Activation of horse liver alcohol dehydrogenase upon substitution of tryptophan 314 at the dimer interface

Horse liver alcohol dehydrogenase contains two tryptophan residues per subunit, Trp-15 on the surface of the catalytic domain and Trp-314 buried in the interface between the subunits of the dimer. We studied the contributions of the tryptophans to fluorescence and catalytic dynamics by substituting Trp-314 with a leucine residue and making two compensatory mutations that were required to obtain a stable protein, leading to the triple mutant M303F-L308I-W314L enzyme. The substitutions increased by two- to sixfold the turnover numbers for ethanol oxidation, acetaldehyde reduction, and the dissociation constants of the coenzymes. The rate of the exponential burst phase for the transient oxidation of ethanol increased slightly, but the rate of dissociation of the enzyme-NADH complex still limited turnover of ethanol, as for wild-type enzyme. The three substitutions at the dimer interface apparently activate the enzyme by allowing more rapid conformational changes that accompany coenzyme binding, probably due to movement of the loop containing residues 293 to 298. The emission spectrum of M303F-L308I-W314L enzyme, which contains Trp-15, was redshifted compared to wild-type enzyme. Time-resolved fluorescence measurements with the triple mutant show that the decay of Trp-15 is dominated by a approximately 7-ns component. In the mutant enzyme with Trp-15 substituted with phenylalanine, the decay of Trp-314 is dominated by a approximately 4-ns component. Solute quenching data for wild-type enzyme and the mutants show that only Trp-15 is exposed to iodide and acrylamide, whereas Trp-314 is inaccessible. The luminescence properties of the tryptophan residues in the mutated enzymes are consistent with conclusions from studies of the wild-type enzyme [M. R. Eftink, 1992, Adv. Biophys. Chem. 2, 81-114].

Incorporation of tryptophan analogues into staphylococcal nuclease: stability toward thermal and guanidine-HCl induced unfolding

The tryptophan analogues, 5-hydroxytryptophan, 7-azatryptophan, 4-fluorotryptophan, 5-fluorotryptophan, and 6-fluorotryptophan, have been biosynthetically incorporated into Staphylococcal nuclease, its V66W mutant, and the Delta 137-149 fragment of the latter mutant. The guanidine-HCl induced unfolding and thermal unfolding of these proteins were studied to characterize the effect of incorporation of these tryptophan analogues on the thermodynamic stability of the proteins. The three proteins have tryptophan residues at positions 140 (in wild type) and 66 (in the Delta 137-149 fragment of V66W) and at both positions (in V66W). The unfolding data show that 5-hydroxytryptophan does not perturb the stability of wild-type nuclease, but it destabilizes the fragment and causes the V66W mutant to unfold in a more cooperative manner. 7-Azatryptophan is found to destabilize all three proteins. 4-Fluorotryptophan is slightly stabilizing of the three proteins, but the other two fluorotryptophans do not alter the stability of the proteins.

Phosphorescence and optically detected magnetic resonance characterization of the environments of tryptophan analogues in staphylococcal nuclease, its V66W mutant, and Delta 137-149 fragment

Phosphorescence and optically detected magnetic resonance (ODMR) measurements are reported on the triplet states of the tryptophan analogues, 7-azatryptophan (7AW), 5-hydroxytryptophan (5HW), and 4-, 5-, and 6-fluorotryptophan (4FW, 5FW, 6FW), when incorporated at position 140 of wild-type Staphylococcal nuclease (7AW-nuclease, etc. ), positions 66 and 140 of its V66W mutant (7AW-V66W, etc.), and the deletion fragment of the latter, Delta 137-149 (7AW-V66W', etc.). These measurements point to the retention of protein structure at position 140 in each of the wild-type nuclease analogues. Substitution of the analogue at both tryptophan sites of V66W leads to structured sites with differentiated triplet-state properties for all analogues except 7AW-V66W, whose structure is destabilized. 5HW-V66W' is the only fragment that apparently lacks structure at position 66. All other V66W' analogues exhibit a structured environment at position 66 (4FW-V66W' was not studied), but in each case this site can be differentiated readily from the corresponding site in intact V66W. 7AW-V66W' is resolved by ODMR into two discrete structures with slightly differing zero field splittings (ZFS). Interaction of the protein with 5HW at position 66 of 5HW-V66W induces a 2-fold increase in the ZFS E parameter, which is reduced to its normal value upon formation of the fragment, 5HW-V66W'. Analogous effects occur for 5FW, but on a smaller scale.

Incorporation of tryptophan analogues into staphylococcal nuclease, its V66W mutant, and Delta 137-149 fragment: spectroscopic studies

We have biosynthetically incorporated several tryptophan analogues into three forms of Staphylococcal nuclease to investigate the spectroscopic characteristics of these "intrinsic" probes and their effect on the structure of the proteins. The set of tryptophan analogues includes 5-hydroxytryptophan, 7-azatryptophan, 4-fluorotryptophan, 5-fluorotryptophan, and 6-fluorotryptophan. 5-Hydroxytryptophan and 7-azatryptophan have red-shifted absorbance spectra, and the latter has a red-shifted fluorescence, which is very sensitive to its environment (being heavily quenched in water). The fluorotryptophans can serve as 19F NMR probes, and 4-fluorotryptophan has a very low fluorescence quantum yield, thus making it a "knock-out" fluorescence analogue. The set of proteins studied includes wild-type nuclease, which has a single tryptophan site at position 140; its V66W mutant, which has a second tryptophan at position 66; and the Delta 137-149 fragment, V66W', which only has a tryptophan at position 66. The environments of positions 66 and 140 are significantly different; position 140 is near the end of the long C-terminal alpha-helix and is moderately solvent-exposed, whereas position 66 is in the beta-barrel core region of the protein and is surrounded by apolar side chains. Absorbance and 19F NMR spectra are used to estimate the extent of analogue incorporation for each protein. Steady-state and time-resolved fluorescence data are reported to characterize the emission of the analogues in these positions in the three proteins and to develop the use of the analogues as probes of protein structure and dynamics. Circular dichroism spectra are reported to show that, in all but a couple of cases, the secondary structure of the proteins containing the analogues is not significantly perturbed by the probes. Additionally, fluorescence anisotropy decay data show the variants of wild-type nuclease to have a rotational correlation time similar to that of tryptophan-containing nuclease.

The use of fluorescence methods to monitor unfolding transitions in proteins

The advantages and some limitations of the use of fluorescence methods for the quantitative determination of the thermodynamics of protein unfolding transitions (i.e., induced by temperature or chemical denaturant) are discussed. Advantages include the sensitivity, multi-dimensional nature of the data, wide amenable concentration range, high signal-to-noise, rapidity of measurement, and adaptability to a variety of sample compartments. Aside from the need for a probe, some problems associated with the method involve the handling of baselines for the pre- and post-transition regions and the difficulty (shared by most other methods) of discerning whether the transition is two-state or multi-state.

