Tryptophan-containing alpha-subunits of the Escherichia coli tryptophan synthase. Enzymatic and urea stability properties

Fluorescence and folding properties of Tyr mutant tryptophan synthase alpha-subunits from Escherichia coli

The fluorescence of tyrosine has been used to monitor a folding process of tryptophan synthase alpha-subunit from Escherichia coli, because this protein has 7 tyrosines, but not tryptophan. Here to assess the contribution of each Tyr to fluorescence properties of this protein during folding, mutant proteins in which Tyr was replaced with Phe were analyzed. The result shows that a change of Tyr fluorescence occurring during folding of this protein is contributed to approximately 40% each by Tyr(4) and Tyr(115), and to the remaining approximately 20% by Tyr(173) and Tyr(175). Y173F and Y175F mutant proteins showed an increase in their fluorescence intensity by approximately 40% and approximately 10%, respectively. These increases appear to be due to multiple effects of increased hydrophobicity, quenching effect of nearby residue Glu(49), and/or energy transfer between Tyrs. Two data for Y173F alpha-subunit of urea-induced unfolding equilibrium monitored by UV and fluorescence were different. This result, together with ANS binding and far UV CD, shows that folding intermediate(s) of Y173F alpha-subunit, contrary to that of wild-type, may contain self-inconsistent properties such as more buried hydrophobicity, highly quenched fluorescence, and different dependencies on urea of UV absorbance, suggesting an ensemble of heterogeneous structures.

Homodimers of mutant tryptophan synthase alpha-subunits in Escherichia coli

Tryptophan synthase alpha-subunit from Escherichia coli functionally exists as a heterotetramer of alpha(2)beta(2) with beta-subunit. While wild-type and mutant (F139W, T24M/F139W, and T24L/F139W) alpha-subunits were expressed as a monomer from recombinant plasmids in Escherichia coli, T24A/F139W, T24S/F139W, and T24K/F139W mutant alpha-subunits were abnormally expressed as soluble homodimers in addition to monomers. Monomers of dimer-forming mutant alpha-subunits retain high affinity to beta-subunit, high activity in stimulating catalytic activities of beta-subunit, and nearly intact content of secondary structure, indicating that the global structures of these monomers are identical to that of F139W alpha-subunit. However, fluorescence spectra of Trp139 and ANS binding indicate that significant perturbations occur in the mutant proteins. Interestingly, these defective properties of monomers caused by residue replacement were partially repaired by the dimer formation. As a result, it is suggested that dimers may be formed by domain or loop swapping, and that residue 24 may play important role in maintaining on-pathway of alpha-subunit folding.

Unfolding/folding studies on cobrotoxin from Taiwan cobra venom: pH and GSH/GSSG govern disulfide isomerization at the C-terminus

Refolding of cobrotoxin was assessed by the exposure degree of its single Trp determined by an acrylamide quenching study. The change in the accessibility of Trp for acrylamide quantitatively reflected the formation of folded cobrotoxin, and the data were confirmed by HPLC and gel electrophoresis analyses. However, the site-specific information provided by quenching Trp fluorescence revealed that the ordered structure in the neighborhood of Trp was attained prior to the complete formation of the tertiary structure of cobrotoxin. HPLC analyses showed that, in addition to refolded cobrotoxin, two novel species (cobrotoxin II and cobrotoxin III) with isomerization of disulfide bonds at the C-terminus of the toxin molecule were produced along the folding reaction. The disulfide pairings in cobrotoxin II and cobrotoxin III were Cys43-Cys55 and Cys54-Cys60 and Cys43-Cys60 and Cys54-Cys55, respectively. Among the three possible two-disulfide species at the C-terminus, the disulfide linkages Cys43-Cys60 and Cys54-Cys55 of cobrotoxin III caused a marked decrease in lethality and resulted in a conformation which was notably different from that observed with the native toxin molecule as evidenced by CD spectra. The refolding reaction was accelerated by the addition of GSH/GSSG, and the resulting products were mostly folded cobrotoxin. However, if GSH/GSSG was not added into the initial folding materials, the yields of cobrotoxin II and cobrotoxin III greatly increased. The conversion of cobrotoxin to its isomers was to be irreversible and pH-dependent: the higher the pH, the faster the rate of conversion. However, this conversion could be partly inhibited by GSH/GSSG. Cobrotoxin II and cobrotoxin III were purified from Taiwan cobra venom as well, and their yields in comparison to that of cobrotoxin in venom were similar to that noted with the folded products in the presence of GSH/GSSG. Moreover, the rate of disulfide isomerization was expected to be slow in venom fluid in which the pH was approximately pH 6.2. Thus, the finding that cobrotoxin represents the predominant neurotoxin species in Taiwan cobra venom is probably associated with the synergistic effects of GSH/GSSG and pH.

Reversible unfolding of sheep liver tetrameric serine hydroxymethyltransferase

Equilibrium unfolding studies of the tetrameric serine hydroxymethyltransferase from sheep liver (SHMT, E.C.2.1.2.1) revealed that the holoenzyme, apoenzyme and the sodium borohydride-reduced holoenzyme had random coil structures in 8 M urea. In the presence of a non-ionic detergent, Brij-35, and polyethylene glycol, the 8 M urea unfolded protein could be completely (> 95%) refolded by a 20-fold dilution. The refolded enzyme was completely active and kinetically similar to the native enzyme. The midpoint of inactivation of the enzyme occurred at a urea concentration that was much below the urea concentration required to bring about a substantial loss of secondary structure. This observation suggested the occurrence of a 'predenaturation transition' in the unfolding pathway. The equilibrium urea-induced denaturation curve of holoSHMT showed two transitions. The midpoint of the first transition was 1.2 M, which was comparable to that required for 50% decrease in enzyme activity. Further, 50% release of the pyridoxal-5'-phosphate (PLP) from the active site, as monitored by decrease in absorbance at 425 nm, also occurred at about 1.2 M urea. Size exclusion chromatography showed that the tetrameric SHMT unfolds via the intermediate formation of dimers. This dissociation occurred at a much lower urea concentration (0.15 M) in the unfolding of the apoenzyme, and at a higher urea concentration (1.2 M) in the unfolding of holoenzyme, thereby demonstrating the involvement of PLP in stabilizing the quaternary structure of the enzyme. Size exclusion chromatography of the refolding intermediates demonstrated that the cofactor shifts the equilibrium towards the formation of the active tetramer. The reduced holoenzyme could also be refolded to its native structure, as observed by fluorescence and CD measurements, indicating that the presence of covalently linked PLP does not affect refolding. The results demonstrate clearly that the dimer is an intermediate in the urea-induced equilibrium unfolding/refolding of sheep liver SHMT; and PLP, in addition to its role in catalysis, is required for the stabilization of the tetrameric structure of the enzyme.

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.