Conformational heterogeneity of the copper binding site in azurin. A time-resolved fluorescence study.

Comparison of the fluorescence spectra and the effect of temperature on the quantum yields of fluorescence of Azurin (from Pseudomonas fluorescens ATCC-13525-2) and 3-methylindole (in methylcyclohexane solution) provides substantive evidence that the tryptophan residue in azurin is completely inaccessible to solvent molecules. The quantum yields of azurin (CuII), azurin (CuI), and apoazurin (lambda ex = 291 nm) were 0.052, 0.054, and 0.31, respectively. Other evidence indicates that there is no energy transfer from tyrosine to tryptophan in any of these proteins. The fluorescence decay behavior of each of the azurin samples was found to be invariant with emission wavelength. The fluorescences of azurin (CuII) and azurin (CuI) decay with dual exponential kinetics (tau 1 = 4.80 ns, tau 2 = 0.18 ns) while that of apoazurin obeys single exponential decay kinetics (tau = 4.90). The ratio of pre-exponentials of azurin (CuII), alpha 1/alpha 2, is found to be 0.25, and this ratio increases to 0.36 on reduction to azurin (CuI). The results are interpreted as originating from different interactions of the tryptophan with two conformers of the copper-ligand complex in azurin.     

The fluorescence decay of tryptophan residues in native and denatured proteins.

The fluorescence decay kinetics at different ranges of the emission spectrum is reported for 17 proteins. Out of eight proteins containing a single tryptophan residue per molecule, seven proteins display multiexponential decay kinetics, suggesting that variability in protein structure may exist for most proteins. Tryptophan residues whose fluorescence spectrum is red shifted may have lifetimes longer than 7 ns. Such long lifetimes have not been detected in any of the denatured proteins studied, indicating that in native proteins the tryptophans having a red-shifted spectrum are affected by the tertiary structure of the protein. The fluorescence decay kinetics of ten denatured proteins studied obey multiexponential decay functions. It is therefore concluded that the tryptophan residues in denatured proteins can be grouped in two classes. The first characterized by a relatively long lifetime of about 4 ns and the second has a short lifetime of about 1.5 ns. The emission spectrum of the group which is characterized by the longer lifetime is red shifted relative to the emission spectrum of the group characterized by the shorter lifetime. A comparison of the decay data with the quantum yield of the proteins raises the possibility that a subgroup of the tryptophan residues is fully quenched. It is noteworthy that despite this heterogeneity in the environment of tryptophan residues in each denatured protein, almost the same decay kinetics has been obtained for all the denatured proteins studied in spite of the vastly different primary structures. It is therefore concluded that each tryptophan residue interacts in a more-or-less random manner with other groups on the polypeptide chain, and that on the average the different tryptophan residues in denatured proteins have a similar type of environment.     

Study of the time-resolved tryptophan fluorescence of crystalline alpha-chymotrypsin

The tryptophan environments in crystalline alpha-chymotrypsin were investigated by fluorescence. The heterogeneous emission from this multitryptophan enzyme was resolved by time-correlated fluorescence spectroscopy. The fluorescence decays at 296-nm laser excitation and various emission wavelengths could be characterized by a triple-exponential function with decay times tau 1 = 150 +/- 50 ps, tau 2 = 1.45 +/- 0.25 ns, and tau 3 = 4.2 +/- 0.4 ns. The corresponding decay-associated emission spectra of the three components had maxima at about 325, 332, and 343 nm. The three decay components in this enzyme can be correlated with X-ray crystallographic data [Birktoft, J.J., & Blow, D.M. (1972) J. Mol. Biol. 68, 187-240]. Inter- and intramolecular tryptophan-tryptophan energy-transfer efficiencies in crystalline alpha-chymotrypsin were computed from the accurately known positions and orientations of all tryptophan residues. These calculations indicate that the three fluorescence decay components in crystalline alpha-chymotrypsin can be assigned to three distinct classes of tryptophyl residues. Because of the different proximity of tryptophan residues to neighboring internal quenching groups, the decay times of the three classes are different. Decay tau 1 can be assigned to Trp-172 and Trp-215 and tau 2 to Trp-51 and Trp-237, while the tryptophyl residues 27, 29, 141, and 207 all have decay time tau 3.     

