Sandor Papp

Tryptophan phosphorescence at room temperature as a tool to study protein structure and dynamics.

Fluorescence and phosphorescence resemble each other and in many ways can give the same type of information. Both originate from a dipolar interaction between light and the molecule. In this regard, both are polarized and subject to the same type of quenching phenomena. In other respects the information which they divulge are complementary. The fluorescence quantum yield is higher for exposed tryptophans and this is expressed in longer lifetime (Grinvald and Steinberg, 1976); in contrast long lifetime of phosphorescence appears to correlate with burial. Phosphorescence, spin-disallowed, is much longer lived than fluorescence. This allows the structural/dynamic characterization of proteins to be studied on a new time regime. A really remarkable finding of studies of protein phosphorescence is that there is such variability both in phosphorescence lifetime and quenchability. We would interpret this to indicate that the tryptophan environment can range from essentially a crystal, almost comparable in rigidity as found at 77 K, to tryptophans in a flexible environment, almost as flexible as free in solution. An interesting task will be to examine the relationship between the yield and lifetime of phosphorescence and details of the tryptophan environment in terms of rigidity and adjacent amino acids among the proteins with known three dimensional structure.

Fluorescence energy transfer as an indicator of Ca2+-ATPase interactions in sarcoplasmic reticulum

Ca2+-ATPase molecules were labeled in intact sarcoplasmic reticulum (SR) vesicles, sequentially with a donor fluorophore, fluorescein-5-isothiocyanate (FITC), and with an acceptor fluorophore, eosin-5-isothiocyanate (EITC), each at a mole ratio of 0.25-0.5 mol/mol of ATPase. The resonance energy transfer was determined from the effect of acceptor on the intensity and lifetime of donor fluorescence. Due to structural similarities, the two dyes compete for the same site(s) on the Ca2+-ATPase, and under optimal conditions each ATPase molecule is labeled either with donor or acceptor fluorophore, but not with both. There is only slight labeling of phospholipids and other proteins in SR, even at concentrations of FITC or EITC higher than those used in the reported experiments. Efficient energy transfer was observed from the covalently bound FITC to EITC that is assumed to reflect interaction between ATPase molecules. Protein denaturing agents (8 M urea and 4 M guanidine) or nonsolubilizing concentrations of detergents (C12E8 or lysolecithin) abolish the energy transfer. These results are consistent with earlier observations that a large portion of the Ca2+-ATPase is present in oligomeric form in the native membrane. The technique is suitable for kinetic analysis of the effect of various treatments on the monomer-oligomer equilibrium of Ca2+-ATPase. A drawback of the method is that the labeled ATPase, although it retains conformational responses, is enzymatically inactive.

Reactions of excited triplet states of metal substituted myoglobin with dioxygen and quinone.

The triplet state absorption and phosphorescence of Zn and Pd derivatives of myoglobin were compared. Both metal derivatives exhibit long triplet state lifetimes at room temperature, but whereas the Pd derivative showed exponential decay and an isosbestic point in the transient absorption spectra, the decay of the Zn derivative was nonsingle exponential and the transient absorption spectra showed evidence of more than one excited state species. No difference was seen in triplet quenching by oxygen for either derivative, indicating that differences in the polypeptide chain between the two derivatives are not large enough to affect oxygen penetrability. Quenching was also observed by anthraquinone sulfonate. In this case, the possibility of long-range transfer by an exchange mechanism is considered.

Effect of calcium on the interactions between Ca2+-ATPase molecules in sarcoplasmic reticulum

The interaction between Ca2+-ATPase molecules in the native sarcoplasmic reticulum membrane and in detergent solutions was analyzed by chemical crosslinking, high performance liquid chromatography (HPLC), and by the polarization of fluorescence of fluorescein 5-isothiocyanate (FITC) covalently attached to the Ca2+-ATPase. Reaction of sarcoplasmic reticulum vesicles with glutaraldehyde causes the crosslinking of Ca2+-ATPase molecules with the formation of dimers, tetramers and higher oligomers. At moderate concentrations of glutaraldehyde solubilization of sarcoplasmic reticulum by C12 E8 or Brij 36T (approximately equal to 4 mg/mg protein) decreased the formation of higher oligomers without significant interference with the appearance of crosslinked ATPase dimers. These observations are consistent with the existence of Ca2+-ATPase dimers in detergent-solubilized sarcoplasmic reticulum. Ca2+ (2-20 mM) and glycerol (10-20%) increased the degree of crosslinking at pH 6.0 both in vesicular and in solubilized sarcoplasmic reticulum, presumably by promoting interactions between ATPase molecules; at pH 7.5 the effect of Ca2+ was less pronounced. In agreement with these observations, high performance liquid chromatography of sarcoplasmic reticulum proteins solubilized by Brij 36T or C12 E10 revealed the presence of components with the expected elution characteristics of Ca2+-ATPase oligomers. The polarization of fluorescence of FITC covalently attached to the Ca2+-ATPase is low in the native sarcoplasmic reticulum due to energy transfer, consistent with the existence of ATPase oligomers (Highsmith, S. and Cohen, J.A. (1987) Biochemistry 26, 154-161); upon solubilization of the sarcoplasmic reticulum by detergents, the polarization of fluorescence increased due to dissociation of ATPase oligomers. Based on its effects on the fluorescence of FITC-ATPase, Ca2+ promoted the interaction between ATPase molecules, both in the native membrane and in detergent solutions.

