Selectivity of tryptophan residues in mediating photolysis of disulfide bridges in goat alpha-lactalbumin

Goat alpha-lactalbumin (GLA) contains four tryptophan (Trp) residues and four disulfide bonds. Illumination with near-UV light results in the cleavage of disulfide bridges and in the formation of free thiols. To obtain information about the reaction products, the illuminated protein was carbamidomethylated and digested with trypsin and the peptides were analyzed by mass spectrometry. Peptides containing Cys120Cam, Cys61Cam, or Cys91Cam were detected, as well as two peptides containing a new Cys-Lys cross-link. In one, Cys6 was cross-linked to Lys122, while the cross-link in the second was either a Cys91-Lys79 or Cys73-Lys93 cross-link; however, the exact linkage could not be defined. The results demonstrate photolytic cleavage of the Cys6-Cys120, Cys61-Cys77, and Cys73-Cys91 disulfide bonds. While photolysis of Cys6-Cys120 and Cys73-Cys91 disulfide bonds in GLA has been reported, cleavage of the Cys61-Cys77 disulfide bonds has not been previously detected. To examine the contribution of the individual Trp residues, we constructed the GLA mutants, W26F, W60F, W104F, and W118F, by replacing single Trp residues with phenylalanine (Phe). The substitution of each Trp residue led to less thiol production compared to that for wild-type GLA, showing that each Trp residue in GLA contributed to the photolytic cleavage of disulfide bridges. The specificity was expressed by the nature of the reaction products. No cleavage of the Cys6-Cys120 disulfide bridge was detected when the W26F mutant was illuminated, and no cleavage of the Cys73-Cys91 disulfide bridge was seen following illumination of W26F or W104F. In contrast, Cys61Cam, resulting from the cleavage of the Cys61-Cys77 disulfide bridge, was found following illumination of any of the mutants.     

Influence of Trp mutation on native, intermediate, and transition states of goat alpha-lactalbumin: an equilibrium and kinetic study

Equilibrium circular dichroism and kinetic stopped-flow fluorescence studies on the stability and the folding kinetics of a set of Trp to Phe mutants of goat alpha-lactalbumin (GLA) were used to characterize the native, intermediate, and transition states of these constructs. GLA contains four tryptophan residues, three of which, Trp26, Trp104, and Trp118, are located in the alpha-domain, while the fourth, Trp60, is located in the beta-domain. Trp26, Trp60, and Trp104 are part of a hydrophobic cluster, whereas Trp118 is situated in a more flexible region near the C-terminal end of the protein. In each case, the mutation leads to a reduction in the overall stability, but only for W26F and W60F is an equilibrium intermediate observed in guanidine hydrochloride-induced unfolding experiments. In kinetic refolding experiments, however, for all samples a burst phase is observed, the amplitude of which depends on the specific mutation. Refolding and unfolding kinetics can adequately be described by a sequential three-state mechanism. phi value analysis showed that the local structure around Trp26, Trp60, and Trp104 is formed in the intermediate and in the transition state of the folding reaction, while around Trp118 no persistent native contacts are observed. From these findings, we conclude that, although hydrophobicity is a major driving force for folding, minor steric changes induced by point mutation can considerably influence the overall stability and the folding process of the protein.     

A theoretical DFT investigation of the lysozyme mechanism: computational evidence for a covalent intermediate pathway

