Biomedical subjects

P M Horowitz

Publications and source records attributed to P M Horowitz.

At least 91 records · Page 5Linked to original sources

PubMed1992

Chaperonin cpn60 from Escherichia coli protects the mitochondrial enzyme rhodanese against heat inactivation and supports folding at elevated temperatures.

The chaperonin protein cpn60 from Escherichia coli protects the monomeric, mitochondrial enzyme rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) against heat inactivation. The thermal inactivation of rhodanese was studied for four different states of the enzyme: native, refolded, bound to cpn60 in the form of a binary complex formed from unfolded rhodanese, and a thermally perturbed state. Thermal stabilization is observed in a range of temperatures from 25 to 48 degrees C. Rhodanese that had been inactivated by incubation at 48 degrees C, in the presence of cpn60 can be reactivated at 25 degrees C, upon addition of cpn10, K+, and MgATP. A recovery of about 80% was achieved after 1 h of the addition of those components. Thus, the enzyme is protected against heat inactivation and kept in a reactivable form if inactivation is attempted using the binary complex formed between rhodanese folding intermediate(s) and cpn60. The chaperonin-assisted refolding of urea-denatured rhodanese is dependent on the temperature of the refolding reaction. However, optimal chaperonin assisted refolding of rhodanese observed at 25 degrees C, which is achieved upon addition of cpn10 and ATP to the cpn60-rhodanese complex, is independent of the temperature of preincubation of the complex, that was formed previously at low temperature. The results are in agreement with a model in which the chaperonin cpn60 interacts with partly folded intermediates by forming a binary complex which is stable to elevated temperatures. In addition, it appears that native rhodanese can be thermally perturbed to produce a state different from that achieved by denaturation that can interact with cpn60.

Bacterial Proteins↗

PubMed1992

Recombinant bovine rhodanese: purification and comparison with bovine liver rhodanese.

Recombinant bovine rhodanese (thiosulfate: cyanide sulfurtransferase, EC 2.8.1.1) has been purified to homogeneity from Escherichia coli BL21(DE3) by cation-exchange chromatography. Recombinant and bovine liver rhodanese coelectrophorese under denaturing conditions, with an apparent subunit molecular weight of 33,000. The amino terminal seven residues of the recombinant protein are identical to those of the bovine enzyme, indicating that E. coli also removes the N-terminal methionine. The Km for thiosulfate is the same for the two proteins. The specific activity of the recombinant enzyme is 12% higher (816 IU/mg) than that of the bovine enzyme (730 IU/mg). The two proteins are indistinguishable as to their ultraviolet absorbance and their intrinsic fluorescence. The ability of the two proteins to refold from 8 M urea to enzymatically active species was similar both for unassisted refolding, and when folding was assisted either by the detergent, lauryl maltoside or by the E. coli chaperonin system composed of cpn60 and cpn10. Bovine rhodanese is known to have multiple electrophoretic forms under native conditions. In contrast, the recombinant protein has only one form, which comigrates with the least negatively charged of the bovine liver isoforms. This is consistent with the retention of the carboxy terminal residues in the recombinant protein that are frequently removed from the bovine liver protein.

Animals↗

PubMed1992

Micelle-assisted protein folding. Denatured rhodanese binding to cardiolipin-containing lauryl maltoside micelles results in slower refolding kinetics but greater enzyme reactivation.