Studies of the unfolding of an unstable mutant of staphylococcal nuclease: evidence for low temperature unfolding and compactness of the high temperature unfolded state

Fluorescence and circular dichroism data as a function of temperature were obtained to characterize the unfolding of nuclease A and two of its less stable mutants. These spectroscopic data were obtained with a modified instrument that enables the nearly simultaneous detection of both fluorescence and CD data on the same sample. A global analysis of these multiple datasets yielded an excellent fit of a model that includes a change in the heat capacity change, deltaC(p), between the unfolded and native states. This analysis gives a deltaC(p) of 2.2 kcal/mol/ x K for thermal unfolding of the WT protein and 1.3 and 1.8 kcal/mol/K for the two mutants. These deltaC(p) values are consistent with significant population of the cold unfolded state at approximately 0 degrees C. Independent evidence for the existence of a cold unfolded state is the observation of a separately migrating peak in size exclusion chromatography. The new chromatographic peak is seen near 0 degrees C, has a partition coefficient corresponding to a larger hydrodynamic radius, and shows a red-shifted fluorescence spectrum, as compared to the native protein. Data also indicate that the high-temperature unfolded form of mutant nuclease is relatively compact. Size exclusion chromatography shows the high temperature unfolded form to have a hydrodynamic radius that is larger than that for the native form, but smaller than that for the urea or pH-induced unfolded forms. Addition of chemical denaturants to the high-temperature unfolded form causes a further unfolding of the protein, as indicated by an increase in the apparent hydrodynamic radius and a decrease in the rotational correlation time for Trp140 (as determined by fluorescence anisotropy decay measurements).

Characterization of the role of side-chain interactions in the binding of ligands to apo trp repressor: pH dependence studies

The pH dependence of the association of apo trp repressor with the series of ligands, tryptophan, tryptamine, indole propionic acid (IPA), and trans-beta-indole acrylic acid (IAA), has been studied using fluorescence titrations and isothermal titration microcalorimetry (ITC). The purpose of such a comparison of ligands and the pH dependency studies is to reveal the role played by the side-chain functional groups in the energetics of the binding of the ligands to the protein. We find that, whereas the binding of tryptamine and IPA have essentially no pH dependence between pH 6 and 10, the binding of tryptophan and IAA depends on pH. For IAA, the affinity drops between pH 6 and 10, consistent with a shift in pKa of some group on the protein from a value of pKa 7.4 to 7.9 upon binding of this ligand. The affinity of IAA also drops below pH 5, but shows saturable binding at pH 2-3, where the protein has previously been found to exist as a partially folded monomeric state. For tryptophan, the pH dependence data indicate that the equilibrium is complicated. We present a model to describe the data in which the alpha-ammonium group of tryptophan has its pKa shifted upward upon binding (i.e. preferential binding of the protonated form of this functional group) and in which the pKa of an unknown group on the protein also has its pKa increased.

Biosynthetic incorporation of tryptophan analogues into staphylococcal nuclease: effect of 5-hydroxytryptophan and 7-azatryptophan on structure and stability

5-Hydroxytryptophan (5HW) and 7-azatryptophan (7AW) are analogue of tryptophan that potentially can be incorporated biosynthetically into proteins and used as spectroscopic probes for studying protein-DNA and protein-protein complexes. The utility of these probes will depend on the extent to which they can be incorporated and the demonstration that they cause minimal perturbation of a protein's structure and stability. To investigate these factors in a model protein, we have incorporated 5HW and 7AW biosynthetically into staphylococcal nuclease A, using a trp auxotroph Escherichia coli expression system containing the temperature-sensitive lambda cI repressor, Both tryptophan analogues are incorporated into the protein with good efficiency. From analysis of absorption spectra, we estimate approximately 95% incorporation of 5HW into position 140 of nuclease, and we estimate approximately 98% incorporation of 7AW, CD spectra of the nuclease variants are similar to that of the tryptophan-containing protein, indicating that the degree of secondary structure is not changed by the tryptophan analogues. Steady-state fluorescence data show emission maxima of 338 nm for 5HW-containing nuclease and 355 nm for 7AW-containing nuclease. Time-resolved fluorescence intensity and anisotropy measurements indicate that the incorporated 5HW residue, like tryptophan at position 140, has a dominant rotational correlation time that is approximately the value expected for global rotation of the protein. Guanidine-hydrochloride-induced unfolding studies show the unfolding transition to be two-state for 5HW-containing protein, with a free energy change for unfolding that is equal to that of the tryptophan-containing protein. In contrast, the guanidine-hydrochloride-induced unfolding of 7AW-containing nuclease appears to show a non-two-state transition, with the apparent stability of the protein being less than that of the tryptophan form.

Global analysis of the acid-induced and urea-induced unfolding of staphylococcal nuclease and two of its variants

We have studied the equilibrium unfolding staphylococcal nuclease and two of its variants, V66W and V66W', over two perturbation axes (acid-induced unfolding as a function of urea concentration and urea-induced unfolding as a function of pH). The transitions were monitored by simultaneous measurements of circular dichroism and fluorescence. With this multidimensional array of data (2 perturbation axes and 2 signals), we present a strategy of performing a global analysis, over as many as 12 individual data sets, to test various models for the unfolding process, to determine with greater confidence the pertinent thermodynamic parameters, and to characterize unfolding intermediates. For example, wildtype nuclease shows a cooperative two-state transition with either urea or pH as denaturant, but the global fits are improved when the model is expanded to include a pH dependence of the urea m value or when two distinct classes of protonic groups are considered. The best fit for wild-type nuclease is with delta G degree 0,UN = 6.4 kcal/mol at pH 7, with the acid-induced unfolding being triggered by protonation of three to five carboxylate groups (with possible contribution from His121), and with the urea m = 2.5 kcal mol-1 M-1. V66W' lacks the last 13 amino acids on the C-terminus, has a tryptophan at position 66, has a predominantly beta-sheet structure, and is less stable than the wild type. For V66W', delta G degree 0,UN = 1.6 kcal/mol, m = 1.2 kcal mol-1 M-1, and there are two or three groups responsible for acid unfolding. V66W, a full-length mutant with two tryptophan residues, unfolds via a three-state mechanism: native reversible intermediate reversible unfolded. It appears that its beta-barrel subdomain retains structure in the intermediate state. Assuming that the unfolding of V66W' and the beta-barrel subdomain of V66W can be described by the same thermodynamic parameters, a global analysis enabled a description of the alpha subdomain of V66W with delta G degree 0,IN = 2.7 kcal/mol, mIN = 1.1 kcal mol-1 M-1, and with the acid unfolding being triggered by protonation of a single group. This group has a pKa around 6 in the unfolded state, suggesting that the state of protonation of a histidine residue may contribute significantly to the stability of V66W.