To test the occurrence of local particularities during the unfolding of Ca2+-loaded goat alpha-lactalbumin (GLA) we replaced Trp60 and -118, either one or both, by Phe. In contrast with alternative studies, our recombinant alpha-lactalbumins are expressed in Pichia pastoris and do not contain the extra N-terminal methionine. The substitution of Trp60 leads to a reduction of the global stability. The effect of the Trp118Phe substitution on the conformation and stability of the mutant, however, is negligible. Comparison of the fluorescence spectra of these mutants makes clear that Trp60 and -118 are strongly quenched in the native state. They both contribute to the quenching of Trp26 and -104 emission. By the interplay of these quenching effects, the fluorescence intensity changes upon thermal unfolding of the mutants behave very differently. This is the reason for a discrepancy of the apparent transition temperatures derived from the shift of the emission maxima (Tm,Fl lambda) and those derived from DSC (Tm,DSC). However, the transition temperatures derived from fluorescence intensity (Tm,Fl int) and from DSC (Tm,DSC), respectively, are quite similar, and thus, no local rearrangements are observed upon heat-induced unfolding. At room temperature, the occurrence of specific local rearrangements upon GdnHCl-induced denaturation of the different mutants is deduced from the apparent free energies of their transition state obtained from stopped-flow fluorescence measurements. By phi-value analysis it appears that, while the surroundings of Trp118 are exposed in the kinetic transition state, the surroundings of Trp60 remain native.     

Structure and dynamics of the alpha-lactalbumin molten globule: fluorescence studies using proteins containing a single tryptophan residue

The fluorescence properties of three variants of alpha-lactalbumin (alpha-LA) containing a single tryptophan residue were investigated under native, molten globule, and unfolded conditions. These proteins have levels of secondary structure and stability similar to those of the wild type. The fluorescence signal in the native state is dominated by that of W104, with the signal of W60 and W118 significantly quenched by the disulfide bonds in their vicinity. In the molten globule state, the magnitude of the fluorescence signal of W60 and W118 increases, due to the loss of rigid, specific side chain packing. In contrast, the magnitude of the signal of W104 decreases in the molten globule state, perhaps due to the protonation of H107 or quenching by D102 or K108. The solvent accessibilities of individual tryptophan residues were investigated by their fluorescence emission maximum and by acrylamide quenching studies. In the native state, the order of solvent accessibility is as follows: W118 > W60 > W104. This order changes to W60 > W104 > W118 in the molten globule state. Remarkably, the solvent accessibility of W118 in the alpha-LA molten globule is lower than that in the native state. The dynamic properties of the three tryptophan residues were examined by time-resolved fluorescence anisotropy decay studies. The overall rotation of the molecule can be observed in both the native and molten globule states. In the molten globule state, there is an increase in the extent of local backbone fluctuations with respect to the native state. However, the fluctuation is not sufficient to result in complete motional averaging. The three tryptophan residues in the native and molten globule states have different degrees of motional freedom, reflecting the folding pattern and dynamic heterogeneity of these states. Taken together, these studies provide new insight into the structure and dynamics of the alpha-LA molten globule, which serves as a prototype for partially folded proteins.     

Conformation of amyloid fibrils of beta2-microglobulin probed by tryptophan mutagenesis

Beta2-microglobulin (beta2-m), a protein responsible for dialysis-related amyloidosis, adopts an immunoglobulin domain fold in its native state. Although beta2-m has Trp residues at positions 60 and 95, both are located near the surface of the domain. Hence, beta2-m does not have a conserved Trp common to other immunoglobulin domains, which is buried in close proximity to the disulfide bond. To study the structure of amyloid fibrils in relation to their native fold, we prepared a series of Trp mutants. Trp60 and Trp95 were both replaced with Phe, and a single Trp was introduced at various positions. Among various mutants, W39-beta2-m, in which a Trp was introduced at the position corresponding to the conserved Trp, exhibited a remarkable quenching of fluorescence in the native state, as observed for other immunoglobulin domains. An x-ray structural analysis revealed that W39-beta2-m assumes the native fold with Trp39 located in the vicinity of the disulfide bond. Comparison of the fluorescence spectra of various mutants for the native and fibrillar forms indicated that, while the Trp residues introduced in the middle of the beta2-m sequence tend to be buried in the fibrils, those located in the C-terminal region are more exposed. In addition, the fluorescence spectra of fibrils prepared at pH 2.5 and 7.0 revealed a large difference in the fluorescence intensity for W60-beta2-m, implying a major structural difference between them.     