Acrylamide and oxygen fluorescence quenching studies with liver alcohol dehydrogenase using steady-state and phase fluorometry

The fluorescence lifetime of liver alcohol dehydrogenase (LADH) has been determined by phase fluorometry at various emission wavelengths and as a function of the concentration of the quencher acrylamide. Acrylamide selectively quenches the fluorescence of the surface tryptophanyl residue Trp-15, thus allowing the fluorescence lifetime of this residue and the buried residue Trp-314 to be evaluated. Values of tau15 = 6.9 ns and tau314 = 3.6 ns are obtained, in qualitative agreement with lifetimes of these residues determined from fluorescence decay studies [Ross, J.B.A., Schmidt, C.J., & Brand, L. (1981) Biochemistry 20, 4369-4377]. The quenching of the fluorescence of LADH by oxygen has also been studied. Quenching by oxygen results in a blue shift in the fluorescence of the protein and a downward-curving Stern-Volmer plot. These data, along with oxygen quenching studies in the presence of 1 M acrylamide, are consistent with a model in which oxygen quenches the fluorescence of Trp-314 and -15 with quenching constants of 3.5 and 25 M-1, respectively. Thus, as in studies with other quenchers, Trp-314 is found to be less accessible to the quencher oxygen than is Trp-15. A lifetime Stern-Volmer plot has also been obtained for the oxygen quenching of LADH. Such a plot deviates somewhat from the intensity Stern-Volmer plot as predicted by simulations of the quenching of two-component systems.     

Time-resolved fluorescence study of the single tryptophans of engineered skeletal muscle troponin C.

The regulatory domain of troponin C (TnC) from chicken skeletal muscle was studied using genetically generated mutants which contained a single tryptophan at positions 22, 52, and 90. The quantum yields of Trp-22 are 0.33 and 0.25 in the presence of Mg2+ (2-Mg state) and Ca2+ (4-Ca state), respectively. The large quantum yield of the 2-Mg state is due to a relatively small nonradiative decay rate and consistent with the emission peak at 331 nm. The intensity decay of this state is monoexponential with a single lifetime of 5.65 ns, independent of wavelength. In the 4-Ca state, the decay is biexponential with the mean of the two lifetimes increasing from 4.54 to 4.92 ns across the emission band. The decay-associated spectrum of the short lifetime is red-shifted by 19 nm relative to the steady-state spectrum. The decay of Trp-52 is biexponential in the 2-Mg state and triexponential in the 4-Ca state. The decay of Trp-90 requires three exponential terms for a satisfactory fit, but can be fitted with two exponential terms in the 4-Ca state. The lower quantum yields (< 0.15) of these two tryptophans are due to a combination of smaller radiative and larger nonradiative decay rates. The results from Trp-22 suggest a homogeneous ground-state indole ring in the absence of bound Ca2+ at the regulatory sites and a ground-state heterogeneity induced by activator Ca2+. The Ca(2+)-induced environmental changes of Trp-52 and Trp-90 deviate from those predicted by a modeled structure of the 4-Ca state. The anisotropy decays of all three tryptophans show two rotational correlation times. The long correlation times (phi 1 = 8.1-8.3 ns) derived from Trp-22 and Trp-90 suggest an asymmetric hydrodynamic shape. TnC becomes more asymmetric upon binding activator Ca2+ (phi 1 = 10.1-11.6 ns). The values of phi 1 obtained from Trp-52 are 3-4 ns shorter than those from Trp-22 and Trp-90, and these reduced correlation times may be related to the mobility of the residue and/or local segmental flexibility.     

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.     

Structural microheterogeneity of a tryptophan residue required for efficient biological electron transfer between putidaredoxin and cytochrome P-450cam