Unfolded (inactive) rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) can be reactivated in the presence of detergents, e.g. lauryl maltoside (LM). Here, we report the reactivation of urea-unfolded rhodanese in the presence of mixed micelles containing LM and the anionic mitochondrial phospholipid, cardiolipin (CL). Reactivation times increased as the number of CL molecules/micelle was increased. A maximum of 94% of the activity was recovered at 2.2 CL/micelle. Only 71% of the activity was recovered in the absence of CL. The major zwitterionic mitochondrial phospholipid, phosphatidylcholine (PC), had no effect on the LM-assisted reactivation of rhodanese. Size exclusion chromatography showed that denatured, but not native, rhodanese apparently binds to micellar amounts of LM and CL/LM, but not to PC/LM micelles. The lifetime of the enzyme-micelle complex increased with the number of CL molecules/micelle. Furthermore, chromatographic fractions containing micelle-bound enzyme had no activity, while renatured rhodanese-containing fractions were active. These results suggest that transient complexes form between enzyme and both LM and CL/LM micelles, and that this complex formation may be necessary for reactivation. For CL/LM micelles, interactions may occur between the positively charged amino-terminal sequence of rhodanese and the negatively charged CL phosphate. Finally, this work shows that there are similarities between "micelle-assisted" and chaperonin-assisted rhodanese refolding.

Animals↗

PubMed1992

Sulfhydryl groups on yeast ribosomal proteins L7 and L26 are significantly more reactive in the 80 S particles than in the 60 S subunits.

The sulfhydryl-directed fluorescent reagent, 5-iodoacetamidofluorescein (IAF), reacts differently with proteins from the 60 S ribosomal subunit of Saccharomyces cerevisiae when this subunit is free as opposed to being contained within the 80 S ribosome. When the 80 S ribosomes and the free 60 S subunits were labeled with IAF, the specific fluorescence intensity (fluorescence intensity unit/A260 60 S subunit) of the subsequently derived 60 S was 16.3 and 5.4, respectively. Gel analysis showed that proteins L7 and L26 were selectively labeled and contained greater than 90% of the total fluorescent label, when 80 S ribosomes were labeled. When free 60 S subunits were labeled, six additional proteins were labeled. Both types of modified 60 S subunits were equally capable to support protein synthesis in vitro. Reassociation of the IAF-labeled derived and free 60 S subunits with unmodified 40 S subunits resulted in a maximum of 5-7% decrease and a 3-fold increase, respectively, in the fluorescence intensity without a shift in the emission maxima. The data suggest that ribosomal proteins L7 and L26 contain SH groups that respond to ribosomal subunit association and become more reactive in the intact ribosome than in the subunit. The environments of some or all of the additionally labeled proteins are also sensitive to subunit reassociation.

Electrophoresis, Gel, Two-Dimensional↗

PubMed1992

Sulfhydryl modification of E. coli Cpn60 leads to loss of its ability to support refolding of rhodanese but not to form a binary complex.

Differential chemical modification of E. coli chaperonin 60 (cpn60) was achieved by using one of several sulfhydryl-directed reagents. For native cpn60, the three cysteines were accessible for reaction with N-ethylmaleimide (NEM), while only two of them are accessible to the larger reagent 4,4'-dipyridyl disulfide (4-PDS). However, no sulfhydryl groups were modified when the even larger reagents 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) or 2-(4'-(iodoacetamido)anilino) naphthalene-6-sulfonic acid (IAANS), were employed, unless the chaperonin was unfolded. The cpn60 that had been covalently modified with NEM or IAANS, was not able to support the chaperonin-assisted refolding of the mitochondrial enzyme rhodanese, which also requires cpn10 and ATP hydrolysis. However, both modified forms of cpn60 were able to form binary complexes with rhodanese, as demonstrated by their ability to arrest the spontaneous refolding of the enzyme. That is, chemical modification with these sulfhydryl-directed reagents produced a species that was not prevented from interaction with partially folded rhodanese, but that was prevented from supporting a subsequent step(s) during the chaperonin-assisted refolding process.

Bacterial Proteins↗

PubMed1992

Immunological evidence for a conformational difference between recombinant bovine rhodanese and rhodanese purified from bovine liver.