Probing local environments of tryptophan residues in proteins: comparison of 19F nuclear magnetic resonance results with the intrinsic fluorescence of soluble human tissue factor

19F nuclear magnetic resonance (19F NMR) of 5-fluorotryptophan (5F-Trp) and tryptophan (Trp) fluorescence both provide information about local environment and solvent exposure of Trp residues. To compare the information provided by these spectroscopies, the four Trp residues in recombinant soluble human tissue factor (sTF) were replaced with 5F-Trp. 19F NMR assignments for the 5F-Trp residues (14, 25, 45, and 158) were based on comparison of the wild-type protein spectrum with the spectra of three single Trp-to-Phe replacement mutants. Previously we showed from fluorescence and absorption difference spectra of mutant versus wild-type sTF that the side chains of Trpl4 and Trp25 are buried, whereas those of Trp45 and Trp158 are partially exposed to bulk solvent (Hasselbacher et al., Biophys J 1995;69:20-29). 19F NMR paramagnetic broadening and solvent-induced isotope-shift experiments show that position 5 of the indole ring of 5F-Trp158 is exposed, whereas that of 5F-Trp45 is essentially inaccessible. Although 5F-Trp incorporation had no discernable effect on the procoagulant cofactor activity of either the wild-type or mutant proteins, 19F NMR chemical shifts showed that the single-Trp mutations are accompanied by subtle changes in the local environments of 5F-Trp residues residing in the same structural domain.     

Probing the dynamics of a mobile loop above the active site of L1, a metallo-beta-lactamase from Stenotrophomonas maltophilia, via site-directed mutagenesis and stopped-flow fluorescence spectroscopy

A structural feature shared by the metallo-beta-lactamases is a flexible loop of amino acids that extends over their active sites and that has been proposed to move during the catalytic cycle of the enzymes, clamping down on substrate. To probe the movement of this loop (residues 152-164), a site-directed mutant of metallo-beta-lactamase L1 was engineered that contained a Trp residue on the loop to serve as a fluorescent probe. It was necessary first, however, to evaluate the contribution of each native Trp residue to the fluorescence changes observed during the catalytic cycle of wild-type L1. Five site-directed mutants of L1 (W39F, W53F, W204F, W206F, and W269F) were prepared and characterized using metal analyses, CD spectroscopy, steady-state kinetics, stopped-flow fluorescence, and fluorescence titrations. All mutants retained the wild-type tertiary structure and bound Zn(II) at levels comparable with wild type and exhibited only slight (<10-fold) decreases in k(cat) values as compared with wild-type L1 for all substrates tested. Fluorescence studies revealed a single mutant, W39F, to be void of the fluorescence changes observed with wild-type L1 during substrate binding and catalysis. Using W39F as a template, a Trp residue was added to the flexile loop over the active site of L1, to generate the double mutant, W39F/D160W. This double mutant retained all the structural and kinetic characteristics of wild-type L1. Stopped-flow fluorescence and rapid-scanning UV-visible studies revealed the motion of the loop (k(obs) = 27 +/- 2 s(-1)) to be similar to the formation rate of a reaction intermediate (k(obs) = 25 +/- 2 s(-1)).     

Photoexcitation of tryptophan groups induces reduction of two disulfide bonds in goat alpha-lactalbumin