The carboxy-terminal tryptophan of putidaredoxin, the Fe2S2.Cys4 iron-sulfur physiological redox partner of cytochrome P-450cam, is essential for maximal biological activity [Davies, M. D., Qin, L., Beck, J. L., Suslick, K. S., Koga, H., Horiuchi, T., & Sligar, S. G. (1990) J. Am. Chem. Soc. 112, 7396-7398]. This single tryptophan-containing protein thus represents an excellent system for studying the solution dynamics of a residue directly implicated in an electron-transfer pathway. Steady-state and time-resolved measurements of the tryptophan fluorescence have been conducted across the emission spectrum as a function of redox state to probe potential structural changes which might be candidates for structural gating phenomena. The steady-state emission spectrum (lambda max = 358 nm) and anisotropy (alpha = 0.04) suggest that Trp-106 is very solvent-exposed and rotating partially free of global protein constraints. The time-resolved fluorescence kinetics for both oxidized and reduced putidaredoxin are fit best with three discrete components of approximately 5, 2, and 0.3 ns. The lifetime components were assigned to physical species with iodide ion quenching experiments, where differential quenching of the longer components was observed (k tau = 2 = 5.9 X 10(8) M-1 s-1, k tau = 5 = 1.3 X 10(8) M-1 s-1). These findings suggest that the multiexponential fluorescence decay results from ground-state conformational microheterogeneity and thus demonstrate that the essential tryptophan exists in at least two distinguishable conformations. Small differences in the relative proportions of the components between redox states were observed but not cleanly resolved.(ABSTRACT TRUNCATED AT 250 WORDS)     

Conformation and dynamics of bovine brain S-100a protein determined by fluorescence spectroscopy.

We have used time-resolved laser fluorescence spectroscopy to investigate the intensity and anisotropy decays of the single tryptophan residue in bovine brain S-100a (alpha beta) protein. The steady-state and acrylamide quenching results indicated that the Trp 90 of the alpha-subunit was partially buried in a relatively nonpolar environment at pH 7.5. Both Ca2+ and pH 8.5 slightly enhanced the exposure of the residue to the solvent, but the residue remained partially buried in the calcium complex at both pH values. The best representation of the intensity decays was a linear combination of three exponential terms, regardless of solvent condition and temperature. The three lifetimes (tau i) were in the range of 0.4-5 ns and insensitive to emission wavelength, but their fractional amplitudes (alpha i) shifted in favor of the shortest component (alpha 1) when the decays were measured at the blue end of the emission spectrum. These results suggest that an excited-state interaction between the indole ring and the side chain of an adjacent residue may be responsible for the observed shortest lifetime. In the presence of Ca2+, the three lifetimes remained relatively unaltered, but the values of alpha 1 decreased by a factor of 2.3 at pH 7.2 and a factor of 1.8 at pH 8.2. This Ca(2+)-induced decrease may be attributed to disruption of the putative excited-state interaction resulting from reorientations of the alpha-helical segments flanking a Ca(2+)-binding loop (residues 62-73). At both pH 7.2 and 8.4, the anisotropy decays of the apoprotein followed a biexponential decay law.(ABSTRACT TRUNCATED AT 250 WORDS)     

Time-resolved fluorescence studies of tryptophan mutants of Escherichia coli glutamine synthetase: conformational analysis of intermediates and transition-state complexes.

Single tryptophan-containing mutants of low adenylylation state Escherichia coli glutamine synthetase have been studied by frequency-domain fluorescence spectroscopy in the presence of various substrates and inhibitors. At pH 6.5, the Mn-bound wild-type enzyme (wild type has two tryptophans/subunit) and the mutant enzymes exhibit heterogeneous fluorescence decay kinetics; the individual tryptophans are adequately described by a triple exponential decay scheme. The recovered lifetime values are 5.9 ns, 2.6 ns, and 0.4 ns for Trp-57 and 5.8 ns, 2.3 ns, and 0.4 ns for Trp-158. These values are nearly identical to the previously reported results at pH 7.5 (Atkins, W.M., Stayton, P.S., & Villafranca, J.J., 1991, Biochemistry 30, 3406-3416). In addition, Trp-57 and Trp-158 both exhibit an ATP-induced increase in the relative fraction of the long lifetime component, whereas only Trp-57 is affected by this ligand at pH 7.5. The transition-state analogue L-methionine-(R,S)-sulfoximine (MSOX) causes a dramatic increase in the fractional intensity of the long lifetime component of Trp-158. This ligand has no effect on the W158S mutant protein and causes a small increase in the fractional intensity of the long lifetime component of the W158F mutant protein. Addition of glutamate to the ATP complex, which affords the gamma-glutamylphosphate-ADP complex, results in the presence of new lifetime components at 7, 3.2, and 0.5 ns for Trp-158, but has no effect on Trp-57. Similar results were obtained when ATP was added to the MSOX complex; Trp-57 exhibits heterogeneous fluorescence decay with lifetimes of 7, 3.5, and 0.8 ns. Decay kinetics of Trp-158 are best fit to a nearly homogeneous decay with a lifetime of 5.5 ns in the MSOX-ATP inactivated complex. These results provide a model for the sequence of structural and dynamic changes that take place at the Trp-57 loop and the central loop (Trp-158) during several intermediate stages of catalysis.     