Rhodanese has been utilized as a model enzyme for the study of protein structure-function relationships. The enzyme has recently been cloned and the recombinant enzyme is now available for investigation. However, prior to use in structure-function studies, the recombinant enzyme must be shown to have the same structure and activity as the bovine liver enzyme used in the previous studies. An immunological study of the conformations of these enzyme conformers is described. Three antibodies (two monoclonal and one polyclonal, site-directed antibody) were shown to detect distinct and nonoverlapping epitopes. The epitopes of the monoclonal antirhodanese antibodies (R207 and MAB11) were mapped to the same CNBr digest fragment of the amino terminal domain of rhodanese, and the epitope of the site-directed antibody prepared against the interdomain tether sequence of rhodanese (PAT-T1) was mapped to that region of rhodanese (residues 142-156). The rhodanese conformers were studied by monitoring the accessibility of the epitopes recognized by each antibody in each conformer using an indirect ELISA. None of the antibodies could detect its epitope on the purified liver enzyme. Two of the antibodies (R207 and PAT-T1) could also not detect their epitopes on the recombinant enzyme. However, MAB11 did detect a conformational difference between the natural and recombinant rhodanese conformers, indicating the conformational difference is localized in the first 73 amino acids of rhodanese. This difference presumably reflects the difference in the histories of the two enzymes and may be due to differences in enzyme folding, differences in the purification procedures, and differences in storage conditions--all of which could influence the final conformation of the enzyme.

Amino Acid Sequence↗

PubMed1992

Yeast 5S rRNA binding to ribosomal protein L1a alters the fluorescence of tryptophan residues lying outside the binding site.

The yeast ribosomal protein L1a contains two tryptophan residues located at positions 95 and 183. Spectrofluorometric analysis showed that the average tryptophan environment is moderately polar. Quenching studies of the yeast 5S rRNA-L1a protein complex (RNP) with acrylamide and iodide revealed tryptophan heterogeneity. The two tryptophan residues are located in the non-RNA-binding region of the L1a molecule. However, dissociation of the yeast 5S rRNA-L1a protein RNP complex to its components resulted in a decline of tryptophan fluorescence. The observation implied that the environment of the tryptophan-containing L1a regions which were not known to be involved in RNA binding was influenced by association with the 5S rRNA molecule.

Acrylamide↗

PubMed1991

Intermediates in the chaperonin-assisted refolding of rhodanese are trapped at low temperature and show a small stoichiometry.

In vitro refolding of the urea-unfolded, monomeric, mitochondrial enzyme rhodanese (thiosulfate sulfur-transferase; EC 2.8.1.1) is facilitated by the chaperonin proteins cpn60 and cpn10 from Escherichia coli at 37 degrees C, but the refolding is strongly inhibited at 10 degrees C. In contrast, the unassisted refolding of rhodanese is efficient at 10 degrees C, but the refolding efficiency decreases as the temperature is raised. These observations provided two measures of the cpn60-rhodanese complex. Thus, we monitored either 1) the cpn60-dependent inhibition of spontaneous folding at 10 degrees C or 2) the recovery of active rhodanese in the complete chaperonin system at 25 degrees C, after first forming a cpn60-rhodanese complex at 10 degrees C. These procedures minimized the aggregation of interactive folding intermediates that tend to overestimate the apparent number of cpn60 14-mers in determining the stoichiometry of protein-cpn60 14-mer interactions. Both procedures used here gave results that were consistent with there being 1 rhodanese binding site/cpn60 tetradecamer. This stoichiometry is significantly less than might be expected from the fact that cpn60 is composed of 14 identical subunits, and it may indicate that rhodanese interacts with a restricted region that is formed when the cpn60 tetradecamer is assembled. The ability to stabilize chaperonin-protein complexes that can subsequently be reactivated will aid studies of the mode of action of the ubiquitous chaperonin proteins.

Adenosine Triphosphate↗

PubMed1991

The quaternary structure of streptavidin in urea.