Illumination of goat alpha-lactalbumin (GLA) with 280 or 295 nm light results in tryptophan-mediated photolysis of disulfide bonds within the protein. The photolysis is not dependent on the absence or presence of Ca(2+) and is observed as well on illumination of native and of partially unfolded GLA. However, photolysis of native GLA results in a partial unfolding of the protein. The latter phenomenon is most clearly observed on fluorescence measurements at low temperatures (near 3 degrees C). The photolysis induces some dimerization and oligomerization, but most GLA molecules remain monomeric. To obtain more information about the reaction products, the illuminated protein is treated with iodoacetamide to label the free thiol groups, it is fragmented with trypsin, and the fragments are analyzed by mass spectrometry. Via this approach, we observe that the cleavage of disulfide bonds is restricted to Cys6-Cys120 and Cys73-Cys91 bonds. The photolytic cleavage of either of these disulfide bonds results in the formation of a single free thiol, a phenomenon restricted to Cys120 and Cys91, respectively. We also found indications that a thioether linkage is formed between Cys73 and Trp60. The alkylsulfenylation of Trp60 presumably results from a combination of primary thiyl and tryptyl radicals.     

Site-directed mutagenesis of conserved C-terminal tyrosine and tryptophan residues of PsbO, the photosystem II manganese-stabilizing protein, alters its activity and fluorescence properties

The extrinsic photosystem II PsbO subunit (manganese-stabilizing protein) contains near-UV CD signals from its complement of aromatic amino acid residues (one Trp, eight Tyr, and 13 Phe residues). Acidification, N-bromosuccinimide modification of Trp, reduction or elimination of a disulfide bond, or deletion of C-terminal amino acids abolishes these signals. Site-directed mutations that substitute Phe for Trp241 and Tyr242, near the C-terminus of PsbO, were used to examine the contribution of these residues to the activity and spectral properties of the protein. Although this substitution is, in theory, conservative, neither mutant binds efficiently to PSII, even though these proteins appear to retain wild-type solution structures. Removal of six residues from the N-terminus of the W241F mutant restores activity to near-wild-type levels. The near-UV CD spectra of the mutants are modified; well-defined Tyr and Trp peaks are lost. Characterizations of the fluorescence spectra of the full-length WF and YF mutants indicate that Y242 contributes significantly to PsbO's Tyr fluorescence emission and that an excited-state tyrosinate could be present in PsbO. Deletion of W241 shows that this residue is a major contributor to PsbO's fluorescence emission. Loss of function is consistent with the proposal that a native C-terminal domain is required for PsbO binding and activity, and restoration of activity by deletion of N-terminal amino acids may provide some insights into the evolution of this important photosynthetic protein.     

Effects of five-tryptophan mutations on structure, stability and function of Escherichia coli dihydrofolate reductase

To elucidate the roles of tryptophan residues in the structure, stability, and function of Escherichia coli dihydrofolate reductase (DHFR), its five tryptophan residues were replaced by site-directed mutagenesis with leucine, phenylalanine or valine (W22F, W22L, W30L, W47L, W74F, W74L, W133F, and W133V). Far-ultraviolet circular dichroism (CD) spectra of these mutants reveal that exciton coupling between Trp47 and Trp74 strongly affects the peptide CD of wild-type DHFR, and that Trp133 also contributes appreciably. No additivity was observed in the contributions of individual tryptophan residues to the fluorescence spectrum of wild-type DHFR, Trp74 having a dominant effect. These single-tryptophan mutations induce large changes in the free energy of urea unfolding, which showed values of 1.79-7.14 kcal/mol, compared with the value for wild-type DHFR of 6.08 kcal/mol. Analysis of CD and fluorescence spectra suggests that thermal unfolding involves an intermediate with the native-like secondary structure, the disrupted Trp47-Trp74 exciton coupling, and the solvent-exposed Trp30 and Trp47 side chains. All the mutants except W22L (13%) retain more than 50% of the enzyme activity of wild-type DHFR. These results demonstrate that the five tryptophan residues of DHFR play important roles in its structure and stability but do not crucially affect its enzymatic function.     