Time-resolved fluorescence studies of genetically engineered Escherichia coli glutamine synthetase. Effects of ATP on the tryptophan-57 loop.

Single-tryptophan-containing mutants of low adenylation state Escherichia coli glutamine synthetase (wild type has two tryptophans at positions 57 and 158) have been constructed and studied by multifrequency phase/modulation fluorescence spectroscopy. The W57L mutant (retains tryptophan at residue 158) and the W158S mutant (retains tryptophan at residue 57) are both characterized by heterogeneous exponential decay kinetics. Global analysis indicates that for the Mn-bound form of the enzyme at pH 7.4 the fluorescence of both tryptophans is best described by a sum of three discrete expontials with recovered lifetimes of 4.77, 1.72, and 0.10 ns for Trp-57 and 5.04, 2.28, and 0.13 ns for Trp-158. The wild-type enzyme also exhibits decay kinetics described by a triple-exponential model with similar lifetime components. The individual tryptophans are distinguishable by the fractional intensities of the resolvable lifetimes. The wild-type and W158S enzymes are dominated by the 5-ns component which provides nearly 60% and 65%, respectively, of the fractional intensity at five wavelengths spanning the emission spectrum. In contrast, the W57L enzyme demonstrates a larger fraction of the 2-ns lifetime species (60%) and only 35% of the longer lifetime component. The substrate ATP induces a shift to approximately 90% of the 5-ns component for the wild-type and W158S enzymes, whereas the W57L protein is essentially unaffected by this ligand. Steady-state quenching studies with iodide indicate that addition of ATP results in a 3.0-3.5-fold decrease in the apparent Stern-Volmer quenching constants for the wild-type and W158S enzymes. Phase/modulation experiments at several iodide concentrations indicate that the median, 2 ns, lifetime component is selectively quenched compared to the 5-ns lifetime component. These results suggest a model where ATP binding results in a shift in the equilibrium distribution of microconformational states populated by Trp-57. ATP shifts this equilibrium nearly completely to the states exhibiting the long-lifetime component which, based on quenching studies, is less solvent-accessible than the conformational states associated with the other lifetime components.     

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.     

Determination of the excited-state lifetimes of the tryptophan residues in barnase, via multifrequency phase fluorometry of tryptophan mutants.

A multifrequency phase fluorometric study is described for wild-type barnase and engineered mutant proteins in which tryptophan residues have been replaced by less fluorescent residues which do not interfere with the determination of the tryptophan emission spectra and lifetimes. The lifetimes of the three tryptophans in the wild-type protein have been resolved. Trp-35 has a single fluorescence lifetime, which varies in the different proteins between 4.3 and 4.8 ns and is pH-independent between pH 5.8 and 8.9. Trp-71 and Trp-94 behave as an energy-transfer couple with both forward and reverse energy transfer. The couple shows two fluorescence lifetimes: 2.42 (+/-0.2) and 0.74 (+/-0.1) ns at pH 8.9, and 0.89 (+/-0.05) and 0.65 (+/-0.05) ns at pH 5.8. In the mutant Trp-94----Phe the lifetime of Trp-71 is 4.73 (+/-0.008) ns at high pH and 4.70 (+/-0.004) ns at low pH. In the mutant Trp-71----Tyr, the lifetime of Trp-94 is 1.57 (+/-0.01) ns at high pH and 0.82 (+/-0.025) ns at low pH. From these lifetimes, one-way energy-transfer efficiencies can be calculated according to Porter [Porter, G.B. (1972) Theor. Chim. Acta 24, 265-270]. At pH 8.9, a 71% efficiency was found for forward transfer (from Trp-71 to Trp-94) and 36% for reverse transfer. At pH 5.8 the transfer efficiency was 86% for forward and 4% for reverse transfer (all +/-2%). These transfer efficiencies correspond fairly well with the ones calculated according to the theory of F?rster [F?rster, T. (1948) Ann. Phys. (Leipzig) 2, 55-75].(ABSTRACT TRUNCATED AT 250 WORDS)     

Application of a reference convolution method to tryptophan fluorescence in proteins. A refined description of rotational dynamics