We report on the interactions of urea and guanidinium salts with streptavidin. Gel filtration chromatography in 0, 4, 6, and 7 M urea indicates that the streptavidin tetramer remains intact in urea. Biotin alters the electrophoretic mobility of streptavidin whether or not 6 M urea is present. The intrinsic fluorescence of streptavidin is increased and blue-shifted in 6 M urea. The fluorescence changes indicate the absence of unfolding. A conformational response to urea is possible, but much of the fluorescence change is due to urea binding as a weak biotin analog (Ka approximately 1.3 M-1). The resistance to structural perturbation by urea reflects the structural stability of streptavidin's anti-parallel beta-barrel motif. Unfolding is sluggish in 6 M guanidinium hydrochloride (half-time, approximately 50 days). After guanidinium thiocyanate unfolding, streptavidin can be refolded, but the unfolding and refolding transitions are centered at different concentrations of perturbant. Slow unfolding, with a 15th power dependence on guanidinium thiocyanate concentration, may be partially responsible for the noncoincidence of the unfolding and refolding processes. Nonequilibrium behavior is also seen in 6 M urea, as native streptavidin does not unfold and guanidinium thiocyanate unfolded streptavidin does not refold. Refolding does occur at lower concentrations of urea. Guanidinium thiocyanate only slowly unfolds the biotin-streptavidin complex. In the presence of biotin, unfolded streptavidin does not refold in 6 M guanidinium thiocyanate or in 6 M urea.

Bacterial Proteins↗

PubMed1991

Unassisted refolding of urea unfolded rhodanese.

In vitro refolding after urea unfolding of the enzyme rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) normally requires the assistance of detergents or chaperonin proteins. No efficient, unassisted, reversible unfolding/folding transition has been demonstrated to date. The detergents or the chaperonin proteins have been proposed to stabilize folding intermediates that kinetically limit folding by aggregating. Based on this hypothesis, we have investigated a number of experimental conditions and have developed a protocol for refolding, without assistants, that gives evidence of a reversible unfolding transition and leads to greater than 80% recovery of native enzyme. In addition to low protein concentration (10 micrograms/ml), low temperatures are required to maximize refolding. Otherwise optimal conditions give less than 10% refolding at 37 degrees C, whereas at 10 degrees C the recovery approaches 80%. The unfolding/refolding phases of the transition curves are most similar in the region of the transition, and refolding yields are significantly reduced when unfolded rhodanese is diluted to low urea concentrations, rather than to concentrations near the transition region. This is consistent with the formation of "sticky" intermediates that can remain soluble close to the transition region. Apparently, nonnative structures, e.g. aggregates, can form rapidly at low denaturant concentrations, and their subsequent conversion to the native structure is slow.

In Vitro Techniques↗

PubMed1991

Chaperonins facilitate the in vitro folding of monomeric mitochondrial rhodanese.

In vitro refolding of the monomeric mitochondrial enzyme, rhodanese (thiosulfate sulfurtransferase; EC 2.8.1.1) is facilitated by molecular chaperonins. The four components: two proteins from Escherichia coli, chaperonin 60 (groEL) and chaperonin 10 (groES), MgATP, and K+, are necessary for the in vitro folding of rhodanese. These were previously shown to be necessary for the in vitro folding of ribulose-1,5-bisphosphate carboxylase at temperatures in excess of 25 degrees C (Viitanen, P. V., Lubben, T. H., Reed, J., Goloubinoff, P., O'Keefe, D. P., and Lorimer, G. H. (1990) Biochemistry 29, 5665-5671). The labile folding intermediate, rhodanese-I, which rapidly aggregates at 37 degrees C in the absence of the chaperonins, can be stabilized by forming a binary complex with chaperonin 60. The discharge of the binary chaperonin 60-rhodanese-I complex, results in the formation of active rhodanese, and requires the presence of chaperonin 10. Optimal refolding is associated with a K(+)-dependent hydrolysis of ATP. At lower protein concentrations and 25 degrees C, where aggregation is reduced, a fraction of the rhodanese refolds to an active form in the absence of the chaperonins. This spontaneous refolding can be arrested by chaperonin 60. There is some refolding (approximately equal to 20%) when ATP is replaced by nonhydrolyzable analogs, but there is no refolding in the presence of ADP or AMP. ATP analogs may interfere with the interaction of rhodanese-I with the chaperonins. Nondenaturing detergents facilitate rhodanese refolding by interacting with exposed hydrophobic surfaces of folding intermediates and thereby prevent aggregation (Tandon, S., and Horowitz, P. (1986) J. Biol. Chem. 261, 15615-15618). The chaperonin proteins appear to play a similar role in as much as they can replace the detergents. Consistent with this view, chaperonin 60, but not chaperonin 10, binds 2-3 molecules of the hydrophobic fluorescent reporter, 1,1'-bi(4-anilino)naphthalene-S,5'-disulfonic acid, indicating the presence of hydrophobic surfaces on chaperonin 60. The number of bound probe molecules is reduced to 1-2 molecules when chaperonin 10 and MgATP are added. The results support a model in which chaperonins facilitate folding, at least in part, by interacting with partly folded intermediates, thus preventing the interactions of hydrophobic surfaces that lead to aggregation.