Unraveling the mechanisms of tryptophan fluorescence quenching in the triosephosphate isomerase from Giardia lamblia

In the native state several proteins exhibit a quenching of fluorescence of their tryptophans. We studied triosephosphate isomerase from Giardia lamblia (GlTIM) to dissect the mechanisms that account for the quenching of fluorescence of its Trp. GlTIM contains four Trp per monomer (Trp75, Trp162, Trp173, and Trp196) distributed throughout the 3D structure. The fluorescence of the denatured enzyme is 3-fold higher than that of native GlTIM. To ascertain the origin of this phenomenon, single and triple mutants of Trp per Phe were made. The intrinsic fluorescence was determined, and the data were interpreted on the basis of the crystal structure of the enzyme. Our data show that the fluorescence of all Trp residues is quenched through two different mechanisms. In one, fluorescence is quenched by aromatic-aromatic interactions due to the proximity and orientation of the indole groups of Trp196 and Trp162. The magnitude of the quenching of fluorescence in Trp162 is higher than in the other three Trp. Fluorescence quenching is also due to energy transfer to the charged residues that surround Trp 75, 173 and 196. Further analysis of the fluorescence of GlTIM showed that, among TIMs from other parasites, Trp at position 12 exhibits rather unique properties.     

Spectral contribution of the individual tryptophan of alphaB-crystallin: a study by site-directed mutagenesis

There are two tryptophan residues in the lens alphaB-crystallin, Trp9 and Trp60. We prepared two Trp --> Phe substituted mutants, W9F and W60F, for use in a spectroscopic study. The two tryptophan residues contribute to Trp fluorescence and near-ultraviolet circular dichroism (UV CD) differently. The major difference in the near-UV CD is the contribution of 1La of Trp: it is positive in W60F but becomes negative in W9F. Further analysis of the near-UV CD shows an increased intensity in the region of 270-280 nm for W60F, suggesting that the Tyr48 is affected by the W60F mutation. It appears that Trp60 is located in a more rigid environment than Trp9, which agrees with a recent structural model in which Trp60 is in a beta-strand.     

Probing folding and fluorescence quenching in human gammaD crystallin Greek key domains using triple tryptophan mutant proteins

Human gammaD crystallin (HgammaD-Crys), a major component of the human eye lens, is a 173-residue, primarily beta-sheet protein, associated with juvenile and mature-onset cataracts. HgammaD-Crys has four tryptophans, with two in each of the homologous Greek key domains, which are conserved throughout the gamma-crystallin family. HgammaD-Crys exhibits native-state fluorescence quenching, despite the absence of ligands or cofactors. The tryptophan absorption and fluorescence quenching may influence the lens response to ultraviolet light or the protection of the retina from ambient ultraviolet damage. To provide fluorescence reporters for each quadrant of the protein, triple mutants, each containing three tryptophan-to-phenylalanine substitutions and one native tryptophan, have been constructed and expressed. Trp 42-only and Trp 130-only exhibited fluorescence quenching between the native and denatured states typical of globular proteins, whereas Trp 68-only and Trp 156-only retained the anomalous quenching pattern of wild-type HgammaD-Crys. The three-dimensional structure of HgammaD-Crys shows Tyr/Tyr/His aromatic cages surrounding Trp 68 and Trp 156 that may be the source of the native-state quenching. During equilibrium refolding/unfolding at 37 degrees C, the tryptophan fluorescence signals indicated that domain I (W42-only and W68-only) unfolded at lower concentrations of GdnHCl than domain II (W130-only and W156-only). Kinetic analysis of both the unfolding and refolding of the triple-mutant tryptophan proteins identified an intermediate along the HgammaD-Crys folding pathway with domain I unfolded and domain II intact. This species is a candidate for the partially folded intermediate in the in vitro aggregation pathway of HgammaD-Crys.     