A reference method for the deconvolution of polarized fluorescence decay data is described. Fluorescence lifetime determinations for p-terphenyl, p-bis[2-(5-phenyloxazolyl)]benzene and N-acetyltryptophanamide (AcTrpNH2) show that with this method more reliable fits of the decays can be made than with the scatterer method, which is most frequently used. Analysis of the AcTrpNH2 decay with p-terphenyl as the reference compound yields an excellent fit with lifetimes of 2.985 ns for AcTrpNH2 and 1.099 ns for p-terphenyl (20 degrees C), whereas the AcTrpNH2 decay cannot be satisfactorily fitted when the scatterer method is used. The frequency of the detected photons is varied to determine the conditions where pulse pile-up starts to affect the measured decays. At detection frequencies of 5 kHz and 15 kHz, which corresponds to 1.7% and 5% respectively of the rate of the excitation photons no effects are found. Decays measured at 30 kHz (10%) are distorted, indicating that pile-up effects play a role at this frequency. The fluorescence and fluorescence anisotropy decays of the tryptophan residues in the proteins human serum albumin, horse liver alcohol dehydrogenase and lysozyme have been reanalysed with the reference method. The single tryptophan residue of the albumin is shown to be characterized by a triple-exponential fluorescence decay. The anisotropy decay of albumin was found to be mono-exponential with a rotational correlation time of 26 ns (20 degrees C). The alcohol dehydrogenase has two different tryptophan residues to which single lifetimes are assigned. It is found that the rotational correlation time for the dehydrogenase changes with excitation wavelength (33 ns for lambda ex = 295 nm and 36 ns for lambda ex = 300 nm at 20 degrees C), indicating a nonspherical protein molecule. Lysozyme has six tryptophan residues, which give rise to a triple-exponential fluorescence decay. A single-exponential decay with a rotational correlation time of 3.8 ns is found for the anisotropy. This correlation time is significantly shorter than that arising from the overall rotation and probably originates from intramolecular, segmental motion.     

The use of site-directed mutagenesis and time-resolved fluorescence spectroscopy to assign the fluorescence contributions of individual tryptophan residues in Bacillus stearothermophilus lactate dehydrogenase

Site-directed mutagenesis has been used to generate two mutant Bacillus stearothermophilus lactate dehydrogenases: in one, Trp-150 has been replaced with a tyrosine residue and, in the other, both Trp-150 and -80 are replaced with tyrosines. Both enzymes are fully catalytically active and their affinities for substrates and coenzymes, and thermal stabilities are very similar to those of the native enzyme. Time-resolved fluorescence measurements using a synchrotron source have shown that all three tryptophans in the native enzyme fluoresce. By comparing the mutant and native enzymes it was possible, for the first time, to assign, unambiguously, lifetimes to the individual tryptophans: Trp-203 (7.4 ns), Trp-80 (2.35 ns) and Trp-150 (less than 0.3 ns). Trp-203 is responsible for 75-80% of the steady-state fluorescence emission, Trp-80 for 20%, and Trp-150 for less than 2%.     

Conformational changes and subunit communication in tryptophan synthase: effect of substrates and substrate analogs.

The transmission of regulatory signals between the alpha- and beta-subunits of the tryptophan synthase alpha 2 beta 2 complex from Salmonella typhimurium has been investigated by monitoring the luminescence properties of the enzyme in the presence and in the absence of the alpha-subunit ligand DL-alpha-glycerol 3-phosphate, the alpha- and beta-subunit substrate indole, and the beta-subunit substrate analog L-histidine. The beta-subunit contains as intrinsic probes Trp-177 and pyridoxal 5'-phosphate, whereas the alpha-subunit has been mutagenized by replacing Ala-129 with a Trp residue. In contrast to the inertness of L-histidine, DL-alpha-glycerol 3-phosphate was found (i) to alter the phosphorescence spectrum of Trp-129, (ii) to shift the fluorescence thermal quenching profile of both Trp-177 and coenzyme to higher temperature, (iii) to slow down the triplet decay kinetics of Trp-177 in fluid solution, and (iv) to affect the equilibrium between different conformations of the enzyme. These findings provide direct evidence that DL-alpha-glycerol 3-phosphate binding affects the structure of the alpha-subunit and, in the presence of coenzyme, induces a conformational change in the beta-subunit that leads to a considerably more rigid structure. As opposed to DL-alpha-glycerol 3-phosphate, the shortening of the phosphorescence lifetime upon indole binding suggests that this substrate increases structural fluctuations in the beta-subunit. Implications for the mechanism of the allosteric regulation between alpha- and beta-subunits are discussed.     