Thermodynamics of protein unfolding: questions pertinent to testing the validity of the two-state model

We discuss a number of questions pertaining to the analysis of data to extract thermodynamic parameters for the reversible unfolding of proteins. Simulations are presented to illustrate problems in trying to test the validity of the two-state model, vis-a-vis a more complicated unfolding model. A conceptual and practical problem is how to consider the unfolded state and how to relate the observed signal to this state. We discuss the idea that the unfolded state can be described as a single macrostate, comprising a distribution of microstates having different degrees of solvent-accessible surface area. We also discuss the possibilities and thermodynamic consequences of having more than one unfolded state and of having a denaturant which both stabilizes and destabilizes the protein's native state.

Thermodynamics of the unfolding and spectroscopic properties of the V66W mutant of Staphylococcal nuclease and its 1-136 fragment

Spectroscopic studies have been performed to characterize the solution structure of the V66W mutant of Staphylococcal nuclease and the corresponding 1-136 fragment, referred to as V66W'. Whereas wild-type nuclease has a single tryptophan residue at position 140, the V66W mutant has a second tryptophan residue at position 66, which is the only such residue in V66W'. Steady-state and time-resolved fluorescence studies show Trp-66 in V66W' to have a blue emission, a relatively large fluorescence quantum yield, a long lifetime, a significant degree of protection from solute quenchers, and to depolarize with a relatively long rotational correlation time. These results characterize Trp-66 in V66W' as being a buried residue, which indicates that this fragment retains some global structure. Circular dichroism (CD) data are consistent with the fragment having lost most of the alpha-helical content of the wild type, while retaining beta-sheet structure. The CD spectrum in the aromatic region also suggests that Trp-66 in the fragment experiences an asymmetric environment, which is not identical to that in the full length mutant, V66W. In addition, optical detection of triplet state magnetic resonance (ODMR) spectroscopy can clearly resolve the tryptophan residues and demonstrates differences between the local environment of Trp-66 in V66W and in V66W', as well as small differences in the Trp-140 environment in wild type and in V66W. Guanidine-HCl induced and thermally induced unfolding studies were performed by simultaneously acquiring CD and fluorescence data as a function of the perturbation and then performing a global analysis of such multiple data sets in terms of two-state and three-state unfolding models. Whereas data for wild-type nuclease and the V66W' fragment are well characterized by a two-state unfolding model, data for the V66W mutant are better characterized by a three-state process. That is, both the denaturant- and temperature-induced unfolding of V66W involves the significant population of an equilibrium unfolding intermediate. Our global analyses yield thermodynamic parameters for the unfolding transitions, and we show that the data for V66W can be described by a constrained three-state model in which the transition of the intermediate to the fully unfolded state is fixed to have the same thermodynamic parameters that describe the unfolding of the V66W' fragment.

Reversible thermal unfolding of ribonuclease T1 in reverse micelles

The reverse micellar system formed by the negatively charged surfactant AOT and the organic solvent isooctane is used to solubilize the protein RNase T1. The physicochemical properties of the entrapped protein have been studied using intrinsic tryptophan fluorescence and far-and near-UV CD. These studies indicate a similar structure for the protein in reverse micelles and in pH 7.0 buffer. Thermal unfolding has been studied as a function of W0, the molar ratio of water to AOT, in the solution. Measuring the change in fluorescence intensity as a function of temperature, we observe a reversible transition for W0 in the range 5-12. Heating rate dependencies carried out on these transitions (0.6-3.0 degrees C/min) indicate that the transition temperature and the apparent van't Hoff enthalpy change depend on the scanning rate as well as on W0. The values of the transition temperature, T(m) and the enthalpy change, delta H degrees(un), extrapolated to an infinitely slow scanning rate, are analyzed considering the electrostatic interaction of the charged residues of the protein with the charges of the surfactant molecules forming reverse micelles, the variation of the size of the reverse micelles, and the relative rates of unfolding, refolding, and irreversible denaturation.

Characterization of the tryptophan binding site of Escherichia coli tryptophan holorepressor by phosphorescence and optical detection of magnetic resonance of a tryptophan-free mutant

The L-tryptophan binding site of the Escherichia coli tryptophan holorepressor (trpR) is characterized by low-temperature phosphorescence and optical detection of magnetic resonance (ODMR) spectroscopy. Measurements are made on a tryptophan-free mutant of trpR, W19/99F, in which both intrinsic tryptophan residues of apo-trpR have been replaced with phenylalanine. Thus, essentially all of the phosphorescence that is observed from trpR originates from the bound L-tryptophan corepressor. The phosphorescence and ODMR results for the bound corepressor agree quite well with those obtained previously for the corepressor site in both single tryptophan-containing mutants, W19F and W99F [Burns, L.E., & Maki, A.H. (1994) J. Fluorescence 4, 217-226]. A red shift of the L-tryptophan phosphorescence origin as well as a decrease in the D-E ODMR frequency result from an increase in the local polarizability upon binding at the corepressor binding site. A large decrease in the ODMR line widths signals a reduction of local heterogeneity upon binding. Subsequent binding of trpR to a self-complementary DNA sequence that mimics the trp operator, 5'-CGTACTAGTTAACTAGTACG-3', produces a further decrease in line widths and additional changes in the ODMR frequencies, attributable to an increase in both the D and E parameters. This result demonstrates that binding of holo-trpR to the operator affects the local environment of the bound corepressor.

Modified spectrophotometer for multi-dimensional circular dichroism/fluorescence data acquisition in titration experiments: application to the pH and guanidine-HCI induced unfolding of apomyoglobin

In a previous paper (Ramsay and Eftink, Biophys. J. 66:516-523) we reported the development of a modified spectrophotometer that can make nearly simultaneous circular dichroism (CD) and fluorescence measurements. This arrangement allows multiple data sets to be collected during a single experiment, resulting in a saving of time and material, and improved correlation between the different types of measurements. The usefulness of the instrument was shown by thermal melting experiments on several different protein systems. This CD/fluorometer spectrophotometer has been further modified by interfacing with a syringe pump and a pH meter. This arrangement allows ligand, pH, and chemical denaturation titration experiments to be performed while monitoring changes in the sample's CD, absorbance, fluorescence, and light scattering properties. Our data acquisition program also has an ability to check whether the signals have approached equilibrium before the data is recorded. For performing pH titrations we have developed a procedure which uses the signal from a pH meter in a feedback circuit in order to collect data at evenly spaced pH intervals. We demonstrate the use of this instrument with studies of the unfolding of sperm whale apomyoglobin, as induced by acid pH and by the addition of guanidine-HCI.