Mechanism of the highly efficient quenching of tryptophan fluorescence in human gammaD-crystallin

Quenching of the fluorescence of buried tryptophans (Trps) is an important reporter of protein conformation. Human gammaD-crystallin (HgammaD-Crys) is a very stable eye lens protein that must remain soluble and folded throughout the human lifetime. Aggregation of non-native or covalently damaged HgammaD-Crys is associated with the prevalent eye disease mature-onset cataract. HgammaD-Crys has two homologous beta-sheet domains, each containing a pair of highly conserved buried tryptophans. The overall fluorescence of the Trps is quenched in the native state despite the absence of the metal ligands or cofactors. We report the results of detailed quantitative measurements of the fluorescence emission spectra and the quantum yields of numerous site-directed mutants of HgammaD-Crys. From fluorescence of triple Trp to Phe mutants, the homologous pair Trp68 and Trp156 were found to be extremely quenched, with quantum yields close to 0.01. The homologous pair Trp42 and Trp130 were moderately fluorescent, with quantum yields of 0.13 and 0.17, respectively. In an attempt to identify quenching and/or electrostatically perturbing residues, a set of 17 candidate amino acids around Trp68 and Trp156 were substituted with neutral or hydrophobic residues. None of these mutants showed significant changes in the fluorescence intensity compared to their own background. Hybrid quantum mechanical-molecular mechanical (QM-MM) simulations with the four different excited Trps as electron donors strongly indicate that electron transfer rates to the amide backbone of Trp68 and Trp156 are extremely fast relative to those for Trp42 and Trp130. This is in agreement with the quantum yields measured experimentally and consistent with the absence of a quenching side chain. Efficient electron transfer to the backbone is possible for Trp68 and Trp156 because of the net favorable location of several charged residues and the orientation of nearby waters, which collectively stabilize electron transfer electrostatically. The fluorescence emission spectra of single and double Trp to Phe mutants provide strong evidence for energy transfer from Trp42 to Trp68 in the N-terminal domain and from Trp130 to Trp156 in the C-terminal domain. The backbone conformation of tryptophans in HgammaD-Crys may have evolved in part to enable the lens to become a very effective UV filter, while the efficient quenching provides an in situ mechanism to protect the tryptophans of the crystallins from photochemical degradation.     

Mutagenesis of Trp(54) and Trp(203) residues on Fibrobacter succinogenes 1,3-1,4-beta-D-glucanase significantly affects catalytic activities of the enzyme

The possible structural and catalytic functions of the nine tryptophan amino acid residues, including Trp(54), Trp(105), Trp(112), Trp(141), Trp(148), Trp(165), Trp(186), Trp(198), and Trp(203) in Fibrobacter succinogenes 1,3-1,4-beta-D-glucanase (Fs beta-glucanase), were characterized using site-directed mutagenesis, initial rate kinetics, fluorescence spectrometry, and structural modeling analysis. Kinetic studies showed that a 5-7-fold increase in K(m) value for lichenan was observed for W141F, W141H, and W203R mutant Fs beta-glucanases, and approximately 72-, 56-, 30-, 29.5-, 4.9-, and 4.3-fold decreases in k(cat) relative to that for the wild-type enzyme were observed for the W54F, W54Y, W141H, W203R, W141F, and W148F mutants, respectively. In contrast, W186F and W203F, unlike the other 12 mutants, exhibited a 1.4- and 4.2-fold increase in k(cat), respectively. W165F and W203R were the only two mutants that exhibited a 4-7-fold higher activity relative to the wild-type enzyme after they were incubated at pH 3.0 for 1 h. Fluorescence spectrometry indicated that all of the mutations on the nine tryptophan amino acid residues retained a folding similar to that of the wild-type enzyme. Structural modeling and kinetic studies suggest that Trp(54), Trp(141), Trp(148), and Trp(203) play important roles in maintaining structural integrity in the substrate-binding cleft and the catalytic efficiency of the enzyme.     

Resolution of the fluorescence equilibrium unfolding profile of trp aporepressor using single tryptophan mutants.