Antisense suppression of proline degradation improves tolerance to freezing and salinity in Arabidopsis thaliana

The location of tryptophan residues in the actin macromolecule was studied on the basis of the known 3D structure. For every tryptophan residue the polarity and packing density of their microenvironments were evaluated. To estimate the accessibility of the tryptophan residues to the solvent molecules it was proposed to analyze the radial dependence of the packing density of atoms in the macromolecule about the geometric center of the indole rings of the tryptophan residues. The proposed analysis revealed that the microenvironment of tryptophan residues Trp-340 and Trp-356 has a very high density. So these residues can be regarded as internal and inaccessible to solvent molecules. Their microenvironment is mainly formed by non-polar groups of protein. Though the packing density of the Trp-86 microenvironment is lower, this tryptophan residue is apparently also inaccessible to solvent molecules, as it is located in the inner region of macromolecule. Tryptophan residue Trp-79 is external and accessible to the solvent. All residues that can affect tryptophan fluorescence were revealed. It was found that in the close vicinity of tryptophan residues Trp-79 and Trp-86 there are a number of sulfur atoms of cysteine and methionine residues that are known to be effective quenchers of tryptophan fluorescence. The most essential is the location of SG atom of Cys-10 near the NE1 atom of the indole ring of tryptophan residue Trp-86. On the basis of microenvironment analysis of these tryptophan residues and the evaluation of energy transfer between them it was concluded that the contribution of tryptophan residues Trp-79 and Trp-86 must be low. Intrinsic fluorescence of actin must be mainly determined by two other tryptophan residues--Trp-340 and Trp-356. It is possible that the unstrained conformation of tryptophan residue Trp-340 and the existence of aromatic rings of tyrosine and phenylalanine and proline residues in the microenvironments of tryptophan residues Trp-340 and Trp-356 are also essential to their blue fluorescence spectrum.     

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)     

Time-resolved intrinsic fluorescence of Enzyme I. The monomer/dimer transition.

Enzyme I of the bacterial phosphotransferase system can exist in a monomer/dimer equilibrium which may have functional significance. Each monomer contains two tryptophan residues. It is demonstrated that the decay of both the monomer and the dimer can be described by a biexponential. The decay times depend on the temperature and at 6 degrees C the decay times are tau 1 = 0.4 ns and tau 2 = 3.2 ns for the monomer and tau 3 = 3.2 ns and tau 4 = 7.2 ns for the dimer form of the enzyme. The changes in the fluorescence decay parameters can be utilized to measure the equilibrium constant for the monomer/dimer transition.     

Time-resolved fluorescence analysis of the recombinant photosystem II antenna complex CP29. Effects of zeaxanthin, pH and phosphorylation.

Nonradiative dissipation of excitation energy is the major photoprotective mechanism in plants. The formation of zeaxanthin in the antenna of photosystem II has been shown to correlate with the onset of nonphotochemical quenching in vivo. We have used recombinant CP29 protein, over-expressed in Escherichia coli and refolded in vitro with purified pigments, to obtain a protein indistinguishable from the native complex extracted from thylakoids, binding either violaxanthin or zeaxanthin together with lutein. These recombinant proteins and the native CP29 were used to measure steady-state chlorophyll fluorescence emission and fluorescence decay kinetics. We found that the presence of zeaxanthin bound to CP29 induces a approximately 35% decrease in fluorescence yield with respect to the control proteins (the native and zeaxanthin-free reconstituted proteins). Fluorescence decay kinetics showed that four components are always present but lifetimes (tau) as well as relative fluorescence quantum yields (rfqy) of the two long-lived components (tau3 and tau4) are modified by the presence of zeaxanthin. The most relevant changes are observed in the rfqy of tau3 and in the average lifetime ( approximately 2.4 ns with zeaxanthin and 3.2-3.4 ns in the control proteins). When studied in vitro, no significant effect of acidic pH (5.2-5.3) is observed on chlorophyll A fluorescence yield or kinetics. The data presented show that recombinant CP29 is able to bind zeaxanthin and this protein-bound zeaxanthin induces a significant quenching effect.