Adenosine Triphosphate↗

PubMed1991

Expression of cloned bovine adrenal rhodanese.

A cDNA for the enzyme rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) has been cloned from a bovine adrenal library. An initiator methionine codon precedes the amino-terminal amino acid found in the isolated protein. Rhodanese is synthesized in the cytoplasm and transferred to the mitochondrial matrix. Thus, any amino-terminal sequence required for organelle import is retained in the mature protein. Furthermore, the DNA sequence shows that there are three additional amino acids, Gly-Lys-Ala, at the carboxyl terminus that are not found by protein sequencing. Additionally, comparison of the published amino acid sequence with that encoded by the open reading frame revealed three differences in the amino acid sequence. Comparison of the bovine and chicken liver sequences shows an overall level of 70% sequence homology, but there is complete identity of all residues that have been implicated in the function of the enzyme. When two mammalian cells, cos-7 and 293 cells, were transiently transfected with a plasmid containing the rhodanese coding region, rhodanese activity in lysates increased approximately 20-fold. Fluorograms of denaturing polyacrylamide gels detected a large increase in a polypeptide that co-migrated with the native protein and reacted with anti-rhodanese antibodies. Nondenaturing gels showed two active species that co-migrated with the two major electrophoretic forms purified by current procedures. Escherichia coli, transformed with a plasmid containing the rhodanese coding region, showed a 15-fold increase in rhodanese activity over baseline values. When the E. coli recombinant protein was analyzed on a nondenaturing gel, only one species was observed that co-electrophoresed with the more electropositive variant seen in purified bovine liver rhodanese. This single variant could be converted by carboxypeptidase B digestion to a form of the enzyme that co-migrated with the more electronegative species isolated from bovine liver. Thus, two major, enzymatically active electrophoretic variants, commonly observed in mammalian cells, can be accounted for by carboxyl-terminal processing without recourse to other post-translational modifications.

Adrenal Glands↗

PubMed1991

The initiation codon AUG binds at a hydrophobic site on yeast 40S ribosomal subunits as revealed by fluorescence studies with bis (1,8-anilinonaphthalenesulfonate).

Binding studies of yeast 40S ribosome with bis (1,8-anilinonaphthalenesulfonate) (bis-ANS) revealed the binding of 3-4 molecules of bis-ANS per ribosome with a dissociation constant (Kd) of 1.45 microM. Binding of AUG to the 40S subunits resulted in a concentration-dependent decrease in the bis-ANS fluorescence without displacing all of the bound bis-ANS from the ribosomes. The residual bis-ANS fluorescence at saturation with AUG corresponds to about 3 molecules of bis-ANS per ribosome. Thus AUG displaces one of the bound bis-ANS molecules. The data suggest that AUG binds at a hydrophobic site on the yeast 40S subunit.

Anilino Naphthalenesulfonates↗

PubMed1990

The detection of kinetic intermediate(s) during refolding of rhodanese.

