two B subunits. The index of dissimilarity for the heterologous E. coli enzyme compared with the homologous S. marcescens enzyme was 2.4, indicating extensive similarity of the two proteins at their surfaces. Comparative antiserum neutralization of B-subunit enzyme activity showed the E. coli enzyme to cross-react 85% as well as the S. marcescens enzyme. With regard to the biochemical and immunochemical parameters used in this study, the S. marcescens and E. coli B subunits were either identical or very similar. These findings support the idea that the trpB cistron of the trp operon is a relatively conserved gene in the Enterobacteriaceae.

Preparation and characterization of a modified form of beta2 subunit of Escherichia coli tryptophan synthetase suitable for investigating protein folding

Globular proteins often appear to consist of distinct compact "domains," and the assumption is frequently implicitly made that these domains correspond to intermediate structures in the folding process. If this assumption is correct, the polypeptide fragment that builds up a domain should be able to spontaneously fold into its native conformation even when isolated. In an attempt to isolate and study such a fragment, the beta2 subunit of tryptophan synthetase [tryptophan synthase, L-serine hydro-lyase (adding indoleglycerol-phosphate), EC 4.2.1.20] has been subjected to controlled proteolysis. The resulting protein is shown to be a dimer, the protomer of which contains two nonoverlapping polypeptide chains of molecular weights 12,000 and 29,000. Though inactive, the nicked protein is shown to be in a conformation that closely resembles that of the original enzyme, since it still can form an enzyme-bound intermediate of the catalytic reactions. The fluorescence of this intermediate is used to characterize the binding sites for the cofactor (pyridoxal-P) and substrates, which are shown to exist on the nicked protein. The possibility is discussed of using the fragments isolated from the nicked protein to study individual steps of the enzymatic reaction, intracistronic complementation, and the folding process in the normal beta2 subunit.

A dimer of a single polypeptide chain catalyzes the terminal four reactions of the L-tryptophan pathway in Euglena gracilis

In Euglena gracilis the terminal four enzyme activities of the tryptophan biosynthetic pathway were found to be associated with a protein with an estimated molecular weight of 325,000 +/- 20,000. The protein was purified approximately 2,000-fold with relatively proportional recoveries of all four enzyme activities. The purified material was homogeneous by the criteria of analytical disc gel electrophoresis and gel isoelectric focusing. Disc gel electrophoresis after denaturation with sodium dodecyl sulfate gave a single protein band with a molecular weight of 155,000 +/- 5,000. Disc gel electrophoresis in 8 M urea also gave rise to a single protein band. We interpret these results as evidence for a single species of subunit. The pathway in Euglena is the only one known to the present in which the terminal enzyme, tryptophan synthase, is not a separate molecular species.

Cooperative and noncooperative binding of pyridoxal 5'-phosphate to tryptophan synthase from Escherichia coli

An improved purification procedure for the beta2 subunit of tryptophan synthase from from Escherichia coli has led to an essentially pure and stable preparation with a specific enzymatic activity that is 30% higher than the previously reported maximum value. Sedimentation analysis shows that the apo-beta2 subunit is monodisperse and dimeric down to a concentration of 0.02 mg of protein/ml. The binding of pyridoxal 5'-phosphate (pyridoxal-P) to the apo-beta2 subunit and to the alpha2-apo-beta2 complex was studied by equilibrium dialysis and spectroscopic titration. Both the beta2 subunit and the alpha2beta2 complex bind 2 mol of pyridoxal-P with no unspecific binding observable at higher concentrations of pyridoxal-P. The binding of pyridoxal-P to the apo-beta2 subunit is cooperative (Hill coefficient nH = 1.7). The data have been fitted to the Adair equation, yielding the apparent microscopic dissociation constants for the complexes with one and two bound ligand molecules. They differ by a factor of 38, suggesting that the apo- and holo-beta2 subunits have distinct conformations. The binding of pyridoxal-P to the alpha2-apo-beta2 complex is noncooperative with a value of the dissociation constant intermediate between the two values of the beta2 subunit. This finding suggests that the alpha subunit may stabilize a third conformational state of the beta2 subunit.