The unfolding of trp aporepressor as a function of pH: evidence for an unfolding intermediate

The urea-induced unfolding of trp aporepressor from Escherichia coli has been studied as a function of pH from 2.5 to 12.0 at 25 degrees C. At pH 7 and above, the unfolding transition, as monitored by changes in the fluorescence intensity at 360 nm, shows a single transition. At low pH, the transition again appears to be a single transition. In the range of 3.5-6.0, the transition is biphasic, indicating the existence of a folding intermediate. The transitions have also been studied using circular dichroism and size exclusion chromatography. The data were fitted by a model in which the dimeric protein first unfolds to form structured monomers, followed by the unfolding of the monomers. From fits with this "folded monomers" model, the free energy change for the dimer<-->monomer dissociation becomes less positive as pH is decreased; the free energy change for the unfolding of the monomers is essentially independent of pH. An alternate model is one in which the dimer first undergoes a transition to a partially unfolded dimeric state, with this intermediate then denaturing to unfolded monomers. Both models give adequate fits to the data obtained at a single protein concentration. From a study of the concentration dependence of the urea-induced unfolding at pH 5, the "folded monomers" model is found to be more consistent with the data. Size exclusion chromatography data support the description of the intermediate state, which is the most populated state at low pH in the absence of urea, as being a relatively compact monomer.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermodynamic studies of the interaction of trp aporepressor with tryptophan analogs

The association of L-tryptophan and some of its analogs, including three conformationally restricted analogs, with trp aporepressor (apo trpR) was studied by isothermal titration microcalorimetry. Contributions of the functional groups of a ligand to the free energy change, delta G degrees', and enthalpy change, delta H degree', of the interaction were evaluated on a molecular basis. Analogs without the alpha-amino group (i.e. desamino analogs) bind with a slightly higher affinity to the protein. On the other hand, descarboxy analogs show weaker binding to the apo trpR. In addition, it is found that there exists enthalpy-entropy compensation for the association of the congener series of ligands with the protein. The entropy change, delta S degree', appears to play a more important role in the binding of the conformationally restricted analogs than in the binding of L-tryptophan and the unlocked ligands.

A multidimensional spectrophotometer for monitoring thermal unfolding transitions of macromolecules

We describe a multidimensional spectrometer that is capable of (nearly) simultaneous measurement of circular dichroism, steady-state fluorescence, and absorbance values on the same sample in a standard 1 x 1 cm cuvette. With a computer controlled thermoelectric cell holder, this instrument is capable of measuring the above types of spectral data at various wavelengths as a function of temperature. We have developed software to control the various acquisition functions and to convert the data files to a format appropriate for use with the nonlinear least squares program, NONLIN (Johnson and Frasier, 1985). We have tested various features of this instrument and we have applied this instrument and data analysis procedure to study the thermal unfolding of ribonuclease A, under conditions were the unfolding of this protein involves an intermediate state.

The use of fluorescence methods to monitor unfolding transitions in proteins

This article discusses several strategies for the use steady-state and time-resolved fluorescence methods to monitor unfolding transitions in proteins. The assumptions and limitations of several methods are discussed. Simulations are presented to show that certain fluorescence observables directly track the population of states in an unfolding transition, whereas other observables skew the transition toward the dominant fluorescing species. Several examples are given, involving the unfolding of Staphylococcal aureus nuclease A, in which thermodynamic information is obtained for the temperature and denaturant induced transitions in this protein.

Luminescence studies with trp repressor and its single-tryptophan mutants

Time-resolved and steady-state fluorescence, low-temperature phosphorescence, and optically detected magnetic resonance (ODMR) measurements have been made to resolve the luminescence contributions of the two intrinsic tryptophan residues in the subunits of trp aporepressor from Escherichia coli. Assignments of spectral information have been confirmed by use of the single-tryptophan mutants W19F and W99F. Solute fluorescence quenching studies show that both Trp19 and Trp99 are exposed to acrylamide and iodide, with Trp99 being the more exposed. Time-resolved and steady-state fluorescence measurements show Trp19 to have a bluer emission, a longer mean fluorescence decay time, a higher quantum yield, and essentially no independent rotational motion with respect to the protein. Trp99 is found to have a redder emission, a shorter mean fluorescence decay time, a lower quantum yield, and a significant degree of rotational freedom. Phosphorescence studies show a clear resolution of 0-0 vibronic transitions for each type of residue, with maxima at 407 and 415 nm that are assigned to Trp19 and Trp99, respectively. ODMR measurements show the zero-field splitting parameters to be quite characteristically different for each tryptophan residue. The existence of resonance energy transfer from Trp19 to Trp99, in the wild-type protein, is indicated by three types of data: comparison of the long-lived decay time (attributed to Trp19) in the absence (W99F) and presence (wild type) of the acceptor Trp99, comparison of the fluorescence quantum yield of the wild-type and mutant proteins, and deviations from the expected phosphorescence intensities for Trp19 and Trp99 in the absence of energy transfer.

Interaction of indoleacrylic acid with Trp aporepressor from Escherichia coli

Trans-beta-Indoleacrylic acid (IAA) binds with moderately high affinity to the dimeric protein, trp aporepressor from Escherichia coli. IAA is itself nonfluorescent, and, upon binding, IAA quenches approximately 95% of the intrinsic tryptophan fluorescence of the protein. Since there is an overlap between the absorbance of IAA and the emission of tryptophan, this quenching appears to be due to resonance energy transfer. To adequately fit binding isotherms, it was necessary to use a model in which the binding of IAA to one subunit is able to quench fluorescence from both subunits of the protein. This model also assumes that binding to the two subunits occurs independently. Binding data were obtained as a function of the concentration of trp aporepressor, over the range of 1 x 10(-8) to 5 x 10(-5) M, in order to test the possibility that protein subunit association or dissociation occurs. When analyzed in terms of the above model, no significant protein concentration dependence is seen for the IAA association constant, indicating that either (i) the protein does not undergo changes in oligomerization over this concentration range or that (ii) different states of aggregation bind IAA with similar affinity. The affinity of IAA was found to increase at higher ionic strength and at lower pH. Also, evidence is presented for a photoinduced change in the configuration of IAA.

Fluorescence studies of a local anesthetic-phospholipid interaction

Steady-state and time-resolved fluorescence data are reported for the local anesthetic dibucaine in the absence and presence of phospholipid vesicles. These vesicles were comprised of dimyristylphosphatidyl choline and approximately 10% dimyristylphosphatidyl glycerol. Solute quenching studies show the bound drug to be protected from collision with iodide ion. The fluorescence lifetime of dibucaine is not significantly changed upon binding to vesicles. The fluorescence anisotropy of dibucaine increases upon association with the vesicles. Anisotropy decay measurements show that the rotational correlation time, phi, of bound dibucaine is increased about one hundred fold over that for free dibucaine. This indicates that the rotational motion of bound dibucaine is slowed by its interaction with the phospholipids. However, we find no evidence that the rotational motion of bound dibucaine is anisotropic.