Recent studies showed that the enzyme rhodanese could be reversibly unfolded in guanidinium chloride (GdmCl) if aggregation and oxidation were minimized. Further, these equilibrium studies suggested the presence of intermediate(s) during refolding (Tandon, S., and Horowitz, P. (1989) J. Biol. Chem. 264, 9859-9866). The present work shows that native and refolded enzymes are very similar in structural and functional characteristics. Kinetics of denaturation/renaturation were used to detect the folding intermediate(s). The shift in fluorescence wavelength maximum was used to monitor the structural changes during the process. First order plots of the structural changes during unfolding and refolding show nonlinear curves. The refolding occurs in at least two phases. The first phase is very fast (t1/2 much less than 30 s) and accounts for the partial regain in the structure but not in the activity. The second phase is slow (t1/2 = 2.9 h) during which the enzyme fully regains its structure along with the activity. The fractional renaturation of rhodanese due to the fast phase, monitored in various concentrations of GdmCl, describes a transition centered at 3.5 M GdmCl which is very similar to the higher of the two transitions observed in the reversible refolding. All of these findings support the presence of detectable intermediate(s) during folding of rhodanese.

Animals↗

PubMed1990

A physical characterization of sulfane sulfurtransferase.

The bacterial enzyme sulfane sulfurtransferase has been studied using spectroscopic techniques. The enzyme was characterized in terms of its near-UV absorption spectrum, molar ellipticity, intrinsic fluorescence spectra and the effects of general and ionic quenching reagents upon its fluorescence. Fluorescence model studies are consistent with sulfane sulfurtransferase having only a single tryptophan residue, which accounts for its low UV absorption coefficient and suggested that this residue is at least partially exposed to solvent. Second derivative absorption spectroscopy studies revealed that most of the bacterial enzyme's tyrosine residues are exposed to solvent. Unlike the better known sulfurtransferase, bovine liver rhodanese, sulfane sulfurtransferase does not undergo a detectable increase in quantum yield when shifting from the sulfur-containing covalent enzyme intermediate to the free enzyme form (which lacks sulfur) during catalysis. CD studies suggest that sulfane sulfurtransferase has a significantly higher proportion of alpha-helix than rhodanese. The renaturation of sulfane sulfurtransferase denatured in 6 M guanidine was shown to be rapid and complete provided that the enzyme had not been oxidized while in the denatured state. Sulfane sulfurtransferase, like rhodanese, catalyzes the transfer of sulfur from thiosulfate to cyanide via a persulfide intermediate, and displays remarkably similar kinetics in this process (Aird, B.A., Heinrikson, R.L. and Westley, J. (1987) J. Biol. Chem 262, 17327-17335). In light of this, the results of the structural studies with sulfane sulfurtransferase are compared and contrasted to data from similar experiments with rhodanese in hopes that they would provide insight about which phenomena observed with rhodanese are intrinsic to the process of transferring sulfur atoms.

Acinetobacter↗

PubMed1990

Stable intermediates can be trapped during the reversible refolding of urea-denatured rhodanese.

The enzyme rhodanese (EC 2.8.1.1) could be reversibly refolded from urea in the presence of lauryl maltoside, beta-mercaptoethanol, and sodium thiosulfate. The unfolding/folding transition monitored using intrinsic fluorescence was resolved into two two-state transitions with midpoints at 3.6 and 5.0 M urea. The analysis assumed an intermediate with an emission maximum at 345 nm. Monitoring anisotropy of intrinsic fluorescence also gave an asymmetric transition. Activity followed one two-state transition centered at 3.6 M urea with no major change of secondary structure. Without thiosulfate or mercaptoethanol, there was one two-state transition at 5.0 M urea giving a species, in dilute urea, with a fluorescence maximum at 345 nm. This intermediate slowly relaxed toward 335 nm (t1/2 = 85 min) if only thiosulfate was absent but without regaining activity. Subsequent addition of thiosulfate led to a first-order recovery of activity (t1/2 = 75 min). Thus, a possible folding intermediate can be trapped which displays increased access of water and solutes to its fluorescent tryptophans. This intermediate conformer, which is flexible, has considerable secondary structure, is inactive, has exposed hydrophobic surfaces, and requires specific reducing conditions to regain full activity. Refolding probably involves an initial, rapid, hydrophobic collapse with acquisition of secondary structure to form the intermediate, followed by slower adjustment to the native global conformation. Final reactivation requires reduction localized at the active site.