Purification and properties of the alpha2beta2 complex of tryptophan synthetase of Proteus mirabilis

A procedure is described for the purification of the tryptophan synthetase alpha2beta2 complex from cell extracts of Proteus mirabilis. A 30-fold purification was achieved with an overall yield of about 23% and a specific activity of 1,600. The complex can be dissociated and the subunits isolated in a pure form. The complex can be reconstituted from the isolated subunits to regain the initial activity. The alpha and beta2 subunits of the tryptophan synthetase complex of P. mirabilis are not significantly different from those of Escherichia coli and other enteric bacteria as to their physical properties, amino acid compositions, and enzymic properties. Complementation studies indicate that the alpha subunit of P. mirabilis hybridizes well with the beta2 subunit from E. coli. Similarly, the beta2 subunit of P. mirabilis readily complexes with the alpha subunits from E. coli, Salmonella typhimurium, and Serratia marcescens. The hybrids formed are all effective in catalyzing the conversion of indoleglycerol phosphate plus serine into tryptophan and glyceraldehyde 3-phosphate. However, these hybrids have reduced or no activity in the other reactions, namely, the condensation of indole and serine to form tryptophan or the aldolytic cleavage of indoleglycerol phosphate.

Purification by immunoadsorbtion chromatography of the normal and a mutant form of the B2 subunit of Escherichia coli tryptophan synthase

Columns containing immobilized immunoglogulin G fractions from normal and immunized animals were used to purify the wild-type beta component of Escherichia coli tryptophan synthase, formula alpha2 beta2, and a mutant form of the beta component. The procedure yielded proteins with no detectable contaminants as measured by analytical acrylamide disc gel electrophoresis and immunodiffusion against proteins subject to sodium dodecylsulfate acrylamide electrophoresis. After elution from the antibody column at pH 11 the normal beta component, by dialysis against pyridoxal phosphate at pH 7.5, could be restored to the enzymatically-active beta2 dimer with a specific activity of 1700 enzyme units/mg protein. This compares with reported values of 2500-3000 enzyme units/mg when the beta2 dimer is purified by conventional means.

The tryptophan synthase from Escherichia coli. An improved purification procedure for the alpha-subunit and binding studies with substrate analogues

An improved method is described for the purification of the alpha-subunit of tryptophan synthase from Escherichia coli. The standard manganese chloride and acid-precipitation steps have been replaced by rapid and efficient chromatographic procedures. Indoleethanol phosphate, indoleprapanol phosphate and indolebutanol phosphate have been synthesized. They are not cleaved by tryptophan synthase and are strictly competitive inhibitors versus indoleglycerol phosphate. The inhibition constant decreases as the number of methylene groups in the side chain increases. This may reflect an improved accommodation of the indole and phosphate moienerated by binding indole, indoleglycerol phosphate and indolepropanol phosphate to the alpha-subunit are very similar. This reflects the transfer of the indole moiety to an hydrophobic environment within the active center. The binding of indolepropanol phosphate to the alpha2beta2-complex perturbs the spectrum of pyridoxal 5'-phosphate located in the beta2-subunit. This demonstrates direct or indirect interactions between the component active sites. Bind studies by spectrophotometric titration and equilibrium dialysis with indolepropanol [32P]phosphate show that there is only one binding site per equivalent of alpha-subunit. Complex formation with the beta2-subunit increases the affinity of the alpha-subunit for indolepropanol phosphate, It is a general consequence of protein-protein interaction in this system.

Tryptophan synthetase alpha(5.7-S): novel molecular species formed within Escherichia coli