Local anesthetic-phospholipid interactions. Effects of ionic strength, temperature, and phospholipid mixtures on the binding of dibucaine to phospholipids

The nature of the interaction of amphipathic drugs, such as dibucaine, with phospholipid bilayer membranes was investigated using equilibrium dialysis. Profiles for the binding of cationic dibucaine to unilamellar vesicles were obtained at different temperature and ionic strengths, and for mixtures of neutral phospholipid dimyristylphosphatidylcholine (DMPC) with negatively charged dimyristylphosphatidylglycerol (DMPG). The degree of binding of the cationic drug at pH 5 was found to be higher at temperatures above the Tm of DMPC (24 degrees C) than below Tm. Also enhanced drug binding was found to occur as the concentration of monovalent salt was increased (0.01-0.85 M) and as the percentage of DMPG was increased. Using the Stern and Guoy-Chapman model, which takes into consideration electrostatic effects, we were able to simultaneously fit all our binding data with a minimum of fitting parameters. These parameters (for data at 45 degrees C) are an association constant, K, of 330 M-1, a maximum possible number of drug molecules absorbed per unit surface of vesicle, sigma m+, of 1.70 x 10(-2) A2, and a surface area per bound drug, gamma D, of 48 A2. The data were fitted equally well by an alternate model in which binding of the drug is described as a partitioning equilibrium, with factors included for electrostatic effects and surface expansion caused by drug intercalation between the fatty acid chains.

Fluorescence studies with human epidermal growth factor

Steady-state and time-resolved fluorescence studies have been performed with human epidermal growth factor, a small globular protein having two adjacent tryptophan residues near its C-terminus. Based on the relatively red fluorescence and accessibility to solute quenchers, the two tryptophan residues are found to be exposed to solvent. Anisotropy decay measurements show the dominant depolarizing process to have a sub-nanosecond rotational correlation time indicating the existence of rapid segmental motion of the fluorescing tryptophan residues. From an analysis of the low-temperature excitation anisotropy spectrum of the protein (and in comparison with that of tryptophan, the peptide melittin, and the dipeptide trp-trp), it is concluded that homo-energy transfer and/or exciton interaction occurs between the adjacent tryptophan residues. A thermal transition in the structure of the protein, which is observed by circular dichroism measurements, is not sensed by the steady-state fluorescence of the protein. This result, in conjunction with the anisotropy decay results, indicates that the two tryptophan residues are in a highly flexible C-terminus segment, which is not an integral part of the three-dimensional structure of the protein. Fluorescence measurements with three site-directed mutants also show very little variation.

Effects of temperature on the fluorescence intensity and anisotropy decays of staphylococcal nuclease and the less stable nuclease-conA-SG28 mutant

Frequency-domain fluorescence spectroscopy was used to investigate the effects of temperature on the intensity and anisotropy decays of the single tryptophan residues of Staphylococcal nuclease A and its nuclease-conA-SG28 mutant. This mutant has the beta-turn forming hexapeptide, Ser-Gly-Asn-Gly-Ser-Pro, substituted for the pentapeptide Tyr-Lys-Gly-Gln-Pro at positions 27-31. The intensity decays were analyzed in terms of a sum of exponentials and with Lorentzian distributions of decay times. The anisotropy decays were analyzed in terms of a sum of exponentials. Both the intensity and anisotropy decay parameters strongly depend on temperature near the thermal transitions of the proteins. Significant differences in the temperature stability of Staphylococcal nuclease and the mutant exist; these proteins show characteristic thermal transition temperatures (Tm) of 51 and 30 degrees C, respectively, at pH 7. The temperature dependence of the intensity decay data are shown to be consistent with a two-state unfolding model. For both proteins, the longer rotational correlation time, due to overall rotational diffusion, decreases dramatically at the transition temperature, and the amplitude of the shorter correlation time increases, indicating increased segmental motions of the single tryptophan residue. The mutant protein appears to have a slightly larger overall rotational correlation time and to show slightly more segmental motion of its Trp than is the case for the wild-type protein.

Fluorescence and conformational stability studies of Staphylococcus nuclease and its mutants, including the less stable nuclease-concanavalin A hybrids

We report steady-state and time-resolved fluorescence studies with the single tryptophan protein, Staphylococcus aureus A, and several of its site-directed mutants. A couple of these mutants, nuclease-conA and nuclease-conA-S28G (which are hybrid proteins containing a six amino acid beta-turn substitute from concanavalin A), are found to have a much lower thermodynamic stability than the wild type. The thermal transition temperatures for nuclease-conA and S28G are 32.8 and 30.5 degrees C, which are about 20 degrees C lower than the Tm for wild-type nuclease A. These mutant proteins also are denatured by a much lower concentration of the denaturants urea and guanidine hydrochloride. We also show that an unfolding transition in the structure of the nuclease-conA hybrids can be induced by relatively low hydrostatic pressure (approximately 700 bar). The free energy for unfolding of nuclease-conA (and nuclease-conA-S28G) is found to be only 1.4 kcal/mol (and 1.2 kcal/mol) by thermal, urea, guanidine hydrochloride, and pressure unfolding. Time-resolved fluorescence intensity and anisotropy measurements with nuclease-conA-S28G show the temperature-, urea-, and pressure-perturbed states each to have a reduced average intensity decay time and to depolarize with a rotational correlation time of approximately 1.0 ns (as compared to a rotational correlation time of 11 ns for the native form of nuclease-conA-S28G at 20 degrees C).

Fluorescence studies with potato carboxypeptidase inhibitor

Potato carboxypeptidase inhibitor (CPI) is a 39-residue globular protein whose X-ray structure is known. The protein's two tryptophan residues (W22 and W28) appear to be on the surface in the crystal structure. The fluorescence spectrum of CPI has a maximum at 344 nm. Acrylamide solute quenching yields an upward curving Stern-Volmer plot with KSV approximately 9 M-1. KI quenching yields a linear plot with KSV approximately 5.5 M-1. These studies indicate that emission occurs from a solvent exposed residue(s). Fluorescence lifetime measurements were fitted to a biexponential decay law with tau 1 = 0.9 ns. f1 = 0.22 and tau 2 = 3.9 ns. Anisotropy decay data were described by a single rotational correlation time of 1.2 ns. This value is somewhat small for the global rotation of a protein of this size. The low temperature phosphorescence of CPI shows a biexponential decay, with a rapidly decaying 0.5-1.0 second component. The triplet state of at least one of the two tryptophan residues must be perturbed by interaction with nearby disulfide bonds.