Dithionitrobenzoic Acid↗

PubMed1990

Intrinsic fluorescence of succinyl-CoA synthetase and four tryptophan mutants. Tryptophan 76 and tryptophan 248 of the beta-subunit are responsive to CoA binding.

Previous studies showed that modification of an average of one of the three tryptophan residues of succinyl-CoA synthetase of Escherichia coli abolished enzyme activity, but did not prevent phosphorylation of the enzyme by ATP [Ybarra, J., Prasad, A. R. S., & Nishimura, J.S. (1986) Biochemistry 25, 7174-7178]. In the present study, single mutations in which each of the three tryptophans (beta-Trp43, beta-Trp76, and beta-Trp248) has been changed to phenylalanine (designated W43F, W76F, and W248F) have been accomplished by the technique of site-directed mutagenesis and the mutant proteins isolated. In addition, a double mutant in which beta-Trp43 and beta-Trp248 were changed to phenylalanines (W43,248F) has also been isolated. Each of the mutant enzymes was practically as active as wild type. Since the emission spectrum of beta-Trp76 reflected a low fluorescence intensity for this residue, it was possible to obtain the emission spectrum of each tryptophan residue by using W43F, W248F, and W43,248F. From the positions of the emission maxima and the results of iodide quenching of fluorescence, it was deduced that beta-Trp248 is a surface residue, beta-Trp43 is buried, and beta-Trp76 is intermediate in location. Coenzyme A, but no other substrate, protected the fluorescence of beta-Trp76 and beta-Trp248, but not of beta-Trp43, against quenching by acrylamide. These results are consistent with an interaction between beta-Trp76 and beta-Trp248 and the binding site for CoA.

Adenosine Diphosphate↗

PubMed1990

Biotin binding changes the conformation and decreases tryptophan accessibility of streptavidin.

Biotin binding reduces the tryptophan fluorescence emissions of streptavidin by 39%, blue shifts the emission peak from 333 to 329 nm, and reduces the bandwidth at half height from 53 to 46 nm. The biotin-induced emission difference spectrum resembles that of a moderately polar tryptophan. Streptavidin fluorescence can be described by two lifetime classes: 2.6 nsec (34%) and 1.3 nsec (66%). With biotin bound, lifetimes are 1.3 nsec (26%) and 0.8 nsec (74%). Biotin binding reduces the average fluorescence lifetime from 1.54 to 0.88 nsec. Biotin does not quench the fluorescence of indoles. The fluorescence changes are consistent with biotin binding causing a conformational change which moves tryptophans into proximity to portions of streptavidin which reduce the quantum yield and lifetimes. Fluorescence quenching by acrylamide revealed two classes of fluorophores. Analysis indicated a shielded component comprising 20-28% of the initial fluorescence with (KSV + V) less than or equal to 0.55 M-1. The more accessible component has a predominance of static quenching. Measurements of fluorescence lifetimes at different acrylamide concentrations confirmed the strong static quenching. Since static quenching could be due to acrylamide binding to streptavidin, a dye displacement assay for acrylamide binding was constructed. Acrylamide does bind to streptavidin (Ka = 5 M-1), and probably binds within the biotin-binding site. In the absence of biotin, none of streptavidin's fluorescence is particularly accessible to iodide. In the presence of biotin, iodide neither quenches fluorescence nor alters emission spectra, and acrylamide access is dramatically reduced. We propose that the three tryptophans which always line the biotin site are sufficiently close to the surface of the binding site to be quenched by bound acrylamide. These tryptophans are shielded from iodide, most probably due to steric or ionic hindrances against diffusion into the binding site. Most of the shielding conferred by biotin binding can be attributed to the direct shielding of these residues and of a fourth tryptophan which moves into the binding site when biotin binds, as shown by X-ray studies (Weber et al., 1989).

Acrylamides↗