A novel molecular species contributes about 5% of the total tryptophan synthetase of Escherichia coli derepressed for the trp operon enzymes. The new species is identified under conditions in which the dissociation of the two nonidentical subunits of the tryptophan synthetase complex is favored. The new species sediments at 5.7S, catalyzes the conversion of indole-3-glycerol phosphate to indole, and has been designated alpha(5.7-S). Although alpha(5.7-S) is not observed in extracts of trpA or trpB mutant strains deficient in the ability to form tryptophan synthetase alpha or beta2 subunits, respectively, a mixture of the two extracts allows the formation of alpha(5.7-S). Similar results are obtained when a homogeneous alpha protein is mixed with an extract of a trpA mutant strain, suggesting that the interaction of alpha and beta2 proteins is obligatory for alpha(5.7-S) formation. One can obtain a beta2 protein preparation that when mixed with a pure alpha protein gives no alpha(5.7-S). Therefore, the interaction of alpha and beta2 proteins alone is not sufficient for the formation of alpha(5.7-S). When a mixture of alpha and beta2 proteins devoid of alpha(5.7-S) is added to extracts of trp deletion mutants, the novel species can be reconstituted in vitro only when deletions are used that carry at least the operator-proximal part of the trpB gene. Therefore, it is concluded that the alpha(5.7-S) species of tryptophan synthetase results from the interaction of the alpha protein, the beta2 protein, and a third component, beta', specified by the deoxyribonucleic acid defined by the end points of two trp deletion mutants.

Tryptophan operon read-through. Isolation and characterization of an abnormally long tryptophan synthetase alpha subunit from a frame-shift mutant of Escherichia coli

A new mutant strain of Escherichia coli, strain ICR-47, contains a frame-shift mutation in the trpA gene, the gene most distal to the operator in the trp operon. Mapping experiments indicate that the lesion is located at a site within 10 to 15% of the end of this gene. The mutation results in "out-of-phase" translation of the distal portion of the trp mRNA; normal translational termination signal(s) are not encountered and a trpA gene product longer than the wild type protein is produced. As with the other enzymes produced from this operon, the in vivo level of the altered protein (the alpha subunit of the tryptophan synthetase enzyme complex) is controlled by exogenous L-tryptophan. The altered alpha subunit from the strain ICR-47 has been isolated and characterized. Molecular weight estimations indicate a molecular weight of approximately 37,000, an increase beyond the wild type enzyme corresponding to an additional 50 to 70 amino acid residues. The protein has a new COOH-terminal amino acid sequence. Results of preliminary hybridization experiments suggest that the ICR-47 mRNA, which is necessarily longer than that needed to code for wild type enzyme, is not detectably different in size from wild type mRNA. The enzymatic properties of the ICR-47 alpha subunit indicates a greatly reduced ability of the mutant subunit to combine functionally with wild type beta2 subunit, the second protein component in the tryptophan synthetase enzyme complex. In contrast, only 40 to 50% of the intrinsic enzymatic activity of the alpha subunit is lost.

Complementation in vitro between mutationally altered beta2 subunits of Escherichia coli tryptophan synthetase

Cross-reacting beta(2) subunits (CRMs) were purified from eight trpB missense mutants to test for complementation in vitro after urea dissociation and reaggregation. One CRM (B290, demonstrating "repairability," i.e., the appearance of enzymatic activity on combination with alpha subunits) was clearly positive with four others, all "non-repairable" CRMs resulting from mutations at three different but neighboring sites. One complementing pair, B290-B248, was studied in more detail and found, upon mixing purified proteins, to give complementation in the absence of denaturants. Complementation activity was low in each case. To study the mechanism of the modest increases in activity, we used a reduced beta(2) subunit as an artificial CRM to form hybrids where both the amount of activity due to complementation and the amount of hybrid could be measured. (In a reduced beta(2) subunit, the two pyridoxal phosphate cofactors have been chemically reduced by sodium borohydride and are covalently attached to lysine residues. This abolishes activity in the tryptophan synthetic reaction and causes the protein to migrate much faster than normal in acrylamide gel electrophoresis.) Reduced beta(2) subunit formed hybrid dimers with the non-repairable CRMs B244 and B248 at pH 6.0, but no enzymatic activity appeared. On the other hand, when reduced beta(2) subunit was mixed with B290 CRM at pH 6.0 to 6.6, an activity increase was seen that was proportional to the amount of hybrid. We conclude that hybrid formation is essential for complementation and that the mechanism of complementation in this system is the correction of a repairable active site on the B290 beta chain by a conformational change occuring when hybrid dimer is formed. This type of complementation must be restricted to a small class of CRMs having a conformationally deformed active site. From the amount of hybrid present and the increase in activity, a specific activity of 50 U/mg was calculated for the hybrid containing reduced and B290 beta chains. This value is slightly less than but close to the activity of the hybrid formed between reduced and normal beta chains, shown earlier to have half the specific activity of the normal dimer.