Fluorescence studies with malate dehydrogenase from Bradyrhizobium japonicum 3I1B-143 bacteroids: a two-tryptophan containing protein

A number of fluorescence studies, both of trp residues and bound NADH, have been reported for porcine malate dehydrogenase (MDH). The large number of trp residues (six) complicates the interpretation of some studies. To circumvent this we have performed studies with a two-tryptophan (per subunit) MDH from Bradyrhizobium japonicum 3I1B-143 bacteroids. We have performed phase/modulation fluorescence lifetime measurements, as a function of temperature and added quencher KI, in order to resolved the 1.2-ns (blue) and 6.5-ns (red) contributions from the two classes of trp residues. Anisotropy decay studies have also been performed. The binding of NADH dynamically quenches the fluorescence of both trp residues, but, unlike mammalian cytoplasmic and mitochondrial MDH, there is not a large enhancement in fluorescence of bound NADH upon forming a ternary complex with either tartronic acid or D-malate.

Effect of micelle diameter on tryptophan dynamics in an amphipathic helical peptide in phosphatidylcholine

The effect of dimyristoylphosphatidylcholine (DMPC) on the conformation and environment of the single tryptophan residue of a model amphipathic helical polypeptide has been investigated by fluorescence quenching with a water-soluble, neutral quencher (acrylamide) and multiple-frequency phase fluorometry. The peptide H-Ser-Ser-Ala-Asp-Trp-Leu-Lys-Ala-Phe-Tyr-Asp-Lys-Val-Ala-Glu-Lys-Leu-Ly s-Glu- Ala-Phe-Ser-Ser-Ser-OH [18As; Kanellis, P., Romans, A.Y., Johnson, B.J., Kercret, H., Chiovetti, R., Jr., Allen, T.M., & Segrest, S.P. (1980) J. Biol. Chem. 255, 11464] was synthesized by solid-phase techniques. Peptide was incubated at 26 degrees C with DMPC at various peptide:lipid weight ratios. The diameter of the resulting disk-shaped micelles increases with increasing lipid concentration from 12.0 +/- 0.4 nm at a 1:1 weight ratio of peptide to lipid to a maximum of 48.7 +/- 1.0 nm at a 1:13 ratio. At a weight ratio of 1:5, the average diameter is 22.7 +/- 0.6 nm. Decreasing the peptide:lipid ratio of the micelle resulted in a blue-shift in the fluorescence emission maximum (from 337 nm at 1:1 to 334 nm at 1:5), an increase in the fluorescence lifetime of the tryptophan measured by the phase shift method at 18 MHz (from 3.12 ns at 1:1 to 3.61 ns at 1:5), a decrease in the rate of fluorescence quenching by acrylamide (from 0.87 x 10(9) M-1 s-1 at 1:1 to 0.42 x 10(9) M-1 s-1 at 1:5), and an increase in the activation energy for quenching (from 6.7 kcal/mol at 1:1 to 12.7 kcal/mol at 1:5).(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence lifetime studies with staphylococcal nuclease and its site-directed mutant. Test of the hypothesis that proline isomerism is the basis for nonexponential decays

Using frequency domain methods, the fluorescence decay of Trp-140 in staphylococcal nuclease and its site-directed mutant (Pro-117----Gly) has been examined. Based on nuclear magnetic resonance (NMR) studies (Evans, P. A., C. M. Dobson, R. A. Kautz, G. Hatfull, and R. O. Fox. 1987. Nature [Lond.]. 329:266-268), it is believed that nuclease exists in two macroscopic, native conformations and that the slow interconversion of these conformations is controlled by the cis----trans isomerization of Pro-117. The above mutant shows only one native conformation in NMR experiments. To test the hypothesis that the biexponential fluorescence decay of Trp-140 of nuclease can also be related to the existence of these conformational states of the protein, we have compared the decay patterns of the wild type and mutant. Essentially no difference was observed, which indicates that there is some other basis for the nonexponential decay of Trp-140. We have used global nonlinear least squares analysis to link the fit of data at several temperatures.

Fluorescence lifetime and solute quenching studies with the single tryptophan containing protein parvalbumin from codfish

The fluorescence decay of cod parvalbumin (both its Ca2+-loaded and Ca2+-depleted forms) is found to be a nonexponential process. The decay data can be fitted either by a double-exponential decay law or by a distribution of decay times. To try to distinguish between the double-exponential and distribution fits, we have collected frequency domain and steady-state fluorescence data as a function of temperature and concentration of the quencher acrylamide. We argue that the correct decay law (i.e., double exponential or distribution) must be consistent with all the data collected as a function of temperature and quencher concentrations. We employ a global analysis procedure to simultaneously fit multiple data sets that are linked by an Arrhenius or Stern-Volmer relationship. For the Ca2+-loaded form of parvalbumin, the distribution model provides a consistent and reasonable fit for all of the frequency domain and steady-state data. The double-exponential model requires more fitting parameters, and some of these assume unreasonable values when this model is fitted to all of the data. For the Ca2+-depleted form of the protein, it is not clear whether the double-exponential or distribution model is superior. For our steady-state solute quenching studies we present a novel analysis in terms of a distribution of quenching constants.

Pressure dependence of fluorescence quenching reactions in proteins

The effect of hydrostatic pressure (0-2.6 kbar) on the acrylamide quenching of the fluorescence of indole derivatives and several single-tryptophan-containing proteins has been studied using phase fluorometry at 25 degrees C. For the model system, N-acetyl-L-tryptophanamide in water, there is essentially no pressure dependence of the quenching rate constant, kappa q. For the internal Trp residue of ribonuclease T1 and cod parvalbumin, there also is essentially no pressure dependence of the apparent kappa q at low pressure. Thus, the activation volume, delta V not equal to, for these quenching processes is approximately zero. Such small delta V not equal to values are expected for diffusion-limited reactions in water at this temperature. The low, apparent delta V not equal to values for the globular proteins characterize these quenching processes as involving very small amplitude fluctuations in the protein structures. Only for the poised tetramer in equilibrium monomer equilibrium of melittin were we able to observe a significant effect of pressure on kappa q and this is due to the pressure-induced shift in the equilibrium position.

Fluorescence studies of phosphoribulokinase, a light-regulated, chloroplastic enzyme

Recent characterization of spinach phosphoribulokinase has revealed that the homodimeric molecule contains only two tryptophans per 44-kDa subunit. We have performed steady-state and frequency domain studies of the intrinsic fluorescence of this protein. The fluorescence properties reflect contributions from both types of tryptophan residues. One of these appears to be relatively exposed to solvent and the quencher, acrylamide; fluoresce with a lambda max of 345 nm; decay with a fluorescence lifetime of 6.3 ns; have a relatively red-shifted absorption spectrum; and have a certain degree of independent motional freedom, with respect to the protein. The other tryptophan residue appears to be more buried; fluoresce with lambda max of 325 nm; have a lifetime of 1.7 ns; have a relatively blue-shifted absorption spectrum; and not to enjoy independent motional freedom. On comparison of phase-resolved spectral data and solute quenching data, we suggest that resonance energy transfer between the blue and red tryptophan residues may occur. We also describe the strategy of simultaneously fitting Stern-Volmer quenching data collected at two emission wavelengths.