The effect of chelating agents on the elemental composition of sarcoplasmic reticulum: the reactivity of SH groups with N-(1-pyrene)maleimide.

Treatment of sarcoplasmic reticulum vesicles with ethylene glycol bis(beta-aminoethyl ether)-N,N-tetraacetic acid (EGTA), Chelex-100, 1,10-phenanthroline, 8-hydroxyquinoline, or 8-hydroxyquinoline sulfonic acid increases the reactivity of SH groups with N-(1-pyrene)maleimide (PMI). The effect of Chelex treatment can be reversed by the addition of 10(-6)-10(-5) M Zn2+ to the Chelex-treated microsomes. The activation of the PMI reaction by EGTA was not reversed by subsequent addition of calcium, although the presence of excess calcium during EGTA treatment abolished the effect. Analysis of the elemental composition of sarcoplasmic reticulum by plasma emission spectroscopy indicates the presence of Zn, Cu, Fe, and Hg in amounts of 1-2 nmol/mg protein; of these only the Zn content is reduced significantly by treatment of microsomes with EGTA or Chelex-100. These observations suggest that Zn2+ may play a role in the regulation of the reactivity of SH groups in sarcoplasmic reticulum either by direct interaction with cysteinyl residues or by an effect upon the conformation of a subpopulation of ATPase molecules.

Intrinsic tryptophan phosphorescence as a marker of conformation and oxygen diffusion in purified cytochrome oxidase.

Cytochrome oxidase exhibits phosphorescence from trytophan in aqueous solution in the absence of oxygen. The lifetime for the resting reduced enzyme suspended in Tween‐20 is around 30 ms at pH 8. The lifetime is longest between pH 7 and 8 and decreases with lowering of pH. Oxygen quenches the phosphorescence with a Stern‐Volmer quenching constant of ∼5 × 107 M−1 s−1 at 5°C whereas cytochrome c has no effect. We interpret these results to indicate that room temperature tryptophan phosphorescence arises from trytophan(s) in structured region(s) remote from the hemes and that the protein does not impose a significant barrier for the diffusion of oxygen.

Tryptophan phosphorescence of the Ca2+-ATPase of sarcoplasmic reticulum.

Phosphorescence of protein tryptophan was analyzed in sarcoplasmic reticulum vesicles, and in the purified Ca2+ transport ATPase in deoxygenated aqueous solutions at room temperature. Upon excitation with light of 295 nm wavelength, the emission maxima of fluorescence and phosphorescence were at 330 nm and at 445 nm, respectively. The phosphorescence decay was multiexponential; the lifetime of the long-lived component of phosphorescence was approximately equal to 22 ms. ATP and vandate significantly reduced the phosphorescence in the presence of either Ca2+ or EGTA; ADP was less effective, while AMP was without effect. The quenching by ATP showed saturation consistent with the idea that the ATP-enzyme complex had a lower phosphorescence yield. Upon exhaustion of ATP, the phosphorescence returned to starting level. Significant quenching of phosphorescence with a decrease in phosphorescence lifetime was also caused by NaNO2, methylvinyl ketone and trichloroacetate, without effect on ATPase activity; this quenching did not show saturation and was therefore probably collisional in nature.

Long-Range Electron Exchange Reactions of Excited Triplet Tryptophan in Proteins

Abstract Ten proteins that exhibit long-lived phosphorescence lifetimes at room temperature were examined for sensitivity to quenching by molecules that are external to the protein. The bimolecular quenching rate constant was found to decrease exponentially with the distance of the tryptophan from the protein surface. Theoretical analysis shows that this behavior is expected for an electron-exchange reaction between the buried tryptophan and quenchers in solution in the rapid diffusion limit. The results allow evaluation of the distance parameter, ρ, for electron transfer through the general protein matrix at 1.0 A, For a unimolecular donor-acceptor pair with ket = ko exp(-r/ρ), ko = 109sec−1.

Reactions of excited triplet states of metal-substituted myoglobins

The small-molecule (i.e. oxygen) penetrability into proteins carries important implications for both the structural character and dynamic properties of proteins. We investigated aspects of protein dynamics by studying triplet-state quenching of Zn and Pd derivatives of myoglobin by oxygen. These two derivatives make an interesting comparison because the Zn derivative of myoglobin is five coordinated and is out-of-plane (as is deoxy myoglobin), whereas Pd porphyrin is planar (as is oxy myoglobin). Therefore relaxation of the polypeptide chain in the excited state analogues of the two functional conformations of myoglobin can be studied. In case of Znmyoglobin, the spectra did not exhibit an isosbestic point and the decay kinetics to the ground state dependtd upon the wavelength of measurement. The decay profiles could be fit by a double exponential function with a good correlation. In the case of Pd-myoglobin an isosbestic point was observed in the emission and the decay profile could be fit with a single exponential function. The comparison of the quenching constants for the two metal substituted derivatives suggests that there is no significant difference between the oxygen penetrability of the Zn and the Pd substituted myoglobins.