Single tryptophan mutants of the trp aporepressor, tryptophan 19-->phenylalanine (W19F) and tryptophan 99-->phenylalanine (W99F), were used in this study to resolve the individual steady-state and time-resolved fluorescence urea unfolding profiles of the two tryptophan residues in this highly intertwined, dimeric protein. The wild-type protein exhibits a large increase in fluorescence intensity and lifetime, as well as a large red shift in the steady-state fluorescence emission spectrum, upon unfolding by urea (Lane, A.N. & Jardetsky, O., 1987, Eur. J. Biochem. 164, 389-396; Gittelman, M.S. & Matthews, C.R., 1990, Biochemistry 29, 7011-7020; Fernando, T. & Royer, C.A., 1992, Biochemistry 31, 6683-6691). Unfolding of the W19F mutant demonstrated that Trp 99 undergoes a large increase in intensity and a red shift upon exposure to solvent. Lifetime studies revealed that the contribution of the dominant 0.5-ns component of this tryptophan tends toward zero with increasing urea, whereas the longer lifetime components increase in importance. This lifting of the quenching of Trp 99 may be due to disruption of the interaction between the two subunits upon denaturation, which abolishes the interaction of Trp 99 on one subunit with the amide quenching group of Asn 32 on the other subunit (Royer, C.A., 1992, Biophys. J. 63, 741-750). On the other hand, Trp 19 is quenched in response to unfolding in the W99F mutant. Exposure to solvent of Trp 19, which is buried at the hydrophobic dimer interface in the native protein, results in a large red shift of the average steady-state emission.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fluorescence of native single-Trp mutants in the lactose permease from Escherichia coli: structural properties and evidence for a substrate-induced conformational change.

Six single-Trp mutants were engineered by individually reintroducing each of the native Trp residues into a functional lactose permease mutant devoid of Trp (Trp-less permease; Menezes ME, Roepe PD, Kaback HR, 1990, Proc Natl Acad Sci USA 87:1638-1642), and fluorescent properties were studied with respect to solvent accessibility, as well as alterations produced by ligand binding. The emission of Trp 33, Trp 78, Trp 171, and Trp 233 is strongly quenched by both acrylamide and iodide, whereas Trp 151 and Trp 10 display a decrease in fluorescence in the presence of acrylamide only and no quenching by iodide. Of the six single-Trp mutants, only Trp 33 exhibits a significant change in fluorescence (ca. 30% enhancement) in the presence of the substrate analog beta,D-galactopyranosyl 1-thio-beta,D-galactopyranoside (TDG). This effect was further characterized by site-directed fluorescent studies with purified single-Cys W33-->C permease labeled with 2-(4'-maleimidylanilino)-naphthalene-6-sulfonic acid (MIANS). Titration of the change in the fluorescence spectrum reveals a 30% enhancement accompanied with a 5-nm blue shift in the emission maximum, and single exponential behavior with an apparent KD of 71 microM. The effect of substrate binding on the rate of MIANS labeling of single-Cys 33 permease was measured in addition to iodide and acrylamide quenching of the MIANS-labeled protein. Complete blockade of labeling is observed in the presence of TDG, as well as a 30% decrease in accessibility to iodide with no change in acrylamide quenching. Overall, the findings are consistent with the proposal (Wu J, Frillingos S, Kaback HR, 1995a, Biochemistry 34:8257-8263) that ligand binding induces a conformational change at the C-terminus of helix I such that Pro 28 and Pro 31, which are on one face, become more accessible to solvent, whereas Trp 33, which is on the opposite face, becomes less accessible to the aqueous phase. The findings regarding accessibility to collisional quenchers are also consistent with the predicted topology of the six native Trp residues in the permease.     

The binding of 14-3-3γ to membranes studied by intrinsic fluorescence spectroscopy

Human 14-3-3 proteins contain two conserved tryptophan residues in each monomer, Trp60 and Trp233 in isoform γ. 14-3-3γ binds to negatively charged membranes and here we show that membrane binding can be monitored by steady-state intrinsic fluorescence spectroscopy. Measurements with W60F and W233F 14-3-3γ mutants revealed that Trp60 is the major contributor to the emission fluorescence, whereas the fluorescence of Trp233, which π-stacks with Tyr184, is quenched. The fluorescence is reduced and red-shifted upon specific binding of a phosphate ligand, and further red-shifted upon binding of 14-3-3γ to the membrane, compatible with solvent exposure of Trp60. Moreover, our results support that membrane binding involves the non-conserved, convex area of 14-3-3γ, and that Trp residues do not intercalate in the bilayer.     