Does the fluorescence quencher acrylamide bind to proteins?

We have studied the protein concentration dependence of the acrylamide quenching of the fluorescence of the proteins, human serum albumin and monellin, and we have found no such dependence for the concentration range of 0.5-20 mg/ml. These quenching studies were performed by fluorescence lifetime measurements using phase/modulation fluorometry. We have also performed equilibrium dialysis studies, which show no large degree of association of acrylamide with serum albumin, and we have found that acrylamide has only a small effect on the activity of selected enzymes. These various studies do not indicate the existence of strong acrylamide-protein interactions and are in discord with a recent report by Blatt et al. in this journal (Blatt, E., Husain, A. and Sawyer, W.H. (1986) Biochim. Biophys. Acta 871, 6-13).

Phase-resolved spectral measurements with several two tryptophan containing proteins

We have used frequency domain fluorescence techniques to resolve the component emission spectra for several two tryptophan containing proteins (e.g., horse liver alcohol dehydrogenase, sperm whale apomyoglobin, yeast 3-phosphoglycerate kinase, apoazurin from Alcaligenes denitricans). We have first performed multifrequency phase/modulation measurements and have found the fluorescence of each of these proteins to be described by a double exponential. Then, using phase-sensitive detection and the algorithm of Gratton and Jameson [Gratton, E., & Jameson, D. M. (1985) Anal. Chem. 57, 1694-1697], we have determined the emission spectrum associated with each decay time for these proteins. We have compared these phase-resolved spectra with the fractional contributions of the component fluorophores determined by selective solute quenching experiments. Reasonably good agreement is seen in most cases, which argues that the individual Trp residues emit independently. In the case of apoazurin, however, a negative amplitude is seen for the phase-resolved spectrum of the short-lifetime component. This pattern is consistent with the occurrence of energy transfer from the internal Trp residue to the surface Trp of this protein. We also present multifrequency lifetime measurements, phase-resolved spectra, and solute quenching data for a few protein-ligand complexes, to illustrate the utility of this approach for the study of changes in the fluorescence of proteins.

Frequency-domain fluorescence studies of an extracellular metalloproteinase of Staphylococcus aureus

A frequency-domain fluorescence study of calcium-binding metalloproteinase from Staphylococcus aureus has shown that this two-tryptophan-containing protein exhibits a double-exponential fluorescence decay. At 10 degrees C in 0.05 M Tris-HCl buffer (pH 9.0) containing 10 mM CaCl2, fluorescence lifetimes of 1.2 and 5.1 ns are observed. Steady-state and frequency-domain solute-quenching studies are consistent with the assignment of the two lifetimes to the two tryptophan residues. The tryptophan residue characterized by a shorter lifetime has a maximum of fluorescence emission at about 317 nm and the second one exhibits a maximum of its emission at 350 nm. These two residues contribute almost equally to the protein's fluorescence. These results, as well as fluorescence-quenching studies using KI and acrylamide as a quencher, indicate that in calcium-loaded metalloproteinase, the tryptophan residue characterized by the shorter lifetime is extensively buried within the protein. The second residue is exposed on the surface of the protein. The tryptophan residues of metalloproteinase have acrylamide dynamic-quenching rate constants, kq values, of 2.3 and 0.26 X 10(9) M-1 X s-1 for the exposed and buried residue, respectively. A study of the temperature dependence of the fluorescence lifetime for the two tryptophan components gives activation energies, Ea values, for thermal quenching of 1.8 and 2.2 kcal/mol for the buried and the exposed residue, respectively. Dissociation of Ca2+ from the protein causes a change in the protein's structure, as can be judged from dramatic changes which occur in the fluorescence properties of the buried tryptophan residue. These changes include an approx. 13 nm red-shift in the maximum of the fluorescence emission and an increase in the acrylamide-quenching rate constant, and they indicate that the removal of Ca2+ results in an increase in the exposure and the polarity of the microenvironment of this 'blue' residue.

Frequency domain fluorescence studies of yeast phosphoglycerate kinase and its ternary complex

A frequency domain fluorescence study of yeast phosphoglycerate kinase has been performed to observe the effect of substrates on the structure and dynamics of the enzyme. At 20 degrees C and pH 7.2, a biexponential decay is observed for tryptophanyl emission. The short fluorescence lifetime (0.4 ns) component is associated with a spectrum having a 329-nm maximum and a 18.4-kJ/mol activation energy, Ea, for thermal quenching. The long-lifetime (3.5 ns) component has a 338-nm maximum and an Ea of only 7.9 kJ/mol. Tentatively we assign the short and long-lifetime components to Trp-333 and Trp-308. Binding of the substrates ATP and 3-phosphoglycerate leads to a significant increase in the fluorescence lifetime, the red shift of the emission spectrum and in the decrease in the Ea for both components. Acrylamide-quenching studies indicate that the two tryptophan residues have about the same degree of kinetic exposure to the quencher and that the binding of the substrates causes a very slight change in the quenching pattern. These fluorescence studies indicate that the binding of the substrates to phosphoglycerate kinase may influence the conformational dynamics around the two tryptophan residues located on one of the protein's domains.

Frequency domain measurements of the fluorescence lifetime of ribonuclease T1

Using multifrequency phase/modulation fluorometry, we have studied the fluorescence decay of the single tryptophan residue of ribonuclease T1 (RNase T1). At neutral pH (7.4) we find that the decay is a double exponential (tau 1 = 3.74 ns, tau 2 = 1.06 ns, f1 = 0.945), in agreement with results from pulsed fluorometry. At pH 5.5 the decay is well described by a single decay time (tau = 3.8 ns). Alternatively, we have fitted the frequency domain data by a distribution of lifetimes. Temperature dependence studies were performed. If analyzed via a double exponential model, the activation energy for the inverse of the short lifetime component (at pH 7.4) is found to be 3.6 kcal/mol, as compared with a value of 1.0 kcal/mol for the activation energy of the inverse of the long lifetime component. If analyzed via the distribution model, the width of the distribution is found to increase at higher temperature. We have also repeated, using lifetime measurements, the temperature dependence of the acrylamide quenching of the fluorescence of RNase T1 at pH 5.5. We find an activation energy of 8 kcal/mol for acrylamide quenching, in agreement with our earlier report.

A fluorescence and NMR relaxation study of thermally-induced conformational changes in liver alcohol dehydrogenase

Fluorescence and NMR relaxation studies have been performed on horse liver alcohol dehydrogenase (alcohol: NAD + oxidoreductase, EC 1.1.1.1) as a function of temperature. Observations of both the intrinsic protein fluorescence and the fluorescence of a noncovalently bound apolar probe, 2-(p-toluidinyl)naphthalene-6-sulfonic acid (TNS), indicate that a significant thermal transition occurs in the protein in the range of temperature 0-40 degrees C, and that there are different temperature-dependent forms of the enzyme. The transition between these forms is affected by the binding of specific ligands to the enzyme's active site. Time-resolved fluorescence studies of the two tryptophan residues in the enzyme suggest that this thermal transition occurs around tryptophan-314, which is buried near the intersubunit region. Binding of nucleotide to the enzyme causes a decrease in spin-lattice relaxation time, T1, which may result from a decrease in the number of water molecules bound to the protein. The observed results may be due to the interactions between the structural domains into which the monomer of the protein is folded.

Viscosity dependence of the solute quenching of the tryptophanyl fluorescence of proteins

We have studied the viscosity dependence of the acrylamide quenching of the fluorescence on the internal tryptophan residues in cod parvalbumin and ribonuclease T1, as well as the model systems. N-acetyl-L-tryptophanamide and glucagon. For the latter systems, the apparent rate constant, kq(app), for acrylamide quenching shows a typical diffusion-limited behavior. For parvalbumin and ribonuclease T1, however, the viscosity dependence of kq(app) is quite different. There is little change in the kq(app) values on increasing the bulk viscosity from 1 to 10 cP (by addition of glycerol), but a further increase from 10 to 100 cP results in a significant reduction in the kq(app). Both an unfolding mechanism and a quencher penetration mechanism are considered to explain the results. Only the penetration mechanism is found to be consistent, and our data are interpreted as indicating that the rate-limiting step for quenching goes from being that of diffusion through the protein matrix, at low viscosity, to diffusion through the bulk solvent, at high viscosity. By also considering the Kramers' relationship in fitting our data, we are able to obtain insight regarding the coupling between internal fluctuations in the structure of the protein and motion of the bulk solvent. For parvalbumin and ribonuclease T1, the internal dynamics are found to be very weakly coupled to the bulk.

Quenching of the intrinsic fluorescence of liver alcohol dehydrogenase by the alkaline transition and by coenzyme binding

The fluorescence of alcohol dehydrogenase is quenched by the acid dissociation of some group on the protein having an apparent pKa of 9.6 at 25 degrees C. The pKa of this alkaline quenching transition is unchanged by the binding of trifluoroethanol or pyrazole to the enzyme or by the selective removal of the active site of Zn2+ ion. This indicates that the ionization of a zinc-bound water molecule is not responsible for the quenching. The binding of NAD+ to the enzyme causes a drop in protein fluorescence and an apparent shift in the alkaline quenching transition to lower pH. In the ternary complex formed with NAD+ and trifluoroethanol the alkaline transition is difficult to discern between pH 6 and pH 11. In the NAD+-pyrazole ternary complex, however, a small but noticeable fluorescence transition is observed with a pKa(app) approximately 9.5. We propose that the alkaline transition centered at pH 9.6 is not shifted to lower pH upon binding NAD+. Instead, the amplitude of the alkaline quenching effect is decreased to the point that it is difficult to detect when NAD+ is bound. We present a model that describes the dependence of the fluorescence of the protein on pH and NAD+ concentration in terms of two independently operating, dynamic quenching mechanisms. Our data and model cast serious doubt on the identification, made previously in the literature, between the alkaline quenching pKa and the pKa of the group whose ionization is coupled to NAD+ binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence lifetime and anisotropy studies with liver alcohol dehydrogenase and its complexes

From measurements of the apparent phase and modulation fluorescence lifetime of liver alcohol dehydrogenase at multiple modulation frequencies (6, 18, and 30 MHz), the individual lifetimes and fractional intensities of Trp-314 and Trp-15 are calculated. Values of tau 314 = 3.6, tau 15 = 7.3, and f314 = 0.56, at 20 degrees C, are found. These values are in general agreement with values previously reported by Ross et al. [Ross, J.B.A., Schmidt, C.J., & Brand, L. (1981) Biochemistry 20, 4369] using pulse-decay methodology. In ternary complexes formed between the enzyme, NAD+ and either pyrazole or trifluoroethanol, the fluorescence lifetime of Trp-314 is found to be reduced, indicating that the binding of these ligands causes a dynamic quenching of this residue. The lifetime of Trp-314 is decreased more in the trifluoroethanol ternary complex than that with pyrazole. Also, the alkaline quenching transition of alcohol dehydrogenase is found to result in the selective, dynamic quenching of Trp-314. No change in the lifetimes of the two Trp residues is found upon selective removal of the active-site zinc atoms. From studies of the fluorescence anisotropy, r, of the enzyme as a function of added acrylamide (which selectively quenches the surface Trp-15 residue), the steady-state anisotropy of each residue is determined to be r314 = 0.26 and r15 = 0.21. In the ternary complexes the anisotropy of each residue increases slightly.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of the pH dependence of the formation of binary and ternary complexes with liver alcohol dehydrogenase

The association of the coenzyme NAD+ to liver alcohol dehydrogenase (LADH) is known to be pH dependent, with the binding being linked to the shift in the pK of some group on the protein from a value of 9-10, in the free enzyme, to 7.5-8 in the LADH-NAD+ binary complex. We have further characterized the nature of this linkage between NAD+ binding and proton dissociation by studying the pH dependence (pH range 6-10) of the proton release, delta n, and enthalpy change, delta Ho(app), for formation of both binary (LADH-NAD+) and ternary (LADH-NAD+-I, where I is pyrazole or trifluoroethanol) complexes. The pH dependence of both delta n and delta Ho(app) is found to be consistent with linkage to a single acid dissociating group, whose pK is perturbed from 9.5 to 8.0 upon NAD+ binding and is further perturbed to approximately 6.0 upon ternary complex formation. The apparent enthalpy change for NAD+ binding is endothermic between pH 7 and pH 10, with a maximum at pH 8.5-9.0. The pH dependence of the delta Ho(app) for both binary and ternary complex formation is consistent with a heat of protonation of -7.5 kcal/mol for the coupled acid dissociating group. The intrinsic enthalpy changes for NAD+ binding and NAD+ plus pyrazole binding to LADH are determined to be approximately 0 and -11.0 kcal/mol, respectively. Enthalpy change data are also presented for the binding of the NAD+ analogues adenosine 5'-diphosphoribose and 3-acetylpyridine adenine dinucleotide.