Sugar binding to recombinant wild-type and mutant glucokinase monitored by kinetic measurement and tryptophan fluorescence

Tryptophan fluorescence was used to study GK (glucokinase), an enzyme that plays a prominent role in glucose homoeostasis which, when inactivated or activated by mutations, causes diabetes mellitus or hypoglycaemia in humans. GK has three tryptophan residues, and binding of D-glucose increases their fluorescence. To assess the contribution of individual tryptophan residues to this effect, we generated GST-GK [GK conjugated to GST (glutathione transferase)] and also pure GK with one, two or three of the tryptophan residues of GK replaced with other amino acids (i.e. W99C, W99R, W167A, W167F, W257F, W99R/W167F, W99R/W257F, W167F/W257F and W99R/W167F/W257F). Enzyme kinetics, binding constants for glucose and several other sugars and fluorescence quantum yields (varphi) were determined and compared with those of wild-type GK retaining its three tryptophan residues. Replacement of all three tryptophan residues resulted in an enzyme that retained all characteristic features of GK, thereby demonstrating the unique usefulness of tryptophan fluorescence as an indicator of GK conformation. Curves of glucose binding to wild-type and mutant GK or GST-GK were hyperbolic, whereas catalysis of wild-type and most mutants exhibited co-operativity with D-glucose. Binding studies showed the following order of affinities for the enzyme variants: N-acetyl-D-glucosamine>D-glucose>D-mannose>D-mannoheptulose>2-deoxy-D-glucose>L-glucose. GK activators increased sugar binding of most enzymes, but not of the mutants Y214A/V452A and C252Y. Contributions to the fluorescence increase from Trp(99) and Trp(167) were large compared with that from Trp(257) and are probably based on distinct mechanisms. The average quantum efficiency of tryptophan fluorescence in the basal and glucose-bound state was modified by activating (Y214A/V452A) or inactivating (C213R and C252Y) mutations and was interpreted as a manifestation of distinct conformational states.     

Effects of tryptophan to phenylalanine substitutions on the structure, stability, and enzyme activity of the IIAB(Man) subunit of the mannose transporter of Escherichia coli.

The hydrophilic subunit of the mannose transporter (IIAB(Man)) of Escherichia coli is a homodimer that contains four tryptophans per monomer, three in the N-terminal domain (Trp12, Trp33, and Trp69) and one in the C-terminal domain (Trp182). Single and double Trp-Phe mutants of IIABMan and of the IIA domain were produced. Fluorescence emission studies revealed that Trp33 and Trp12 are the major fluorescence emitters, Trp69 is strongly quenched in the native protein and Trp182 strongly blue shifted, indicative of a hydrophobic environment. Stabilities of the Trp mutants of dimeric IIA(Man) and IIAB(Man) were estimated from midpoints of the GdmHCl-induced unfolding transitions and from the amount of dimers that resisted dissociation by SDS (sodium dodecyl sulfate), respectively. W12F exhibited increased stability, but only 6% of the wild-type phosphotransferase activity, whereas W33F was marginally and W69F significantly destabilized, but fully active. Second site mutations W33F and W69F in the background of the W12F mutation reduced protein stability and suppressed the functional defect of W12F. These results suggest that flexibility is required for the adjustments of protein-protein contacts necessary for the phosphoryltransfer between the phosphorylcarrier protein HPr, IIA(Man), IIB(Man), and the incoming mannose bound to the transmembrane IIC(Man)-IID(Man) complex.